Java tutorial
/* * Copyright (c) 2008, 2019, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.lang.invoke; import jdk.internal.access.SharedSecrets; import jdk.internal.module.IllegalAccessLogger; import jdk.internal.org.objectweb.asm.ClassReader; import jdk.internal.reflect.CallerSensitive; import jdk.internal.reflect.Reflection; import jdk.internal.vm.annotation.ForceInline; import sun.invoke.util.ValueConversions; import sun.invoke.util.VerifyAccess; import sun.invoke.util.Wrapper; import sun.reflect.misc.ReflectUtil; import sun.security.util.SecurityConstants; import java.lang.invoke.LambdaForm.BasicType; import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.Member; import java.lang.reflect.Method; import java.lang.reflect.Modifier; import java.lang.reflect.ReflectPermission; import java.nio.ByteOrder; import java.security.AccessController; import java.security.PrivilegedAction; import java.security.ProtectionDomain; import java.util.ArrayList; import java.util.Arrays; import java.util.BitSet; import java.util.Iterator; import java.util.List; import java.util.Objects; import java.util.Set; import java.util.concurrent.ConcurrentHashMap; import java.util.stream.Collectors; import java.util.stream.Stream; import static java.lang.invoke.MethodHandleImpl.Intrinsic; import static java.lang.invoke.MethodHandleNatives.Constants.*; import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException; import static java.lang.invoke.MethodType.methodType; /** * This class consists exclusively of static methods that operate on or return * method handles. They fall into several categories: * <ul> * <li>Lookup methods which help create method handles for methods and fields. * <li>Combinator methods, which combine or transform pre-existing method handles into new ones. * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns. * </ul> * A lookup, combinator, or factory method will fail and throw an * {@code IllegalArgumentException} if the created method handle's type * would have <a href="MethodHandle.html#maxarity">too many parameters</a>. * * @author John Rose, JSR 292 EG * @since 1.7 */ public class MethodHandles { private MethodHandles() { } // do not instantiate static final MemberName.Factory IMPL_NAMES = MemberName.getFactory(); // See IMPL_LOOKUP below. //// Method handle creation from ordinary methods. /** * Returns a {@link Lookup lookup object} with * full capabilities to emulate all supported bytecode behaviors of the caller. * These capabilities include <a href="MethodHandles.Lookup.html#privacc">private access</a> to the caller. * Factory methods on the lookup object can create * <a href="MethodHandleInfo.html#directmh">direct method handles</a> * for any member that the caller has access to via bytecodes, * including protected and private fields and methods. * This lookup object is a <em>capability</em> which may be delegated to trusted agents. * Do not store it in place where untrusted code can access it. * <p> * This method is caller sensitive, which means that it may return different * values to different callers. * @return a lookup object for the caller of this method, with private access */ @CallerSensitive @ForceInline // to ensure Reflection.getCallerClass optimization public static Lookup lookup() { return new Lookup(Reflection.getCallerClass()); } /** * This reflected$lookup method is the alternate implementation of * the lookup method when being invoked by reflection. */ @CallerSensitive private static Lookup reflected$lookup() { Class<?> caller = Reflection.getCallerClass(); if (caller.getClassLoader() == null) { throw newIllegalArgumentException("illegal lookupClass: " + caller); } return new Lookup(caller); } /** * Returns a {@link Lookup lookup object} which is trusted minimally. * The lookup has the {@code PUBLIC} and {@code UNCONDITIONAL} modes. * It can only be used to create method handles to public members of * public classes in packages that are exported unconditionally. * <p> * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class} * of this lookup object will be {@link java.lang.Object}. * * @apiNote The use of Object is conventional, and because the lookup modes are * limited, there is no special access provided to the internals of Object, its package * or its module. Consequently, the lookup context of this lookup object will be the * bootstrap class loader, which means it cannot find user classes. * * <p style="font-size:smaller;"> * <em>Discussion:</em> * The lookup class can be changed to any other class {@code C} using an expression of the form * {@link Lookup#in publicLookup().in(C.class)}. * but may change the lookup context by virtue of changing the class loader. * A public lookup object is always subject to * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>. * Also, it cannot access * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>. * @return a lookup object which is trusted minimally * * @revised 9 * @spec JPMS */ public static Lookup publicLookup() { return Lookup.PUBLIC_LOOKUP; } /** * Returns a {@link Lookup lookup object} with full capabilities to emulate all * supported bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc"> * private access</a>, on a target class. * This method checks that a caller, specified as a {@code Lookup} object, is allowed to * do <em>deep reflection</em> on the target class. If {@code m1} is the module containing * the {@link Lookup#lookupClass() lookup class}, and {@code m2} is the module containing * the target class, then this check ensures that * <ul> * <li>{@code m1} {@link Module#canRead reads} {@code m2}.</li> * <li>{@code m2} {@link Module#isOpen(String,Module) opens} the package containing * the target class to at least {@code m1}.</li> * <li>The lookup has the {@link Lookup#MODULE MODULE} lookup mode.</li> * </ul> * <p> * If there is a security manager, its {@code checkPermission} method is called to * check {@code ReflectPermission("suppressAccessChecks")}. * @apiNote The {@code MODULE} lookup mode serves to authenticate that the lookup object * was created by code in the caller module (or derived from a lookup object originally * created by the caller). A lookup object with the {@code MODULE} lookup mode can be * shared with trusted parties without giving away {@code PRIVATE} and {@code PACKAGE} * access to the caller. * @param targetClass the target class * @param lookup the caller lookup object * @return a lookup object for the target class, with private access * @throws IllegalArgumentException if {@code targetClass} is a primitve type or array class * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null} * @throws IllegalAccessException if the access check specified above fails * @throws SecurityException if denied by the security manager * @since 9 * @spec JPMS * @see Lookup#dropLookupMode */ public static Lookup privateLookupIn(Class<?> targetClass, Lookup lookup) throws IllegalAccessException { SecurityManager sm = System.getSecurityManager(); if (sm != null) sm.checkPermission(ACCESS_PERMISSION); if (targetClass.isPrimitive()) throw new IllegalArgumentException(targetClass + " is a primitive class"); if (targetClass.isArray()) throw new IllegalArgumentException(targetClass + " is an array class"); Module targetModule = targetClass.getModule(); Module callerModule = lookup.lookupClass().getModule(); if (!callerModule.canRead(targetModule)) throw new IllegalAccessException(callerModule + " does not read " + targetModule); if (targetModule.isNamed()) { String pn = targetClass.getPackageName(); assert !pn.isEmpty() : "unnamed package cannot be in named module"; if (!targetModule.isOpen(pn, callerModule)) throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule); } if ((lookup.lookupModes() & Lookup.MODULE) == 0) throw new IllegalAccessException("lookup does not have MODULE lookup mode"); if (!callerModule.isNamed() && targetModule.isNamed()) { IllegalAccessLogger logger = IllegalAccessLogger.illegalAccessLogger(); if (logger != null) { logger.logIfOpenedForIllegalAccess(lookup, targetClass); } } return new Lookup(targetClass); } /** * Performs an unchecked "crack" of a * <a href="MethodHandleInfo.html#directmh">direct method handle</a>. * The result is as if the user had obtained a lookup object capable enough * to crack the target method handle, called * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect} * on the target to obtain its symbolic reference, and then called * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs} * to resolve the symbolic reference to a member. * <p> * If there is a security manager, its {@code checkPermission} method * is called with a {@code ReflectPermission("suppressAccessChecks")} permission. * @param <T> the desired type of the result, either {@link Member} or a subtype * @param target a direct method handle to crack into symbolic reference components * @param expected a class object representing the desired result type {@code T} * @return a reference to the method, constructor, or field object * @exception SecurityException if the caller is not privileged to call {@code setAccessible} * @exception NullPointerException if either argument is {@code null} * @exception IllegalArgumentException if the target is not a direct method handle * @exception ClassCastException if the member is not of the expected type * @since 1.8 */ public static <T extends Member> T reflectAs(Class<T> expected, MethodHandle target) { SecurityManager smgr = System.getSecurityManager(); if (smgr != null) smgr.checkPermission(ACCESS_PERMISSION); Lookup lookup = Lookup.IMPL_LOOKUP; // use maximally privileged lookup return lookup.revealDirect(target).reflectAs(expected, lookup); } // Copied from AccessibleObject, as used by Method.setAccessible, etc.: private static final java.security.Permission ACCESS_PERMISSION = new ReflectPermission("suppressAccessChecks"); /** * A <em>lookup object</em> is a factory for creating method handles, * when the creation requires access checking. * Method handles do not perform * access checks when they are called, but rather when they are created. * Therefore, method handle access * restrictions must be enforced when a method handle is created. * The caller class against which those restrictions are enforced * is known as the {@linkplain #lookupClass() lookup class}. * <p> * A lookup class which needs to create method handles will call * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself. * When the {@code Lookup} factory object is created, the identity of the lookup class is * determined, and securely stored in the {@code Lookup} object. * The lookup class (or its delegates) may then use factory methods * on the {@code Lookup} object to create method handles for access-checked members. * This includes all methods, constructors, and fields which are allowed to the lookup class, * even private ones. * * <h2><a id="lookups"></a>Lookup Factory Methods</h2> * The factory methods on a {@code Lookup} object correspond to all major * use cases for methods, constructors, and fields. * Each method handle created by a factory method is the functional * equivalent of a particular <em>bytecode behavior</em>. * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.) * Here is a summary of the correspondence between these factory methods and * the behavior of the resulting method handles: * <table class="striped"> * <caption style="display:none">lookup method behaviors</caption> * <thead> * <tr> * <th scope="col"><a id="equiv"></a>lookup expression</th> * <th scope="col">member</th> * <th scope="col">bytecode behavior</th> * </tr> * </thead> * <tbody> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th> * <td>{@code FT f;}</td><td>{@code (T) this.f;}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th> * <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th> * <td>{@code FT f;}</td><td>{@code this.f = x;}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th> * <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th> * <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th> * <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th> * <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th> * <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th> * <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th> * <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th> * <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th> * <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th> * <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td> * </tr> * <tr> * <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th> * <td>{@code class C { ... }}</td><td>{@code C.class;}</td> * </tr> * </tbody> * </table> * * Here, the type {@code C} is the class or interface being searched for a member, * documented as a parameter named {@code refc} in the lookup methods. * The method type {@code MT} is composed from the return type {@code T} * and the sequence of argument types {@code A*}. * The constructor also has a sequence of argument types {@code A*} and * is deemed to return the newly-created object of type {@code C}. * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}. * The formal parameter {@code this} stands for the self-reference of type {@code C}; * if it is present, it is always the leading argument to the method handle invocation. * (In the case of some {@code protected} members, {@code this} may be * restricted in type to the lookup class; see below.) * The name {@code arg} stands for all the other method handle arguments. * In the code examples for the Core Reflection API, the name {@code thisOrNull} * stands for a null reference if the accessed method or field is static, * and {@code this} otherwise. * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand * for reflective objects corresponding to the given members. * <p> * The bytecode behavior for a {@code findClass} operation is a load of a constant class, * as if by {@code ldc CONSTANT_Class}. * The behavior is represented, not as a method handle, but directly as a {@code Class} constant. * <p> * In cases where the given member is of variable arity (i.e., a method or constructor) * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}. * In all other cases, the returned method handle will be of fixed arity. * <p style="font-size:smaller;"> * <em>Discussion:</em> * The equivalence between looked-up method handles and underlying * class members and bytecode behaviors * can break down in a few ways: * <ul style="font-size:smaller;"> * <li>If {@code C} is not symbolically accessible from the lookup class's loader, * the lookup can still succeed, even when there is no equivalent * Java expression or bytecoded constant. * <li>Likewise, if {@code T} or {@code MT} * is not symbolically accessible from the lookup class's loader, * the lookup can still succeed. * For example, lookups for {@code MethodHandle.invokeExact} and * {@code MethodHandle.invoke} will always succeed, regardless of requested type. * <li>If there is a security manager installed, it can forbid the lookup * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>). * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle} * constant is not subject to security manager checks. * <li>If the looked-up method has a * <a href="MethodHandle.html#maxarity">very large arity</a>, * the method handle creation may fail with an * {@code IllegalArgumentException}, due to the method handle type having * <a href="MethodHandle.html#maxarity">too many parameters.</a> * </ul> * * <h2><a id="access"></a>Access checking</h2> * Access checks are applied in the factory methods of {@code Lookup}, * when a method handle is created. * This is a key difference from the Core Reflection API, since * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke} * performs access checking against every caller, on every call. * <p> * All access checks start from a {@code Lookup} object, which * compares its recorded lookup class against all requests to * create method handles. * A single {@code Lookup} object can be used to create any number * of access-checked method handles, all checked against a single * lookup class. * <p> * A {@code Lookup} object can be shared with other trusted code, * such as a metaobject protocol. * A shared {@code Lookup} object delegates the capability * to create method handles on private members of the lookup class. * Even if privileged code uses the {@code Lookup} object, * the access checking is confined to the privileges of the * original lookup class. * <p> * A lookup can fail, because * the containing class is not accessible to the lookup class, or * because the desired class member is missing, or because the * desired class member is not accessible to the lookup class, or * because the lookup object is not trusted enough to access the member. * In the case of a field setter function on a {@code final} field, * finality enforcement is treated as a kind of access control, * and the lookup will fail, except in special cases of * {@link Lookup#unreflectSetter Lookup.unreflectSetter}. * In any of these cases, a {@code ReflectiveOperationException} will be * thrown from the attempted lookup. The exact class will be one of * the following: * <ul> * <li>NoSuchMethodException — if a method is requested but does not exist * <li>NoSuchFieldException — if a field is requested but does not exist * <li>IllegalAccessException — if the member exists but an access check fails * </ul> * <p> * In general, the conditions under which a method handle may be * looked up for a method {@code M} are no more restrictive than the conditions * under which the lookup class could have compiled, verified, and resolved a call to {@code M}. * Where the JVM would raise exceptions like {@code NoSuchMethodError}, * a method handle lookup will generally raise a corresponding * checked exception, such as {@code NoSuchMethodException}. * And the effect of invoking the method handle resulting from the lookup * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a> * to executing the compiled, verified, and resolved call to {@code M}. * The same point is true of fields and constructors. * <p style="font-size:smaller;"> * <em>Discussion:</em> * Access checks only apply to named and reflected methods, * constructors, and fields. * Other method handle creation methods, such as * {@link MethodHandle#asType MethodHandle.asType}, * do not require any access checks, and are used * independently of any {@code Lookup} object. * <p> * If the desired member is {@code protected}, the usual JVM rules apply, * including the requirement that the lookup class must either be in the * same package as the desired member, or must inherit that member. * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.) * In addition, if the desired member is a non-static field or method * in a different package, the resulting method handle may only be applied * to objects of the lookup class or one of its subclasses. * This requirement is enforced by narrowing the type of the leading * {@code this} parameter from {@code C} * (which will necessarily be a superclass of the lookup class) * to the lookup class itself. * <p> * The JVM imposes a similar requirement on {@code invokespecial} instruction, * that the receiver argument must match both the resolved method <em>and</em> * the current class. Again, this requirement is enforced by narrowing the * type of the leading parameter to the resulting method handle. * (See the Java Virtual Machine Specification, section 4.10.1.9.) * <p> * The JVM represents constructors and static initializer blocks as internal methods * with special names ({@code "<init>"} and {@code "<clinit>"}). * The internal syntax of invocation instructions allows them to refer to such internal * methods as if they were normal methods, but the JVM bytecode verifier rejects them. * A lookup of such an internal method will produce a {@code NoSuchMethodException}. * <p> * If the relationship between nested types is expressed directly through the * {@code NestHost} and {@code NestMembers} attributes * (see the Java Virtual Machine Specification, sections 4.7.28 and 4.7.29), * then the associated {@code Lookup} object provides direct access to * the lookup class and all of its nestmates * (see {@link java.lang.Class#getNestHost Class.getNestHost}). * Otherwise, access between nested classes is obtained by the Java compiler creating * a wrapper method to access a private method of another class in the same nest. * For example, a nested class {@code C.D} * can access private members within other related classes such as * {@code C}, {@code C.D.E}, or {@code C.B}, * but the Java compiler may need to generate wrapper methods in * those related classes. In such cases, a {@code Lookup} object on * {@code C.E} would be unable to access those private members. * A workaround for this limitation is the {@link Lookup#in Lookup.in} method, * which can transform a lookup on {@code C.E} into one on any of those other * classes, without special elevation of privilege. * <p> * The accesses permitted to a given lookup object may be limited, * according to its set of {@link #lookupModes lookupModes}, * to a subset of members normally accessible to the lookup class. * For example, the {@link MethodHandles#publicLookup publicLookup} * method produces a lookup object which is only allowed to access * public members in public classes of exported packages. * The caller sensitive method {@link MethodHandles#lookup lookup} * produces a lookup object with full capabilities relative to * its caller class, to emulate all supported bytecode behaviors. * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object * with fewer access modes than the original lookup object. * * <p style="font-size:smaller;"> * <a id="privacc"></a> * <em>Discussion of private access:</em> * We say that a lookup has <em>private access</em> * if its {@linkplain #lookupModes lookup modes} * include the possibility of accessing {@code private} members * (which includes the private members of nestmates). * As documented in the relevant methods elsewhere, * only lookups with private access possess the following capabilities: * <ul style="font-size:smaller;"> * <li>access private fields, methods, and constructors of the lookup class and its nestmates * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods, * such as {@code Class.forName} * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a> * for classes accessible to the lookup class * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes * within the same package member * </ul> * <p style="font-size:smaller;"> * Each of these permissions is a consequence of the fact that a lookup object * with private access can be securely traced back to an originating class, * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions * can be reliably determined and emulated by method handles. * * <h2><a id="secmgr"></a>Security manager interactions</h2> * Although bytecode instructions can only refer to classes in * a related class loader, this API can search for methods in any * class, as long as a reference to its {@code Class} object is * available. Such cross-loader references are also possible with the * Core Reflection API, and are impossible to bytecode instructions * such as {@code invokestatic} or {@code getfield}. * There is a {@linkplain java.lang.SecurityManager security manager API} * to allow applications to check such cross-loader references. * These checks apply to both the {@code MethodHandles.Lookup} API * and the Core Reflection API * (as found on {@link java.lang.Class Class}). * <p> * If a security manager is present, member and class lookups are subject to * additional checks. * From one to three calls are made to the security manager. * Any of these calls can refuse access by throwing a * {@link java.lang.SecurityException SecurityException}. * Define {@code smgr} as the security manager, * {@code lookc} as the lookup class of the current lookup object, * {@code refc} as the containing class in which the member * is being sought, and {@code defc} as the class in which the * member is actually defined. * (If a class or other type is being accessed, * the {@code refc} and {@code defc} values are the class itself.) * The value {@code lookc} is defined as <em>not present</em> * if the current lookup object does not have * <a href="MethodHandles.Lookup.html#privacc">private access</a>. * The calls are made according to the following rules: * <ul> * <li><b>Step 1:</b> * If {@code lookc} is not present, or if its class loader is not * the same as or an ancestor of the class loader of {@code refc}, * then {@link SecurityManager#checkPackageAccess * smgr.checkPackageAccess(refcPkg)} is called, * where {@code refcPkg} is the package of {@code refc}. * <li><b>Step 2a:</b> * If the retrieved member is not public and * {@code lookc} is not present, then * {@link SecurityManager#checkPermission smgr.checkPermission} * with {@code RuntimePermission("accessDeclaredMembers")} is called. * <li><b>Step 2b:</b> * If the retrieved class has a {@code null} class loader, * and {@code lookc} is not present, then * {@link SecurityManager#checkPermission smgr.checkPermission} * with {@code RuntimePermission("getClassLoader")} is called. * <li><b>Step 3:</b> * If the retrieved member is not public, * and if {@code lookc} is not present, * and if {@code defc} and {@code refc} are different, * then {@link SecurityManager#checkPackageAccess * smgr.checkPackageAccess(defcPkg)} is called, * where {@code defcPkg} is the package of {@code defc}. * </ul> * Security checks are performed after other access checks have passed. * Therefore, the above rules presuppose a member or class that is public, * or else that is being accessed from a lookup class that has * rights to access the member or class. * * <h2><a id="callsens"></a>Caller sensitive methods</h2> * A small number of Java methods have a special property called caller sensitivity. * A <em>caller-sensitive</em> method can behave differently depending on the * identity of its immediate caller. * <p> * If a method handle for a caller-sensitive method is requested, * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply, * but they take account of the lookup class in a special way. * The resulting method handle behaves as if it were called * from an instruction contained in the lookup class, * so that the caller-sensitive method detects the lookup class. * (By contrast, the invoker of the method handle is disregarded.) * Thus, in the case of caller-sensitive methods, * different lookup classes may give rise to * differently behaving method handles. * <p> * In cases where the lookup object is * {@link MethodHandles#publicLookup() publicLookup()}, * or some other lookup object without * <a href="MethodHandles.Lookup.html#privacc">private access</a>, * the lookup class is disregarded. * In such cases, no caller-sensitive method handle can be created, * access is forbidden, and the lookup fails with an * {@code IllegalAccessException}. * <p style="font-size:smaller;"> * <em>Discussion:</em> * For example, the caller-sensitive method * {@link java.lang.Class#forName(String) Class.forName(x)} * can return varying classes or throw varying exceptions, * depending on the class loader of the class that calls it. * A public lookup of {@code Class.forName} will fail, because * there is no reasonable way to determine its bytecode behavior. * <p style="font-size:smaller;"> * If an application caches method handles for broad sharing, * it should use {@code publicLookup()} to create them. * If there is a lookup of {@code Class.forName}, it will fail, * and the application must take appropriate action in that case. * It may be that a later lookup, perhaps during the invocation of a * bootstrap method, can incorporate the specific identity * of the caller, making the method accessible. * <p style="font-size:smaller;"> * The function {@code MethodHandles.lookup} is caller sensitive * so that there can be a secure foundation for lookups. * Nearly all other methods in the JSR 292 API rely on lookup * objects to check access requests. * * @revised 9 */ public static final class Lookup { /** The class on behalf of whom the lookup is being performed. */ private final Class<?> lookupClass; /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */ private final int allowedModes; static { Reflection.registerFieldsToFilter(Lookup.class, Set.of("lookupClass", "allowedModes")); } /** A single-bit mask representing {@code public} access, * which may contribute to the result of {@link #lookupModes lookupModes}. * The value, {@code 0x01}, happens to be the same as the value of the * {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}. */ public static final int PUBLIC = Modifier.PUBLIC; /** A single-bit mask representing {@code private} access, * which may contribute to the result of {@link #lookupModes lookupModes}. * The value, {@code 0x02}, happens to be the same as the value of the * {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}. */ public static final int PRIVATE = Modifier.PRIVATE; /** A single-bit mask representing {@code protected} access, * which may contribute to the result of {@link #lookupModes lookupModes}. * The value, {@code 0x04}, happens to be the same as the value of the * {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}. */ public static final int PROTECTED = Modifier.PROTECTED; /** A single-bit mask representing {@code package} access (default access), * which may contribute to the result of {@link #lookupModes lookupModes}. * The value is {@code 0x08}, which does not correspond meaningfully to * any particular {@linkplain java.lang.reflect.Modifier modifier bit}. */ public static final int PACKAGE = Modifier.STATIC; /** A single-bit mask representing {@code module} access (default access), * which may contribute to the result of {@link #lookupModes lookupModes}. * The value is {@code 0x10}, which does not correspond meaningfully to * any particular {@linkplain java.lang.reflect.Modifier modifier bit}. * In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup} * with this lookup mode can access all public types in the module of the * lookup class and public types in packages exported by other modules * to the module of the lookup class. * @since 9 * @spec JPMS */ public static final int MODULE = PACKAGE << 1; /** A single-bit mask representing {@code unconditional} access * which may contribute to the result of {@link #lookupModes lookupModes}. * The value is {@code 0x20}, which does not correspond meaningfully to * any particular {@linkplain java.lang.reflect.Modifier modifier bit}. * A {@code Lookup} with this lookup mode assumes {@linkplain * java.lang.Module#canRead(java.lang.Module) readability}. * In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup} * with this lookup mode can access all public members of public types * of all modules where the type is in a package that is {@link * java.lang.Module#isExported(String) exported unconditionally}. * @since 9 * @spec JPMS * @see #publicLookup() */ public static final int UNCONDITIONAL = PACKAGE << 2; private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL); private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL); private static final int TRUSTED = -1; private static int fixmods(int mods) { mods &= (ALL_MODES - PACKAGE - MODULE - UNCONDITIONAL); return (mods != 0) ? mods : (PACKAGE | MODULE | UNCONDITIONAL); } /** Tells which class is performing the lookup. It is this class against * which checks are performed for visibility and access permissions. * <p> * The class implies a maximum level of access permission, * but the permissions may be additionally limited by the bitmask * {@link #lookupModes lookupModes}, which controls whether non-public members * can be accessed. * @return the lookup class, on behalf of which this lookup object finds members */ public Class<?> lookupClass() { return lookupClass; } // This is just for calling out to MethodHandleImpl. private Class<?> lookupClassOrNull() { return (allowedModes == TRUSTED) ? null : lookupClass; } /** Tells which access-protection classes of members this lookup object can produce. * The result is a bit-mask of the bits * {@linkplain #PUBLIC PUBLIC (0x01)}, * {@linkplain #PRIVATE PRIVATE (0x02)}, * {@linkplain #PROTECTED PROTECTED (0x04)}, * {@linkplain #PACKAGE PACKAGE (0x08)}, * {@linkplain #MODULE MODULE (0x10)}, * and {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)}. * <p> * A freshly-created lookup object * on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has * all possible bits set, except {@code UNCONDITIONAL}. * A lookup object on a new lookup class * {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object} * may have some mode bits set to zero. * Mode bits can also be * {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}. * Once cleared, mode bits cannot be restored from the downgraded lookup object. * The purpose of this is to restrict access via the new lookup object, * so that it can access only names which can be reached by the original * lookup object, and also by the new lookup class. * @return the lookup modes, which limit the kinds of access performed by this lookup object * @see #in * @see #dropLookupMode * * @revised 9 * @spec JPMS */ public int lookupModes() { return allowedModes & ALL_MODES; } /** Embody the current class (the lookupClass) as a lookup class * for method handle creation. * Must be called by from a method in this package, * which in turn is called by a method not in this package. */ Lookup(Class<?> lookupClass) { this(lookupClass, FULL_POWER_MODES); // make sure we haven't accidentally picked up a privileged class: checkUnprivilegedlookupClass(lookupClass); } private Lookup(Class<?> lookupClass, int allowedModes) { this.lookupClass = lookupClass; this.allowedModes = allowedModes; } /** * Creates a lookup on the specified new lookup class. * The resulting object will report the specified * class as its own {@link #lookupClass() lookupClass}. * <p> * However, the resulting {@code Lookup} object is guaranteed * to have no more access capabilities than the original. * In particular, access capabilities can be lost as follows:<ul> * <li>If the old lookup class is in a {@link Module#isNamed() named} module, and * the new lookup class is in a different module {@code M}, then no members, not * even public members in {@code M}'s exported packages, will be accessible. * The exception to this is when this lookup is {@link #publicLookup() * publicLookup}, in which case {@code PUBLIC} access is not lost. * <li>If the old lookup class is in an unnamed module, and the new lookup class * is a different module then {@link #MODULE MODULE} access is lost. * <li>If the new lookup class differs from the old one then {@code UNCONDITIONAL} is lost. * <li>If the new lookup class is in a different package * than the old one, protected and default (package) members will not be accessible. * <li>If the new lookup class is not within the same package member * as the old one, private members will not be accessible, and protected members * will not be accessible by virtue of inheritance. * (Protected members may continue to be accessible because of package sharing.) * <li>If the new lookup class is not accessible to the old lookup class, * then no members, not even public members, will be accessible. * (In all other cases, public members will continue to be accessible.) * </ul> * <p> * The resulting lookup's capabilities for loading classes * (used during {@link #findClass} invocations) * are determined by the lookup class' loader, * which may change due to this operation. * * @param requestedLookupClass the desired lookup class for the new lookup object * @return a lookup object which reports the desired lookup class, or the same object * if there is no change * @throws NullPointerException if the argument is null * * @revised 9 * @spec JPMS */ public Lookup in(Class<?> requestedLookupClass) { Objects.requireNonNull(requestedLookupClass); if (allowedModes == TRUSTED) // IMPL_LOOKUP can make any lookup at all return new Lookup(requestedLookupClass, FULL_POWER_MODES); if (requestedLookupClass == this.lookupClass) return this; // keep same capabilities int newModes = (allowedModes & FULL_POWER_MODES); if (!VerifyAccess.isSameModule(this.lookupClass, requestedLookupClass)) { // Need to drop all access when teleporting from a named module to another // module. The exception is publicLookup where PUBLIC is not lost. if (this.lookupClass.getModule().isNamed() && (this.allowedModes & UNCONDITIONAL) == 0) newModes = 0; else newModes &= ~(MODULE | PACKAGE | PRIVATE | PROTECTED); } if ((newModes & PACKAGE) != 0 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) { newModes &= ~(PACKAGE | PRIVATE | PROTECTED); } // Allow nestmate lookups to be created without special privilege: if ((newModes & PRIVATE) != 0 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) { newModes &= ~(PRIVATE | PROTECTED); } if ((newModes & PUBLIC) != 0 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) { // The requested class it not accessible from the lookup class. // No permissions. newModes = 0; } checkUnprivilegedlookupClass(requestedLookupClass); return new Lookup(requestedLookupClass, newModes); } /** * Creates a lookup on the same lookup class which this lookup object * finds members, but with a lookup mode that has lost the given lookup mode. * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED} or {@link #PRIVATE PRIVATE}. * {@link #PROTECTED PROTECTED} and {@link #UNCONDITIONAL UNCONDITIONAL} are always * dropped and so the resulting lookup mode will never have these access capabilities. * When dropping {@code PACKAGE} then the resulting lookup will not have {@code PACKAGE} * or {@code PRIVATE} access. When dropping {@code MODULE} then the resulting lookup will * not have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access. If {@code PUBLIC} * is dropped then the resulting lookup has no access. * @param modeToDrop the lookup mode to drop * @return a lookup object which lacks the indicated mode, or the same object if there is no change * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC}, * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE} or {@code UNCONDITIONAL} * @see MethodHandles#privateLookupIn * @since 9 */ public Lookup dropLookupMode(int modeToDrop) { int oldModes = lookupModes(); int newModes = oldModes & ~(modeToDrop | PROTECTED | UNCONDITIONAL); switch (modeToDrop) { case PUBLIC: newModes &= ~(ALL_MODES); break; case MODULE: newModes &= ~(PACKAGE | PRIVATE); break; case PACKAGE: newModes &= ~(PRIVATE); break; case PROTECTED: case PRIVATE: case UNCONDITIONAL: break; default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop"); } if (newModes == oldModes) return this; // return self if no change return new Lookup(lookupClass(), newModes); } /** * Defines a class to the same class loader and in the same runtime package and * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's * {@linkplain #lookupClass() lookup class}. * * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include * {@link #PACKAGE PACKAGE} access as default (package) members will be * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate * that the lookup object was created by a caller in the runtime package (or derived * from a lookup originally created by suitably privileged code to a target class in * the runtime package). </p> * * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined * by the <em>The Java Virtual Machine Specification</em>) with a class name in the * same package as the lookup class. </p> * * <p> This method does not run the class initializer. The class initializer may * run at a later time, as detailed in section 12.4 of the <em>The Java Language * Specification</em>. </p> * * <p> If there is a security manager, its {@code checkPermission} method is first called * to check {@code RuntimePermission("defineClass")}. </p> * * @param bytes the class bytes * @return the {@code Class} object for the class * @throws IllegalArgumentException the bytes are for a class in a different package * to the lookup class * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access * @throws LinkageError if the class is malformed ({@code ClassFormatError}), cannot be * verified ({@code VerifyError}), is already defined, or another linkage error occurs * @throws SecurityException if denied by the security manager * @throws NullPointerException if {@code bytes} is {@code null} * @since 9 * @spec JPMS * @see Lookup#privateLookupIn * @see Lookup#dropLookupMode * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain) */ public Class<?> defineClass(byte[] bytes) throws IllegalAccessException { SecurityManager sm = System.getSecurityManager(); if (sm != null) sm.checkPermission(new RuntimePermission("defineClass")); if ((lookupModes() & PACKAGE) == 0) throw new IllegalAccessException("Lookup does not have PACKAGE access"); assert (lookupModes() & (MODULE | PUBLIC)) != 0; // parse class bytes to get class name (in internal form) bytes = bytes.clone(); String name; try { ClassReader reader = new ClassReader(bytes); name = reader.getClassName(); } catch (RuntimeException e) { // ASM exceptions are poorly specified ClassFormatError cfe = new ClassFormatError(); cfe.initCause(e); throw cfe; } // get package and class name in binary form String cn, pn; int index = name.lastIndexOf('/'); if (index == -1) { cn = name; pn = ""; } else { cn = name.replace('/', '.'); pn = cn.substring(0, index); } if (!pn.equals(lookupClass.getPackageName())) { throw new IllegalArgumentException("Class not in same package as lookup class"); } // invoke the class loader's defineClass method ClassLoader loader = lookupClass.getClassLoader(); ProtectionDomain pd = (loader != null) ? lookupClassProtectionDomain() : null; String source = "__Lookup_defineClass__"; Class<?> clazz = SharedSecrets.getJavaLangAccess().defineClass(loader, cn, bytes, pd, source); return clazz; } private ProtectionDomain lookupClassProtectionDomain() { ProtectionDomain pd = cachedProtectionDomain; if (pd == null) { cachedProtectionDomain = pd = protectionDomain(lookupClass); } return pd; } private ProtectionDomain protectionDomain(Class<?> clazz) { PrivilegedAction<ProtectionDomain> pa = clazz::getProtectionDomain; return AccessController.doPrivileged(pa); } // cached protection domain private volatile ProtectionDomain cachedProtectionDomain; // Make sure outer class is initialized first. static { IMPL_NAMES.getClass(); } /** Package-private version of lookup which is trusted. */ static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED); /** Version of lookup which is trusted minimally. * It can only be used to create method handles to publicly accessible * members in packages that are exported unconditionally. */ static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, (PUBLIC | UNCONDITIONAL)); private static void checkUnprivilegedlookupClass(Class<?> lookupClass) { String name = lookupClass.getName(); if (name.startsWith("java.lang.invoke.")) throw newIllegalArgumentException("illegal lookupClass: " + lookupClass); } /** * Displays the name of the class from which lookups are to be made. * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.) * If there are restrictions on the access permitted to this lookup, * this is indicated by adding a suffix to the class name, consisting * of a slash and a keyword. The keyword represents the strongest * allowed access, and is chosen as follows: * <ul> * <li>If no access is allowed, the suffix is "/noaccess". * <li>If only public access to types in exported packages is allowed, the suffix is "/public". * <li>If only public access and unconditional access are allowed, the suffix is "/publicLookup". * <li>If only public and module access are allowed, the suffix is "/module". * <li>If only public, module and package access are allowed, the suffix is "/package". * <li>If only public, module, package, and private access are allowed, the suffix is "/private". * </ul> * If none of the above cases apply, it is the case that full * access (public, module, package, private, and protected) is allowed. * In this case, no suffix is added. * This is true only of an object obtained originally from * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}. * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in} * always have restricted access, and will display a suffix. * <p> * (It may seem strange that protected access should be * stronger than private access. Viewed independently from * package access, protected access is the first to be lost, * because it requires a direct subclass relationship between * caller and callee.) * @see #in * * @revised 9 * @spec JPMS */ @Override public String toString() { String cname = lookupClass.getName(); switch (allowedModes) { case 0: // no privileges return cname + "/noaccess"; case PUBLIC: return cname + "/public"; case PUBLIC | UNCONDITIONAL: return cname + "/publicLookup"; case PUBLIC | MODULE: return cname + "/module"; case PUBLIC | MODULE | PACKAGE: return cname + "/package"; case FULL_POWER_MODES & ~PROTECTED: return cname + "/private"; case FULL_POWER_MODES: return cname; case TRUSTED: return "/trusted"; // internal only; not exported default: // Should not happen, but it's a bitfield... cname = cname + "/" + Integer.toHexString(allowedModes); assert (false) : cname; return cname; } } /** * Produces a method handle for a static method. * The type of the method handle will be that of the method. * (Since static methods do not take receivers, there is no * additional receiver argument inserted into the method handle type, * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.) * The method and all its argument types must be accessible to the lookup object. * <p> * The returned method handle will have * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if * the method's variable arity modifier bit ({@code 0x0080}) is set. * <p> * If the returned method handle is invoked, the method's class will * be initialized, if it has not already been initialized. * <p><b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle MH_asList = publicLookup().findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)); assertEquals("[x, y]", MH_asList.invoke("x", "y").toString()); * }</pre></blockquote> * @param refc the class from which the method is accessed * @param name the name of the method * @param type the type of the method * @return the desired method handle * @throws NoSuchMethodException if the method does not exist * @throws IllegalAccessException if access checking fails, * or if the method is not {@code static}, * or if the method's variable arity modifier bit * is set and {@code asVarargsCollector} fails * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null */ public MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException { MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type); return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method)); } /** * Produces a method handle for a virtual method. * The type of the method handle will be that of the method, * with the receiver type (usually {@code refc}) prepended. * The method and all its argument types must be accessible to the lookup object. * <p> * When called, the handle will treat the first argument as a receiver * and, for non-private methods, dispatch on the receiver's type to determine which method * implementation to enter. * For private methods the named method in {@code refc} will be invoked on the receiver. * (The dispatching action is identical with that performed by an * {@code invokevirtual} or {@code invokeinterface} instruction.) * <p> * The first argument will be of type {@code refc} if the lookup * class has full privileges to access the member. Otherwise * the member must be {@code protected} and the first argument * will be restricted in type to the lookup class. * <p> * The returned method handle will have * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if * the method's variable arity modifier bit ({@code 0x0080}) is set. * <p> * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual} * instructions and method handles produced by {@code findVirtual}, * if the class is {@code MethodHandle} and the name string is * {@code invokeExact} or {@code invoke}, the resulting * method handle is equivalent to one produced by * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker} * with the same {@code type} argument. * <p> * If the class is {@code VarHandle} and the name string corresponds to * the name of a signature-polymorphic access mode method, the resulting * method handle is equivalent to one produced by * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with * the access mode corresponding to the name string and with the same * {@code type} arguments. * <p> * <b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle MH_concat = publicLookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class, "hashCode", methodType(int.class)); MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class, "hashCode", methodType(int.class)); assertEquals("xy", (String) MH_concat.invokeExact("x", "y")); assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy")); assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy")); // interface method: MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class, "subSequence", methodType(CharSequence.class, int.class, int.class)); assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString()); // constructor "internal method" must be accessed differently: MethodType MT_newString = methodType(void.class); //()V for new String() try { assertEquals("impossible", lookup() .findVirtual(String.class, "<init>", MT_newString)); } catch (NoSuchMethodException ex) { } // OK MethodHandle MH_newString = publicLookup() .findConstructor(String.class, MT_newString); assertEquals("", (String) MH_newString.invokeExact()); * }</pre></blockquote> * * @param refc the class or interface from which the method is accessed * @param name the name of the method * @param type the type of the method, with the receiver argument omitted * @return the desired method handle * @throws NoSuchMethodException if the method does not exist * @throws IllegalAccessException if access checking fails, * or if the method is {@code static}, * or if the method's variable arity modifier bit * is set and {@code asVarargsCollector} fails * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null */ public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException { if (refc == MethodHandle.class) { MethodHandle mh = findVirtualForMH(name, type); if (mh != null) return mh; } else if (refc == VarHandle.class) { MethodHandle mh = findVirtualForVH(name, type); if (mh != null) return mh; } byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual); MemberName method = resolveOrFail(refKind, refc, name, type); return getDirectMethod(refKind, refc, method, findBoundCallerClass(method)); } private MethodHandle findVirtualForMH(String name, MethodType type) { // these names require special lookups because of the implicit MethodType argument if ("invoke".equals(name)) return invoker(type); if ("invokeExact".equals(name)) return exactInvoker(type); assert (!MemberName.isMethodHandleInvokeName(name)); return null; } private MethodHandle findVirtualForVH(String name, MethodType type) { try { return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type); } catch (IllegalArgumentException e) { return null; } } /** * Produces a method handle which creates an object and initializes it, using * the constructor of the specified type. * The parameter types of the method handle will be those of the constructor, * while the return type will be a reference to the constructor's class. * The constructor and all its argument types must be accessible to the lookup object. * <p> * The requested type must have a return type of {@code void}. * (This is consistent with the JVM's treatment of constructor type descriptors.) * <p> * The returned method handle will have * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if * the constructor's variable arity modifier bit ({@code 0x0080}) is set. * <p> * If the returned method handle is invoked, the constructor's class will * be initialized, if it has not already been initialized. * <p><b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle MH_newArrayList = publicLookup().findConstructor( ArrayList.class, methodType(void.class, Collection.class)); Collection orig = Arrays.asList("x", "y"); Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig); assert(orig != copy); assertEquals(orig, copy); // a variable-arity constructor: MethodHandle MH_newProcessBuilder = publicLookup().findConstructor( ProcessBuilder.class, methodType(void.class, String[].class)); ProcessBuilder pb = (ProcessBuilder) MH_newProcessBuilder.invoke("x", "y", "z"); assertEquals("[x, y, z]", pb.command().toString()); * }</pre></blockquote> * @param refc the class or interface from which the method is accessed * @param type the type of the method, with the receiver argument omitted, and a void return type * @return the desired method handle * @throws NoSuchMethodException if the constructor does not exist * @throws IllegalAccessException if access checking fails * or if the method's variable arity modifier bit * is set and {@code asVarargsCollector} fails * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null */ public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException { if (refc.isArray()) { throw new NoSuchMethodException("no constructor for array class: " + refc.getName()); } String name = "<init>"; MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type); return getDirectConstructor(refc, ctor); } /** * Looks up a class by name from the lookup context defined by this {@code Lookup} object. The static * initializer of the class is not run. * <p> * The lookup context here is determined by the {@linkplain #lookupClass() lookup class}, its class * loader, and the {@linkplain #lookupModes() lookup modes}. In particular, the method first attempts to * load the requested class, and then determines whether the class is accessible to this lookup object. * * @param targetName the fully qualified name of the class to be looked up. * @return the requested class. * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws LinkageError if the linkage fails * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader. * @throws IllegalAccessException if the class is not accessible, using the allowed access * modes. * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @since 9 */ public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException { Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader()); return accessClass(targetClass); } /** * Determines if a class can be accessed from the lookup context defined by this {@code Lookup} object. The * static initializer of the class is not run. * <p> * The lookup context here is determined by the {@linkplain #lookupClass() lookup class} and the * {@linkplain #lookupModes() lookup modes}. * * @param targetClass the class to be access-checked * * @return the class that has been access-checked * * @throws IllegalAccessException if the class is not accessible from the lookup class, using the allowed access * modes. * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @since 9 */ public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException { if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, allowedModes)) { throw new MemberName(targetClass).makeAccessException("access violation", this); } checkSecurityManager(targetClass, null); return targetClass; } /** * Produces an early-bound method handle for a virtual method. * It will bypass checks for overriding methods on the receiver, * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial} * instruction from within the explicitly specified {@code specialCaller}. * The type of the method handle will be that of the method, * with a suitably restricted receiver type prepended. * (The receiver type will be {@code specialCaller} or a subtype.) * The method and all its argument types must be accessible * to the lookup object. * <p> * Before method resolution, * if the explicitly specified caller class is not identical with the * lookup class, or if this lookup object does not have * <a href="MethodHandles.Lookup.html#privacc">private access</a> * privileges, the access fails. * <p> * The returned method handle will have * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if * the method's variable arity modifier bit ({@code 0x0080}) is set. * <p style="font-size:smaller;"> * <em>(Note: JVM internal methods named {@code "<init>"} are not visible to this API, * even though the {@code invokespecial} instruction can refer to them * in special circumstances. Use {@link #findConstructor findConstructor} * to access instance initialization methods in a safe manner.)</em> * <p><b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... static class Listie extends ArrayList { public String toString() { return "[wee Listie]"; } static Lookup lookup() { return MethodHandles.lookup(); } } ... // no access to constructor via invokeSpecial: MethodHandle MH_newListie = Listie.lookup() .findConstructor(Listie.class, methodType(void.class)); Listie l = (Listie) MH_newListie.invokeExact(); try { assertEquals("impossible", Listie.lookup().findSpecial( Listie.class, "<init>", methodType(void.class), Listie.class)); } catch (NoSuchMethodException ex) { } // OK // access to super and self methods via invokeSpecial: MethodHandle MH_super = Listie.lookup().findSpecial( ArrayList.class, "toString" , methodType(String.class), Listie.class); MethodHandle MH_this = Listie.lookup().findSpecial( Listie.class, "toString" , methodType(String.class), Listie.class); MethodHandle MH_duper = Listie.lookup().findSpecial( Object.class, "toString" , methodType(String.class), Listie.class); assertEquals("[]", (String) MH_super.invokeExact(l)); assertEquals(""+l, (String) MH_this.invokeExact(l)); assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method try { assertEquals("inaccessible", Listie.lookup().findSpecial( String.class, "toString", methodType(String.class), Listie.class)); } catch (IllegalAccessException ex) { } // OK Listie subl = new Listie() { public String toString() { return "[subclass]"; } }; assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method * }</pre></blockquote> * * @param refc the class or interface from which the method is accessed * @param name the name of the method (which must not be "<init>") * @param type the type of the method, with the receiver argument omitted * @param specialCaller the proposed calling class to perform the {@code invokespecial} * @return the desired method handle * @throws NoSuchMethodException if the method does not exist * @throws IllegalAccessException if access checking fails, * or if the method is {@code static}, * or if the method's variable arity modifier bit * is set and {@code asVarargsCollector} fails * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null */ public MethodHandle findSpecial(Class<?> refc, String name, MethodType type, Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException { checkSpecialCaller(specialCaller, refc); Lookup specialLookup = this.in(specialCaller); MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type); return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method)); } /** * Produces a method handle giving read access to a non-static field. * The type of the method handle will have a return type of the field's * value type. * The method handle's single argument will be the instance containing * the field. * Access checking is performed immediately on behalf of the lookup class. * @param refc the class or interface from which the method is accessed * @param name the field's name * @param type the field's type * @return a method handle which can load values from the field * @throws NoSuchFieldException if the field does not exist * @throws IllegalAccessException if access checking fails, or if the field is {@code static} * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null * @see #findVarHandle(Class, String, Class) */ public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException { MemberName field = resolveOrFail(REF_getField, refc, name, type); return getDirectField(REF_getField, refc, field); } /** * Produces a method handle giving write access to a non-static field. * The type of the method handle will have a void return type. * The method handle will take two arguments, the instance containing * the field, and the value to be stored. * The second argument will be of the field's value type. * Access checking is performed immediately on behalf of the lookup class. * @param refc the class or interface from which the method is accessed * @param name the field's name * @param type the field's type * @return a method handle which can store values into the field * @throws NoSuchFieldException if the field does not exist * @throws IllegalAccessException if access checking fails, or if the field is {@code static} * or {@code final} * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null * @see #findVarHandle(Class, String, Class) */ public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException { MemberName field = resolveOrFail(REF_putField, refc, name, type); return getDirectField(REF_putField, refc, field); } /** * Produces a VarHandle giving access to a non-static field {@code name} * of type {@code type} declared in a class of type {@code recv}. * The VarHandle's variable type is {@code type} and it has one * coordinate type, {@code recv}. * <p> * Access checking is performed immediately on behalf of the lookup * class. * <p> * Certain access modes of the returned VarHandle are unsupported under * the following conditions: * <ul> * <li>if the field is declared {@code final}, then the write, atomic * update, numeric atomic update, and bitwise atomic update access * modes are unsupported. * <li>if the field type is anything other than {@code byte}, * {@code short}, {@code char}, {@code int}, {@code long}, * {@code float}, or {@code double} then numeric atomic update * access modes are unsupported. * <li>if the field type is anything other than {@code boolean}, * {@code byte}, {@code short}, {@code char}, {@code int} or * {@code long} then bitwise atomic update access modes are * unsupported. * </ul> * <p> * If the field is declared {@code volatile} then the returned VarHandle * will override access to the field (effectively ignore the * {@code volatile} declaration) in accordance to its specified * access modes. * <p> * If the field type is {@code float} or {@code double} then numeric * and atomic update access modes compare values using their bitwise * representation (see {@link Float#floatToRawIntBits} and * {@link Double#doubleToRawLongBits}, respectively). * @apiNote * Bitwise comparison of {@code float} values or {@code double} values, * as performed by the numeric and atomic update access modes, differ * from the primitive {@code ==} operator and the {@link Float#equals} * and {@link Double#equals} methods, specifically with respect to * comparing NaN values or comparing {@code -0.0} with {@code +0.0}. * Care should be taken when performing a compare and set or a compare * and exchange operation with such values since the operation may * unexpectedly fail. * There are many possible NaN values that are considered to be * {@code NaN} in Java, although no IEEE 754 floating-point operation * provided by Java can distinguish between them. Operation failure can * occur if the expected or witness value is a NaN value and it is * transformed (perhaps in a platform specific manner) into another NaN * value, and thus has a different bitwise representation (see * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more * details). * The values {@code -0.0} and {@code +0.0} have different bitwise * representations but are considered equal when using the primitive * {@code ==} operator. Operation failure can occur if, for example, a * numeric algorithm computes an expected value to be say {@code -0.0} * and previously computed the witness value to be say {@code +0.0}. * @param recv the receiver class, of type {@code R}, that declares the * non-static field * @param name the field's name * @param type the field's type, of type {@code T} * @return a VarHandle giving access to non-static fields. * @throws NoSuchFieldException if the field does not exist * @throws IllegalAccessException if access checking fails, or if the field is {@code static} * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null * @since 9 */ public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException { MemberName getField = resolveOrFail(REF_getField, recv, name, type); MemberName putField = resolveOrFail(REF_putField, recv, name, type); return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField); } /** * Produces a method handle giving read access to a static field. * The type of the method handle will have a return type of the field's * value type. * The method handle will take no arguments. * Access checking is performed immediately on behalf of the lookup class. * <p> * If the returned method handle is invoked, the field's class will * be initialized, if it has not already been initialized. * @param refc the class or interface from which the method is accessed * @param name the field's name * @param type the field's type * @return a method handle which can load values from the field * @throws NoSuchFieldException if the field does not exist * @throws IllegalAccessException if access checking fails, or if the field is not {@code static} * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null */ public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException { MemberName field = resolveOrFail(REF_getStatic, refc, name, type); return getDirectField(REF_getStatic, refc, field); } /** * Produces a method handle giving write access to a static field. * The type of the method handle will have a void return type. * The method handle will take a single * argument, of the field's value type, the value to be stored. * Access checking is performed immediately on behalf of the lookup class. * <p> * If the returned method handle is invoked, the field's class will * be initialized, if it has not already been initialized. * @param refc the class or interface from which the method is accessed * @param name the field's name * @param type the field's type * @return a method handle which can store values into the field * @throws NoSuchFieldException if the field does not exist * @throws IllegalAccessException if access checking fails, or if the field is not {@code static} * or is {@code final} * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null */ public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException { MemberName field = resolveOrFail(REF_putStatic, refc, name, type); return getDirectField(REF_putStatic, refc, field); } /** * Produces a VarHandle giving access to a static field {@code name} of * type {@code type} declared in a class of type {@code decl}. * The VarHandle's variable type is {@code type} and it has no * coordinate types. * <p> * Access checking is performed immediately on behalf of the lookup * class. * <p> * If the returned VarHandle is operated on, the declaring class will be * initialized, if it has not already been initialized. * <p> * Certain access modes of the returned VarHandle are unsupported under * the following conditions: * <ul> * <li>if the field is declared {@code final}, then the write, atomic * update, numeric atomic update, and bitwise atomic update access * modes are unsupported. * <li>if the field type is anything other than {@code byte}, * {@code short}, {@code char}, {@code int}, {@code long}, * {@code float}, or {@code double}, then numeric atomic update * access modes are unsupported. * <li>if the field type is anything other than {@code boolean}, * {@code byte}, {@code short}, {@code char}, {@code int} or * {@code long} then bitwise atomic update access modes are * unsupported. * </ul> * <p> * If the field is declared {@code volatile} then the returned VarHandle * will override access to the field (effectively ignore the * {@code volatile} declaration) in accordance to its specified * access modes. * <p> * If the field type is {@code float} or {@code double} then numeric * and atomic update access modes compare values using their bitwise * representation (see {@link Float#floatToRawIntBits} and * {@link Double#doubleToRawLongBits}, respectively). * @apiNote * Bitwise comparison of {@code float} values or {@code double} values, * as performed by the numeric and atomic update access modes, differ * from the primitive {@code ==} operator and the {@link Float#equals} * and {@link Double#equals} methods, specifically with respect to * comparing NaN values or comparing {@code -0.0} with {@code +0.0}. * Care should be taken when performing a compare and set or a compare * and exchange operation with such values since the operation may * unexpectedly fail. * There are many possible NaN values that are considered to be * {@code NaN} in Java, although no IEEE 754 floating-point operation * provided by Java can distinguish between them. Operation failure can * occur if the expected or witness value is a NaN value and it is * transformed (perhaps in a platform specific manner) into another NaN * value, and thus has a different bitwise representation (see * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more * details). * The values {@code -0.0} and {@code +0.0} have different bitwise * representations but are considered equal when using the primitive * {@code ==} operator. Operation failure can occur if, for example, a * numeric algorithm computes an expected value to be say {@code -0.0} * and previously computed the witness value to be say {@code +0.0}. * @param decl the class that declares the static field * @param name the field's name * @param type the field's type, of type {@code T} * @return a VarHandle giving access to a static field * @throws NoSuchFieldException if the field does not exist * @throws IllegalAccessException if access checking fails, or if the field is not {@code static} * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null * @since 9 */ public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException { MemberName getField = resolveOrFail(REF_getStatic, decl, name, type); MemberName putField = resolveOrFail(REF_putStatic, decl, name, type); return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField); } /** * Produces an early-bound method handle for a non-static method. * The receiver must have a supertype {@code defc} in which a method * of the given name and type is accessible to the lookup class. * The method and all its argument types must be accessible to the lookup object. * The type of the method handle will be that of the method, * without any insertion of an additional receiver parameter. * The given receiver will be bound into the method handle, * so that every call to the method handle will invoke the * requested method on the given receiver. * <p> * The returned method handle will have * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if * the method's variable arity modifier bit ({@code 0x0080}) is set * <em>and</em> the trailing array argument is not the only argument. * (If the trailing array argument is the only argument, * the given receiver value will be bound to it.) * <p> * This is almost equivalent to the following code, with some differences noted below: * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle mh0 = lookup().findVirtual(defc, name, type); MethodHandle mh1 = mh0.bindTo(receiver); mh1 = mh1.withVarargs(mh0.isVarargsCollector()); return mh1; * }</pre></blockquote> * where {@code defc} is either {@code receiver.getClass()} or a super * type of that class, in which the requested method is accessible * to the lookup class. * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity. * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and * the receiver is restricted by {@code findVirtual} to the lookup class.) * @param receiver the object from which the method is accessed * @param name the name of the method * @param type the type of the method, with the receiver argument omitted * @return the desired method handle * @throws NoSuchMethodException if the method does not exist * @throws IllegalAccessException if access checking fails * or if the method's variable arity modifier bit * is set and {@code asVarargsCollector} fails * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws NullPointerException if any argument is null * @see MethodHandle#bindTo * @see #findVirtual */ public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException { Class<? extends Object> refc = receiver.getClass(); // may get NPE MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type); MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerClass(method)); if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) { throw new IllegalAccessException( "The restricted defining class " + mh.type().leadingReferenceParameter().getName() + " is not assignable from receiver class " + receiver.getClass().getName()); } return mh.bindArgumentL(0, receiver).setVarargs(method); } /** * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a> * to <i>m</i>, if the lookup class has permission. * If <i>m</i> is non-static, the receiver argument is treated as an initial argument. * If <i>m</i> is virtual, overriding is respected on every call. * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped. * The type of the method handle will be that of the method, * with the receiver type prepended (but only if it is non-static). * If the method's {@code accessible} flag is not set, * access checking is performed immediately on behalf of the lookup class. * If <i>m</i> is not public, do not share the resulting handle with untrusted parties. * <p> * The returned method handle will have * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if * the method's variable arity modifier bit ({@code 0x0080}) is set. * <p> * If <i>m</i> is static, and * if the returned method handle is invoked, the method's class will * be initialized, if it has not already been initialized. * @param m the reflected method * @return a method handle which can invoke the reflected method * @throws IllegalAccessException if access checking fails * or if the method's variable arity modifier bit * is set and {@code asVarargsCollector} fails * @throws NullPointerException if the argument is null */ public MethodHandle unreflect(Method m) throws IllegalAccessException { if (m.getDeclaringClass() == MethodHandle.class) { MethodHandle mh = unreflectForMH(m); if (mh != null) return mh; } if (m.getDeclaringClass() == VarHandle.class) { MethodHandle mh = unreflectForVH(m); if (mh != null) return mh; } MemberName method = new MemberName(m); byte refKind = method.getReferenceKind(); if (refKind == REF_invokeSpecial) refKind = REF_invokeVirtual; assert (method.isMethod()); @SuppressWarnings("deprecation") Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this; return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerClass(method)); } private MethodHandle unreflectForMH(Method m) { // these names require special lookups because they throw UnsupportedOperationException if (MemberName.isMethodHandleInvokeName(m.getName())) return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m)); return null; } private MethodHandle unreflectForVH(Method m) { // these names require special lookups because they throw UnsupportedOperationException if (MemberName.isVarHandleMethodInvokeName(m.getName())) return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m)); return null; } /** * Produces a method handle for a reflected method. * It will bypass checks for overriding methods on the receiver, * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial} * instruction from within the explicitly specified {@code specialCaller}. * The type of the method handle will be that of the method, * with a suitably restricted receiver type prepended. * (The receiver type will be {@code specialCaller} or a subtype.) * If the method's {@code accessible} flag is not set, * access checking is performed immediately on behalf of the lookup class, * as if {@code invokespecial} instruction were being linked. * <p> * Before method resolution, * if the explicitly specified caller class is not identical with the * lookup class, or if this lookup object does not have * <a href="MethodHandles.Lookup.html#privacc">private access</a> * privileges, the access fails. * <p> * The returned method handle will have * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if * the method's variable arity modifier bit ({@code 0x0080}) is set. * @param m the reflected method * @param specialCaller the class nominally calling the method * @return a method handle which can invoke the reflected method * @throws IllegalAccessException if access checking fails, * or if the method is {@code static}, * or if the method's variable arity modifier bit * is set and {@code asVarargsCollector} fails * @throws NullPointerException if any argument is null */ public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException { checkSpecialCaller(specialCaller, null); Lookup specialLookup = this.in(specialCaller); MemberName method = new MemberName(m, true); assert (method.isMethod()); // ignore m.isAccessible: this is a new kind of access return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerClass(method)); } /** * Produces a method handle for a reflected constructor. * The type of the method handle will be that of the constructor, * with the return type changed to the declaring class. * The method handle will perform a {@code newInstance} operation, * creating a new instance of the constructor's class on the * arguments passed to the method handle. * <p> * If the constructor's {@code accessible} flag is not set, * access checking is performed immediately on behalf of the lookup class. * <p> * The returned method handle will have * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if * the constructor's variable arity modifier bit ({@code 0x0080}) is set. * <p> * If the returned method handle is invoked, the constructor's class will * be initialized, if it has not already been initialized. * @param c the reflected constructor * @return a method handle which can invoke the reflected constructor * @throws IllegalAccessException if access checking fails * or if the method's variable arity modifier bit * is set and {@code asVarargsCollector} fails * @throws NullPointerException if the argument is null */ public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException { MemberName ctor = new MemberName(c); assert (ctor.isConstructor()); @SuppressWarnings("deprecation") Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this; return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor); } /** * Produces a method handle giving read access to a reflected field. * The type of the method handle will have a return type of the field's * value type. * If the field is {@code static}, the method handle will take no arguments. * Otherwise, its single argument will be the instance containing * the field. * If the {@code Field} object's {@code accessible} flag is not set, * access checking is performed immediately on behalf of the lookup class. * <p> * If the field is static, and * if the returned method handle is invoked, the field's class will * be initialized, if it has not already been initialized. * @param f the reflected field * @return a method handle which can load values from the reflected field * @throws IllegalAccessException if access checking fails * @throws NullPointerException if the argument is null */ public MethodHandle unreflectGetter(Field f) throws IllegalAccessException { return unreflectField(f, false); } /** * Produces a method handle giving write access to a reflected field. * The type of the method handle will have a void return type. * If the field is {@code static}, the method handle will take a single * argument, of the field's value type, the value to be stored. * Otherwise, the two arguments will be the instance containing * the field, and the value to be stored. * If the {@code Field} object's {@code accessible} flag is not set, * access checking is performed immediately on behalf of the lookup class. * <p> * If the field is {@code final}, write access will not be * allowed and access checking will fail, except under certain * narrow circumstances documented for {@link Field#set Field.set}. * A method handle is returned only if a corresponding call to * the {@code Field} object's {@code set} method could return * normally. In particular, fields which are both {@code static} * and {@code final} may never be set. * <p> * If the field is {@code static}, and * if the returned method handle is invoked, the field's class will * be initialized, if it has not already been initialized. * @param f the reflected field * @return a method handle which can store values into the reflected field * @throws IllegalAccessException if access checking fails, * or if the field is {@code final} and write access * is not enabled on the {@code Field} object * @throws NullPointerException if the argument is null */ public MethodHandle unreflectSetter(Field f) throws IllegalAccessException { return unreflectField(f, true); } private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException { MemberName field = new MemberName(f, isSetter); if (isSetter && field.isStatic() && field.isFinal()) throw field.makeAccessException("static final field has no write access", this); assert (isSetter ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind()) : MethodHandleNatives.refKindIsGetter(field.getReferenceKind())); @SuppressWarnings("deprecation") Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this; return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field); } /** * Produces a VarHandle giving access to a reflected field {@code f} * of type {@code T} declared in a class of type {@code R}. * The VarHandle's variable type is {@code T}. * If the field is non-static the VarHandle has one coordinate type, * {@code R}. Otherwise, the field is static, and the VarHandle has no * coordinate types. * <p> * Access checking is performed immediately on behalf of the lookup * class, regardless of the value of the field's {@code accessible} * flag. * <p> * If the field is static, and if the returned VarHandle is operated * on, the field's declaring class will be initialized, if it has not * already been initialized. * <p> * Certain access modes of the returned VarHandle are unsupported under * the following conditions: * <ul> * <li>if the field is declared {@code final}, then the write, atomic * update, numeric atomic update, and bitwise atomic update access * modes are unsupported. * <li>if the field type is anything other than {@code byte}, * {@code short}, {@code char}, {@code int}, {@code long}, * {@code float}, or {@code double} then numeric atomic update * access modes are unsupported. * <li>if the field type is anything other than {@code boolean}, * {@code byte}, {@code short}, {@code char}, {@code int} or * {@code long} then bitwise atomic update access modes are * unsupported. * </ul> * <p> * If the field is declared {@code volatile} then the returned VarHandle * will override access to the field (effectively ignore the * {@code volatile} declaration) in accordance to its specified * access modes. * <p> * If the field type is {@code float} or {@code double} then numeric * and atomic update access modes compare values using their bitwise * representation (see {@link Float#floatToRawIntBits} and * {@link Double#doubleToRawLongBits}, respectively). * @apiNote * Bitwise comparison of {@code float} values or {@code double} values, * as performed by the numeric and atomic update access modes, differ * from the primitive {@code ==} operator and the {@link Float#equals} * and {@link Double#equals} methods, specifically with respect to * comparing NaN values or comparing {@code -0.0} with {@code +0.0}. * Care should be taken when performing a compare and set or a compare * and exchange operation with such values since the operation may * unexpectedly fail. * There are many possible NaN values that are considered to be * {@code NaN} in Java, although no IEEE 754 floating-point operation * provided by Java can distinguish between them. Operation failure can * occur if the expected or witness value is a NaN value and it is * transformed (perhaps in a platform specific manner) into another NaN * value, and thus has a different bitwise representation (see * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more * details). * The values {@code -0.0} and {@code +0.0} have different bitwise * representations but are considered equal when using the primitive * {@code ==} operator. Operation failure can occur if, for example, a * numeric algorithm computes an expected value to be say {@code -0.0} * and previously computed the witness value to be say {@code +0.0}. * @param f the reflected field, with a field of type {@code T}, and * a declaring class of type {@code R} * @return a VarHandle giving access to non-static fields or a static * field * @throws IllegalAccessException if access checking fails * @throws NullPointerException if the argument is null * @since 9 */ public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException { MemberName getField = new MemberName(f, false); MemberName putField = new MemberName(f, true); return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(), f.getDeclaringClass(), getField, putField); } /** * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a> * created by this lookup object or a similar one. * Security and access checks are performed to ensure that this lookup object * is capable of reproducing the target method handle. * This means that the cracking may fail if target is a direct method handle * but was created by an unrelated lookup object. * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> * and was created by a lookup object for a different class. * @param target a direct method handle to crack into symbolic reference components * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object * @exception SecurityException if a security manager is present and it * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a> * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails * @exception NullPointerException if the target is {@code null} * @see MethodHandleInfo * @since 1.8 */ public MethodHandleInfo revealDirect(MethodHandle target) { MemberName member = target.internalMemberName(); if (member == null || (!member.isResolved() && !member.isMethodHandleInvoke() && !member.isVarHandleMethodInvoke())) throw newIllegalArgumentException("not a direct method handle"); Class<?> defc = member.getDeclaringClass(); byte refKind = member.getReferenceKind(); assert (MethodHandleNatives.refKindIsValid(refKind)); if (refKind == REF_invokeSpecial && !target.isInvokeSpecial()) // Devirtualized method invocation is usually formally virtual. // To avoid creating extra MemberName objects for this common case, // we encode this extra degree of freedom using MH.isInvokeSpecial. refKind = REF_invokeVirtual; if (refKind == REF_invokeVirtual && defc.isInterface()) // Symbolic reference is through interface but resolves to Object method (toString, etc.) refKind = REF_invokeInterface; // Check SM permissions and member access before cracking. try { checkAccess(refKind, defc, member); checkSecurityManager(defc, member); } catch (IllegalAccessException ex) { throw new IllegalArgumentException(ex); } if (allowedModes != TRUSTED && member.isCallerSensitive()) { Class<?> callerClass = target.internalCallerClass(); if (!hasPrivateAccess() || callerClass != lookupClass()) throw new IllegalArgumentException("method handle is caller sensitive: " + callerClass); } // Produce the handle to the results. return new InfoFromMemberName(this, member, refKind); } /// Helper methods, all package-private. MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException { checkSymbolicClass(refc); // do this before attempting to resolve Objects.requireNonNull(name); Objects.requireNonNull(type); return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), NoSuchFieldException.class); } MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException { checkSymbolicClass(refc); // do this before attempting to resolve Objects.requireNonNull(name); Objects.requireNonNull(type); checkMethodName(refKind, name); // NPE check on name return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), NoSuchMethodException.class); } MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException { checkSymbolicClass(member.getDeclaringClass()); // do this before attempting to resolve Objects.requireNonNull(member.getName()); Objects.requireNonNull(member.getType()); return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(), ReflectiveOperationException.class); } MemberName resolveOrNull(byte refKind, MemberName member) { // do this before attempting to resolve if (!isClassAccessible(member.getDeclaringClass())) { return null; } Objects.requireNonNull(member.getName()); Objects.requireNonNull(member.getType()); return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull()); } void checkSymbolicClass(Class<?> refc) throws IllegalAccessException { if (!isClassAccessible(refc)) { throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this); } } boolean isClassAccessible(Class<?> refc) { Objects.requireNonNull(refc); Class<?> caller = lookupClassOrNull(); return caller == null || VerifyAccess.isClassAccessible(refc, caller, allowedModes); } /** Check name for an illegal leading "<" character. */ void checkMethodName(byte refKind, String name) throws NoSuchMethodException { if (name.startsWith("<") && refKind != REF_newInvokeSpecial) throw new NoSuchMethodException("illegal method name: " + name); } /** * Find my trustable caller class if m is a caller sensitive method. * If this lookup object has private access, then the caller class is the lookupClass. * Otherwise, if m is caller-sensitive, throw IllegalAccessException. */ Class<?> findBoundCallerClass(MemberName m) throws IllegalAccessException { Class<?> callerClass = null; if (MethodHandleNatives.isCallerSensitive(m)) { // Only lookups with private access are allowed to resolve caller-sensitive methods if (hasPrivateAccess()) { callerClass = lookupClass; } else { throw new IllegalAccessException( "Attempt to lookup caller-sensitive method using restricted lookup object"); } } return callerClass; } /** * Returns {@code true} if this lookup has {@code PRIVATE} access. * @return {@code true} if this lookup has {@code PRIVATE} access. * @since 9 */ public boolean hasPrivateAccess() { return (allowedModes & PRIVATE) != 0; } /** * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>. * Determines a trustable caller class to compare with refc, the symbolic reference class. * If this lookup object has private access, then the caller class is the lookupClass. */ void checkSecurityManager(Class<?> refc, MemberName m) { SecurityManager smgr = System.getSecurityManager(); if (smgr == null) return; if (allowedModes == TRUSTED) return; // Step 1: boolean fullPowerLookup = hasPrivateAccess(); if (!fullPowerLookup || !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) { ReflectUtil.checkPackageAccess(refc); } if (m == null) { // findClass or accessClass // Step 2b: if (!fullPowerLookup) { smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION); } return; } // Step 2a: if (m.isPublic()) return; if (!fullPowerLookup) { smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION); } // Step 3: Class<?> defc = m.getDeclaringClass(); if (!fullPowerLookup && defc != refc) { ReflectUtil.checkPackageAccess(defc); } } void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException { boolean wantStatic = (refKind == REF_invokeStatic); String message; if (m.isConstructor()) message = "expected a method, not a constructor"; else if (!m.isMethod()) message = "expected a method"; else if (wantStatic != m.isStatic()) message = wantStatic ? "expected a static method" : "expected a non-static method"; else { checkAccess(refKind, refc, m); return; } throw m.makeAccessException(message, this); } void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException { boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind); String message; if (wantStatic != m.isStatic()) message = wantStatic ? "expected a static field" : "expected a non-static field"; else { checkAccess(refKind, refc, m); return; } throw m.makeAccessException(message, this); } /** Check public/protected/private bits on the symbolic reference class and its member. */ void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException { assert (m.referenceKindIsConsistentWith(refKind) && MethodHandleNatives.refKindIsValid(refKind) && (MethodHandleNatives.refKindIsField(refKind) == m.isField())); int allowedModes = this.allowedModes; if (allowedModes == TRUSTED) return; int mods = m.getModifiers(); if (Modifier.isProtected(mods) && refKind == REF_invokeVirtual && m.getDeclaringClass() == Object.class && m.getName().equals("clone") && refc.isArray()) { // The JVM does this hack also. // (See ClassVerifier::verify_invoke_instructions // and LinkResolver::check_method_accessability.) // Because the JVM does not allow separate methods on array types, // there is no separate method for int[].clone. // All arrays simply inherit Object.clone. // But for access checking logic, we make Object.clone // (normally protected) appear to be public. // Later on, when the DirectMethodHandle is created, // its leading argument will be restricted to the // requested array type. // N.B. The return type is not adjusted, because // that is *not* the bytecode behavior. mods ^= Modifier.PROTECTED | Modifier.PUBLIC; } if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) { // cannot "new" a protected ctor in a different package mods ^= Modifier.PROTECTED; } if (Modifier.isFinal(mods) && MethodHandleNatives.refKindIsSetter(refKind)) throw m.makeAccessException("unexpected set of a final field", this); int requestedModes = fixmods(mods); // adjust 0 => PACKAGE if ((requestedModes & allowedModes) != 0) { if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(), mods, lookupClass(), allowedModes)) return; } else { // Protected members can also be checked as if they were package-private. if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0 && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass())) return; } throw m.makeAccessException(accessFailedMessage(refc, m), this); } String accessFailedMessage(Class<?> refc, MemberName m) { Class<?> defc = m.getDeclaringClass(); int mods = m.getModifiers(); // check the class first: boolean classOK = (Modifier.isPublic(defc.getModifiers()) && (defc == refc || Modifier.isPublic(refc.getModifiers()))); if (!classOK && (allowedModes & PACKAGE) != 0) { classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), FULL_POWER_MODES) && (defc == refc || VerifyAccess.isClassAccessible(refc, lookupClass(), FULL_POWER_MODES))); } if (!classOK) return "class is not public"; if (Modifier.isPublic(mods)) return "access to public member failed"; // (how?, module not readable?) if (Modifier.isPrivate(mods)) return "member is private"; if (Modifier.isProtected(mods)) return "member is protected"; return "member is private to package"; } private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException { int allowedModes = this.allowedModes; if (allowedModes == TRUSTED) return; if (!hasPrivateAccess() || (specialCaller != lookupClass() // ensure non-abstract methods in superinterfaces can be special-invoked && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller)))) throw new MemberName(specialCaller).makeAccessException("no private access for invokespecial", this); } private boolean restrictProtectedReceiver(MemberName method) { // The accessing class only has the right to use a protected member // on itself or a subclass. Enforce that restriction, from JVMS 5.4.4, etc. if (!method.isProtected() || method.isStatic() || allowedModes == TRUSTED || method.getDeclaringClass() == lookupClass() || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass())) return false; return true; } private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException { assert (!method.isStatic()); // receiver type of mh is too wide; narrow to caller if (!method.getDeclaringClass().isAssignableFrom(caller)) { throw method.makeAccessException("caller class must be a subclass below the method", caller); } MethodType rawType = mh.type(); if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow MethodType narrowType = rawType.changeParameterType(0, caller); assert (!mh.isVarargsCollector()); // viewAsType will lose varargs-ness assert (mh.viewAsTypeChecks(narrowType, true)); return mh.copyWith(narrowType, mh.form); } /** Check access and get the requested method. */ private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException { final boolean doRestrict = true; final boolean checkSecurity = true; return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, boundCallerClass); } /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */ private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException { final boolean doRestrict = false; final boolean checkSecurity = true; return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, boundCallerClass); } /** Check access and get the requested method, eliding security manager checks. */ private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException { final boolean doRestrict = true; final boolean checkSecurity = false; // not needed for reflection or for linking CONSTANT_MH constants return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, boundCallerClass); } /** Common code for all methods; do not call directly except from immediately above. */ private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method, boolean checkSecurity, boolean doRestrict, Class<?> boundCallerClass) throws IllegalAccessException { checkMethod(refKind, refc, method); // Optionally check with the security manager; this isn't needed for unreflect* calls. if (checkSecurity) checkSecurityManager(refc, method); assert (!method.isMethodHandleInvoke()); if (refKind == REF_invokeSpecial && refc != lookupClass() && !refc.isInterface() && refc != lookupClass().getSuperclass() && refc.isAssignableFrom(lookupClass())) { assert (!method.getName().equals("<init>")); // not this code path // Per JVMS 6.5, desc. of invokespecial instruction: // If the method is in a superclass of the LC, // and if our original search was above LC.super, // repeat the search (symbolic lookup) from LC.super // and continue with the direct superclass of that class, // and so forth, until a match is found or no further superclasses exist. // FIXME: MemberName.resolve should handle this instead. Class<?> refcAsSuper = lookupClass(); MemberName m2; do { refcAsSuper = refcAsSuper.getSuperclass(); m2 = new MemberName(refcAsSuper, method.getName(), method.getMethodType(), REF_invokeSpecial); m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull()); } while (m2 == null && // no method is found yet refc != refcAsSuper); // search up to refc if (m2 == null) throw new InternalError(method.toString()); method = m2; refc = refcAsSuper; // redo basic checks checkMethod(refKind, refc, method); } DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass()); MethodHandle mh = dmh; // Optionally narrow the receiver argument to lookupClass using restrictReceiver. if ((doRestrict && refKind == REF_invokeSpecial) || (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(method))) { mh = restrictReceiver(method, dmh, lookupClass()); } mh = maybeBindCaller(method, mh, boundCallerClass); mh = mh.setVarargs(method); return mh; } private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh, Class<?> boundCallerClass) throws IllegalAccessException { if (allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method)) return mh; Class<?> hostClass = lookupClass; if (!hasPrivateAccess()) // caller must have private access hostClass = boundCallerClass; // boundCallerClass came from a security manager style stack walk MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, hostClass); // Note: caller will apply varargs after this step happens. return cbmh; } /** Check access and get the requested field. */ private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException { final boolean checkSecurity = true; return getDirectFieldCommon(refKind, refc, field, checkSecurity); } /** Check access and get the requested field, eliding security manager checks. */ private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException { final boolean checkSecurity = false; // not needed for reflection or for linking CONSTANT_MH constants return getDirectFieldCommon(refKind, refc, field, checkSecurity); } /** Common code for all fields; do not call directly except from immediately above. */ private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field, boolean checkSecurity) throws IllegalAccessException { checkField(refKind, refc, field); // Optionally check with the security manager; this isn't needed for unreflect* calls. if (checkSecurity) checkSecurityManager(refc, field); DirectMethodHandle dmh = DirectMethodHandle.make(refc, field); boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(field)); if (doRestrict) return restrictReceiver(field, dmh, lookupClass()); return dmh; } private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind, Class<?> refc, MemberName getField, MemberName putField) throws IllegalAccessException { final boolean checkSecurity = true; return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity); } private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind, Class<?> refc, MemberName getField, MemberName putField) throws IllegalAccessException { final boolean checkSecurity = false; return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity); } private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind, Class<?> refc, MemberName getField, MemberName putField, boolean checkSecurity) throws IllegalAccessException { assert getField.isStatic() == putField.isStatic(); assert getField.isGetter() && putField.isSetter(); assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives .refKindIsStatic(putRefKind); assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind); checkField(getRefKind, refc, getField); if (checkSecurity) checkSecurityManager(refc, getField); if (!putField.isFinal()) { // A VarHandle does not support updates to final fields, any // such VarHandle to a final field will be read-only and // therefore the following write-based accessibility checks are // only required for non-final fields checkField(putRefKind, refc, putField); if (checkSecurity) checkSecurityManager(refc, putField); } boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) && restrictProtectedReceiver(getField)); if (doRestrict) { assert !getField.isStatic(); // receiver type of VarHandle is too wide; narrow to caller if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) { throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass()); } refc = lookupClass(); } return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(), this.allowedModes == TRUSTED); } /** Check access and get the requested constructor. */ private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException { final boolean checkSecurity = true; return getDirectConstructorCommon(refc, ctor, checkSecurity); } /** Check access and get the requested constructor, eliding security manager checks. */ private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException { final boolean checkSecurity = false; // not needed for reflection or for linking CONSTANT_MH constants return getDirectConstructorCommon(refc, ctor, checkSecurity); } /** Common code for all constructors; do not call directly except from immediately above. */ private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor, boolean checkSecurity) throws IllegalAccessException { assert (ctor.isConstructor()); checkAccess(REF_newInvokeSpecial, refc, ctor); // Optionally check with the security manager; this isn't needed for unreflect* calls. if (checkSecurity) checkSecurityManager(refc, ctor); assert (!MethodHandleNatives.isCallerSensitive(ctor)); // maybeBindCaller not relevant here return DirectMethodHandle.make(ctor).setVarargs(ctor); } /** Hook called from the JVM (via MethodHandleNatives) to link MH constants: */ /*non-public*/ MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type) throws ReflectiveOperationException { if (!(type instanceof Class || type instanceof MethodType)) throw new InternalError("unresolved MemberName"); MemberName member = new MemberName(refKind, defc, name, type); MethodHandle mh = LOOKASIDE_TABLE.get(member); if (mh != null) { checkSymbolicClass(defc); return mh; } if (defc == MethodHandle.class && refKind == REF_invokeVirtual) { // Treat MethodHandle.invoke and invokeExact specially. mh = findVirtualForMH(member.getName(), member.getMethodType()); if (mh != null) { return mh; } } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) { // Treat signature-polymorphic methods on VarHandle specially. mh = findVirtualForVH(member.getName(), member.getMethodType()); if (mh != null) { return mh; } } MemberName resolved = resolveOrFail(refKind, member); mh = getDirectMethodForConstant(refKind, defc, resolved); if (mh instanceof DirectMethodHandle && canBeCached(refKind, defc, resolved)) { MemberName key = mh.internalMemberName(); if (key != null) { key = key.asNormalOriginal(); } if (member.equals(key)) { // better safe than sorry LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh); } } return mh; } private boolean canBeCached(byte refKind, Class<?> defc, MemberName member) { if (refKind == REF_invokeSpecial) { return false; } if (!Modifier.isPublic(defc.getModifiers()) || !Modifier.isPublic(member.getDeclaringClass().getModifiers()) || !member.isPublic() || member.isCallerSensitive()) { return false; } ClassLoader loader = defc.getClassLoader(); if (loader != null) { ClassLoader sysl = ClassLoader.getSystemClassLoader(); boolean found = false; while (sysl != null) { if (loader == sysl) { found = true; break; } sysl = sysl.getParent(); } if (!found) { return false; } } try { MemberName resolved2 = publicLookup().resolveOrNull(refKind, new MemberName(refKind, defc, member.getName(), member.getType())); if (resolved2 == null) { return false; } checkSecurityManager(defc, resolved2); } catch (SecurityException ex) { return false; } return true; } private MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member) throws ReflectiveOperationException { if (MethodHandleNatives.refKindIsField(refKind)) { return getDirectFieldNoSecurityManager(refKind, defc, member); } else if (MethodHandleNatives.refKindIsMethod(refKind)) { return getDirectMethodNoSecurityManager(refKind, defc, member, lookupClass); } else if (refKind == REF_newInvokeSpecial) { return getDirectConstructorNoSecurityManager(defc, member); } // oops throw newIllegalArgumentException("bad MethodHandle constant #" + member); } static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>(); } /** * Produces a method handle constructing arrays of a desired type, * as if by the {@code anewarray} bytecode. * The return type of the method handle will be the array type. * The type of its sole argument will be {@code int}, which specifies the size of the array. * * <p> If the returned method handle is invoked with a negative * array size, a {@code NegativeArraySizeException} will be thrown. * * @param arrayClass an array type * @return a method handle which can create arrays of the given type * @throws NullPointerException if the argument is {@code null} * @throws IllegalArgumentException if {@code arrayClass} is not an array type * @see java.lang.reflect.Array#newInstance(Class, int) * @jvms 6.5 {@code anewarray} Instruction * @since 9 */ public static MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException { if (!arrayClass.isArray()) { throw newIllegalArgumentException("not an array class: " + arrayClass.getName()); } MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance) .bindTo(arrayClass.getComponentType()); return ani.asType(ani.type().changeReturnType(arrayClass)); } /** * Produces a method handle returning the length of an array, * as if by the {@code arraylength} bytecode. * The type of the method handle will have {@code int} as return type, * and its sole argument will be the array type. * * <p> If the returned method handle is invoked with a {@code null} * array reference, a {@code NullPointerException} will be thrown. * * @param arrayClass an array type * @return a method handle which can retrieve the length of an array of the given array type * @throws NullPointerException if the argument is {@code null} * @throws IllegalArgumentException if arrayClass is not an array type * @jvms 6.5 {@code arraylength} Instruction * @since 9 */ public static MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException { return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH); } /** * Produces a method handle giving read access to elements of an array, * as if by the {@code aaload} bytecode. * The type of the method handle will have a return type of the array's * element type. Its first argument will be the array type, * and the second will be {@code int}. * * <p> When the returned method handle is invoked, * the array reference and array index are checked. * A {@code NullPointerException} will be thrown if the array reference * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be * thrown if the index is negative or if it is greater than or equal to * the length of the array. * * @param arrayClass an array type * @return a method handle which can load values from the given array type * @throws NullPointerException if the argument is null * @throws IllegalArgumentException if arrayClass is not an array type * @jvms 6.5 {@code aaload} Instruction */ public static MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException { return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET); } /** * Produces a method handle giving write access to elements of an array, * as if by the {@code astore} bytecode. * The type of the method handle will have a void return type. * Its last argument will be the array's element type. * The first and second arguments will be the array type and int. * * <p> When the returned method handle is invoked, * the array reference and array index are checked. * A {@code NullPointerException} will be thrown if the array reference * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be * thrown if the index is negative or if it is greater than or equal to * the length of the array. * * @param arrayClass the class of an array * @return a method handle which can store values into the array type * @throws NullPointerException if the argument is null * @throws IllegalArgumentException if arrayClass is not an array type * @jvms 6.5 {@code aastore} Instruction */ public static MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException { return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET); } /** * Produces a VarHandle giving access to elements of an array of type * {@code arrayClass}. The VarHandle's variable type is the component type * of {@code arrayClass} and the list of coordinate types is * {@code (arrayClass, int)}, where the {@code int} coordinate type * corresponds to an argument that is an index into an array. * <p> * Certain access modes of the returned VarHandle are unsupported under * the following conditions: * <ul> * <li>if the component type is anything other than {@code byte}, * {@code short}, {@code char}, {@code int}, {@code long}, * {@code float}, or {@code double} then numeric atomic update access * modes are unsupported. * <li>if the field type is anything other than {@code boolean}, * {@code byte}, {@code short}, {@code char}, {@code int} or * {@code long} then bitwise atomic update access modes are * unsupported. * </ul> * <p> * If the component type is {@code float} or {@code double} then numeric * and atomic update access modes compare values using their bitwise * representation (see {@link Float#floatToRawIntBits} and * {@link Double#doubleToRawLongBits}, respectively). * * <p> When the returned {@code VarHandle} is invoked, * the array reference and array index are checked. * A {@code NullPointerException} will be thrown if the array reference * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be * thrown if the index is negative or if it is greater than or equal to * the length of the array. * * @apiNote * Bitwise comparison of {@code float} values or {@code double} values, * as performed by the numeric and atomic update access modes, differ * from the primitive {@code ==} operator and the {@link Float#equals} * and {@link Double#equals} methods, specifically with respect to * comparing NaN values or comparing {@code -0.0} with {@code +0.0}. * Care should be taken when performing a compare and set or a compare * and exchange operation with such values since the operation may * unexpectedly fail. * There are many possible NaN values that are considered to be * {@code NaN} in Java, although no IEEE 754 floating-point operation * provided by Java can distinguish between them. Operation failure can * occur if the expected or witness value is a NaN value and it is * transformed (perhaps in a platform specific manner) into another NaN * value, and thus has a different bitwise representation (see * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more * details). * The values {@code -0.0} and {@code +0.0} have different bitwise * representations but are considered equal when using the primitive * {@code ==} operator. Operation failure can occur if, for example, a * numeric algorithm computes an expected value to be say {@code -0.0} * and previously computed the witness value to be say {@code +0.0}. * @param arrayClass the class of an array, of type {@code T[]} * @return a VarHandle giving access to elements of an array * @throws NullPointerException if the arrayClass is null * @throws IllegalArgumentException if arrayClass is not an array type * @since 9 */ public static VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException { return VarHandles.makeArrayElementHandle(arrayClass); } /** * Produces a VarHandle giving access to elements of a {@code byte[]} array * viewed as if it were a different primitive array type, such as * {@code int[]} or {@code long[]}. * The VarHandle's variable type is the component type of * {@code viewArrayClass} and the list of coordinate types is * {@code (byte[], int)}, where the {@code int} coordinate type * corresponds to an argument that is an index into a {@code byte[]} array. * The returned VarHandle accesses bytes at an index in a {@code byte[]} * array, composing bytes to or from a value of the component type of * {@code viewArrayClass} according to the given endianness. * <p> * The supported component types (variables types) are {@code short}, * {@code char}, {@code int}, {@code long}, {@code float} and * {@code double}. * <p> * Access of bytes at a given index will result in an * {@code IndexOutOfBoundsException} if the index is less than {@code 0} * or greater than the {@code byte[]} array length minus the size (in bytes) * of {@code T}. * <p> * Access of bytes at an index may be aligned or misaligned for {@code T}, * with respect to the underlying memory address, {@code A} say, associated * with the array and index. * If access is misaligned then access for anything other than the * {@code get} and {@code set} access modes will result in an * {@code IllegalStateException}. In such cases atomic access is only * guaranteed with respect to the largest power of two that divides the GCD * of {@code A} and the size (in bytes) of {@code T}. * If access is aligned then following access modes are supported and are * guaranteed to support atomic access: * <ul> * <li>read write access modes for all {@code T}, with the exception of * access modes {@code get} and {@code set} for {@code long} and * {@code double} on 32-bit platforms. * <li>atomic update access modes for {@code int}, {@code long}, * {@code float} or {@code double}. * (Future major platform releases of the JDK may support additional * types for certain currently unsupported access modes.) * <li>numeric atomic update access modes for {@code int} and {@code long}. * (Future major platform releases of the JDK may support additional * numeric types for certain currently unsupported access modes.) * <li>bitwise atomic update access modes for {@code int} and {@code long}. * (Future major platform releases of the JDK may support additional * numeric types for certain currently unsupported access modes.) * </ul> * <p> * Misaligned access, and therefore atomicity guarantees, may be determined * for {@code byte[]} arrays without operating on a specific array. Given * an {@code index}, {@code T} and it's corresponding boxed type, * {@code T_BOX}, misalignment may be determined as follows: * <pre>{@code * int sizeOfT = T_BOX.BYTES; // size in bytes of T * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]). * alignmentOffset(0, sizeOfT); * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT; * boolean isMisaligned = misalignedAtIndex != 0; * }</pre> * <p> * If the variable type is {@code float} or {@code double} then atomic * update access modes compare values using their bitwise representation * (see {@link Float#floatToRawIntBits} and * {@link Double#doubleToRawLongBits}, respectively). * @param viewArrayClass the view array class, with a component type of * type {@code T} * @param byteOrder the endianness of the view array elements, as * stored in the underlying {@code byte} array * @return a VarHandle giving access to elements of a {@code byte[]} array * viewed as if elements corresponding to the components type of the view * array class * @throws NullPointerException if viewArrayClass or byteOrder is null * @throws IllegalArgumentException if viewArrayClass is not an array type * @throws UnsupportedOperationException if the component type of * viewArrayClass is not supported as a variable type * @since 9 */ public static VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass, ByteOrder byteOrder) throws IllegalArgumentException { Objects.requireNonNull(byteOrder); return VarHandles.byteArrayViewHandle(viewArrayClass, byteOrder == ByteOrder.BIG_ENDIAN); } /** * Produces a VarHandle giving access to elements of a {@code ByteBuffer} * viewed as if it were an array of elements of a different primitive * component type to that of {@code byte}, such as {@code int[]} or * {@code long[]}. * The VarHandle's variable type is the component type of * {@code viewArrayClass} and the list of coordinate types is * {@code (ByteBuffer, int)}, where the {@code int} coordinate type * corresponds to an argument that is an index into a {@code byte[]} array. * The returned VarHandle accesses bytes at an index in a * {@code ByteBuffer}, composing bytes to or from a value of the component * type of {@code viewArrayClass} according to the given endianness. * <p> * The supported component types (variables types) are {@code short}, * {@code char}, {@code int}, {@code long}, {@code float} and * {@code double}. * <p> * Access will result in a {@code ReadOnlyBufferException} for anything * other than the read access modes if the {@code ByteBuffer} is read-only. * <p> * Access of bytes at a given index will result in an * {@code IndexOutOfBoundsException} if the index is less than {@code 0} * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of * {@code T}. * <p> * Access of bytes at an index may be aligned or misaligned for {@code T}, * with respect to the underlying memory address, {@code A} say, associated * with the {@code ByteBuffer} and index. * If access is misaligned then access for anything other than the * {@code get} and {@code set} access modes will result in an * {@code IllegalStateException}. In such cases atomic access is only * guaranteed with respect to the largest power of two that divides the GCD * of {@code A} and the size (in bytes) of {@code T}. * If access is aligned then following access modes are supported and are * guaranteed to support atomic access: * <ul> * <li>read write access modes for all {@code T}, with the exception of * access modes {@code get} and {@code set} for {@code long} and * {@code double} on 32-bit platforms. * <li>atomic update access modes for {@code int}, {@code long}, * {@code float} or {@code double}. * (Future major platform releases of the JDK may support additional * types for certain currently unsupported access modes.) * <li>numeric atomic update access modes for {@code int} and {@code long}. * (Future major platform releases of the JDK may support additional * numeric types for certain currently unsupported access modes.) * <li>bitwise atomic update access modes for {@code int} and {@code long}. * (Future major platform releases of the JDK may support additional * numeric types for certain currently unsupported access modes.) * </ul> * <p> * Misaligned access, and therefore atomicity guarantees, may be determined * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an * {@code index}, {@code T} and it's corresponding boxed type, * {@code T_BOX}, as follows: * <pre>{@code * int sizeOfT = T_BOX.BYTES; // size in bytes of T * ByteBuffer bb = ... * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT); * boolean isMisaligned = misalignedAtIndex != 0; * }</pre> * <p> * If the variable type is {@code float} or {@code double} then atomic * update access modes compare values using their bitwise representation * (see {@link Float#floatToRawIntBits} and * {@link Double#doubleToRawLongBits}, respectively). * @param viewArrayClass the view array class, with a component type of * type {@code T} * @param byteOrder the endianness of the view array elements, as * stored in the underlying {@code ByteBuffer} (Note this overrides the * endianness of a {@code ByteBuffer}) * @return a VarHandle giving access to elements of a {@code ByteBuffer} * viewed as if elements corresponding to the components type of the view * array class * @throws NullPointerException if viewArrayClass or byteOrder is null * @throws IllegalArgumentException if viewArrayClass is not an array type * @throws UnsupportedOperationException if the component type of * viewArrayClass is not supported as a variable type * @since 9 */ public static VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass, ByteOrder byteOrder) throws IllegalArgumentException { Objects.requireNonNull(byteOrder); return VarHandles.makeByteBufferViewHandle(viewArrayClass, byteOrder == ByteOrder.BIG_ENDIAN); } /// method handle invocation (reflective style) /** * Produces a method handle which will invoke any method handle of the * given {@code type}, with a given number of trailing arguments replaced by * a single trailing {@code Object[]} array. * The resulting invoker will be a method handle with the following * arguments: * <ul> * <li>a single {@code MethodHandle} target * <li>zero or more leading values (counted by {@code leadingArgCount}) * <li>an {@code Object[]} array containing trailing arguments * </ul> * <p> * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with * the indicated {@code type}. * That is, if the target is exactly of the given {@code type}, it will behave * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType} * is used to convert the target to the required {@code type}. * <p> * The type of the returned invoker will not be the given {@code type}, but rather * will have all parameters except the first {@code leadingArgCount} * replaced by a single array of type {@code Object[]}, which will be * the final parameter. * <p> * Before invoking its target, the invoker will spread the final array, apply * reference casts as necessary, and unbox and widen primitive arguments. * If, when the invoker is called, the supplied array argument does * not have the correct number of elements, the invoker will throw * an {@link IllegalArgumentException} instead of invoking the target. * <p> * This method is equivalent to the following code (though it may be more efficient): * <blockquote><pre>{@code MethodHandle invoker = MethodHandles.invoker(type); int spreadArgCount = type.parameterCount() - leadingArgCount; invoker = invoker.asSpreader(Object[].class, spreadArgCount); return invoker; * }</pre></blockquote> * This method throws no reflective or security exceptions. * @param type the desired target type * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target * @return a method handle suitable for invoking any method handle of the given type * @throws NullPointerException if {@code type} is null * @throws IllegalArgumentException if {@code leadingArgCount} is not in * the range from 0 to {@code type.parameterCount()} inclusive, * or if the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> */ public static MethodHandle spreadInvoker(MethodType type, int leadingArgCount) { if (leadingArgCount < 0 || leadingArgCount > type.parameterCount()) throw newIllegalArgumentException("bad argument count", leadingArgCount); type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount); return type.invokers().spreadInvoker(leadingArgCount); } /** * Produces a special <em>invoker method handle</em> which can be used to * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}. * The resulting invoker will have a type which is * exactly equal to the desired type, except that it will accept * an additional leading argument of type {@code MethodHandle}. * <p> * This method is equivalent to the following code (though it may be more efficient): * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)} * * <p style="font-size:smaller;"> * <em>Discussion:</em> * Invoker method handles can be useful when working with variable method handles * of unknown types. * For example, to emulate an {@code invokeExact} call to a variable method * handle {@code M}, extract its type {@code T}, * look up the invoker method {@code X} for {@code T}, * and call the invoker method, as {@code X.invoke(T, A...)}. * (It would not work to call {@code X.invokeExact}, since the type {@code T} * is unknown.) * If spreading, collecting, or other argument transformations are required, * they can be applied once to the invoker {@code X} and reused on many {@code M} * method handle values, as long as they are compatible with the type of {@code X}. * <p style="font-size:smaller;"> * <em>(Note: The invoker method is not available via the Core Reflection API. * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke} * on the declared {@code invokeExact} or {@code invoke} method will raise an * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em> * <p> * This method throws no reflective or security exceptions. * @param type the desired target type * @return a method handle suitable for invoking any method handle of the given type * @throws IllegalArgumentException if the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> */ public static MethodHandle exactInvoker(MethodType type) { return type.invokers().exactInvoker(); } /** * Produces a special <em>invoker method handle</em> which can be used to * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}. * The resulting invoker will have a type which is * exactly equal to the desired type, except that it will accept * an additional leading argument of type {@code MethodHandle}. * <p> * Before invoking its target, if the target differs from the expected type, * the invoker will apply reference casts as * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}. * Similarly, the return value will be converted as necessary. * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle}, * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}. * <p> * This method is equivalent to the following code (though it may be more efficient): * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)} * <p style="font-size:smaller;"> * <em>Discussion:</em> * A {@linkplain MethodType#genericMethodType general method type} is one which * mentions only {@code Object} arguments and return values. * An invoker for such a type is capable of calling any method handle * of the same arity as the general type. * <p style="font-size:smaller;"> * <em>(Note: The invoker method is not available via the Core Reflection API. * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke} * on the declared {@code invokeExact} or {@code invoke} method will raise an * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em> * <p> * This method throws no reflective or security exceptions. * @param type the desired target type * @return a method handle suitable for invoking any method handle convertible to the given type * @throws IllegalArgumentException if the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> */ public static MethodHandle invoker(MethodType type) { return type.invokers().genericInvoker(); } /** * Produces a special <em>invoker method handle</em> which can be used to * invoke a signature-polymorphic access mode method on any VarHandle whose * associated access mode type is compatible with the given type. * The resulting invoker will have a type which is exactly equal to the * desired given type, except that it will accept an additional leading * argument of type {@code VarHandle}. * * @param accessMode the VarHandle access mode * @param type the desired target type * @return a method handle suitable for invoking an access mode method of * any VarHandle whose access mode type is of the given type. * @since 9 */ static public MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) { return type.invokers().varHandleMethodExactInvoker(accessMode); } /** * Produces a special <em>invoker method handle</em> which can be used to * invoke a signature-polymorphic access mode method on any VarHandle whose * associated access mode type is compatible with the given type. * The resulting invoker will have a type which is exactly equal to the * desired given type, except that it will accept an additional leading * argument of type {@code VarHandle}. * <p> * Before invoking its target, if the access mode type differs from the * desired given type, the invoker will apply reference casts as necessary * and box, unbox, or widen primitive values, as if by * {@link MethodHandle#asType asType}. Similarly, the return value will be * converted as necessary. * <p> * This method is equivalent to the following code (though it may be more * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)} * * @param accessMode the VarHandle access mode * @param type the desired target type * @return a method handle suitable for invoking an access mode method of * any VarHandle whose access mode type is convertible to the given * type. * @since 9 */ static public MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) { return type.invokers().varHandleMethodInvoker(accessMode); } static /*non-public*/ MethodHandle basicInvoker(MethodType type) { return type.invokers().basicInvoker(); } /// method handle modification (creation from other method handles) /** * Produces a method handle which adapts the type of the * given method handle to a new type by pairwise argument and return type conversion. * The original type and new type must have the same number of arguments. * The resulting method handle is guaranteed to report a type * which is equal to the desired new type. * <p> * If the original type and new type are equal, returns target. * <p> * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType}, * and some additional conversions are also applied if those conversions fail. * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied * if possible, before or instead of any conversions done by {@code asType}: * <ul> * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type, * then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast. * (This treatment of interfaces follows the usage of the bytecode verifier.) * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive, * the boolean is converted to a byte value, 1 for true, 0 for false. * (This treatment follows the usage of the bytecode verifier.) * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive, * <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5), * and the low order bit of the result is tested, as if by {@code (x & 1) != 0}. * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean, * then a Java casting conversion (JLS 5.5) is applied. * (Specifically, <em>T0</em> will convert to <em>T1</em> by * widening and/or narrowing.) * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing * conversion will be applied at runtime, possibly followed * by a Java casting conversion (JLS 5.5) on the primitive value, * possibly followed by a conversion from byte to boolean by testing * the low-order bit. * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, * and if the reference is null at runtime, a zero value is introduced. * </ul> * @param target the method handle to invoke after arguments are retyped * @param newType the expected type of the new method handle * @return a method handle which delegates to the target after performing * any necessary argument conversions, and arranges for any * necessary return value conversions * @throws NullPointerException if either argument is null * @throws WrongMethodTypeException if the conversion cannot be made * @see MethodHandle#asType */ public static MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) { explicitCastArgumentsChecks(target, newType); // use the asTypeCache when possible: MethodType oldType = target.type(); if (oldType == newType) return target; if (oldType.explicitCastEquivalentToAsType(newType)) { return target.asFixedArity().asType(newType); } return MethodHandleImpl.makePairwiseConvert(target, newType, false); } private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) { if (target.type().parameterCount() != newType.parameterCount()) { throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType); } } /** * Produces a method handle which adapts the calling sequence of the * given method handle to a new type, by reordering the arguments. * The resulting method handle is guaranteed to report a type * which is equal to the desired new type. * <p> * The given array controls the reordering. * Call {@code #I} the number of incoming parameters (the value * {@code newType.parameterCount()}, and call {@code #O} the number * of outgoing parameters (the value {@code target.type().parameterCount()}). * Then the length of the reordering array must be {@code #O}, * and each element must be a non-negative number less than {@code #I}. * For every {@code N} less than {@code #O}, the {@code N}-th * outgoing argument will be taken from the {@code I}-th incoming * argument, where {@code I} is {@code reorder[N]}. * <p> * No argument or return value conversions are applied. * The type of each incoming argument, as determined by {@code newType}, * must be identical to the type of the corresponding outgoing parameter * or parameters in the target method handle. * The return type of {@code newType} must be identical to the return * type of the original target. * <p> * The reordering array need not specify an actual permutation. * An incoming argument will be duplicated if its index appears * more than once in the array, and an incoming argument will be dropped * if its index does not appear in the array. * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments}, * incoming arguments which are not mentioned in the reordering array * may be of any type, as determined only by {@code newType}. * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodType intfn1 = methodType(int.class, int.class); MethodType intfn2 = methodType(int.class, int.class, int.class); MethodHandle sub = ... (int x, int y) -> (x-y) ...; assert(sub.type().equals(intfn2)); MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1); MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0); assert((int)rsub.invokeExact(1, 100) == 99); MethodHandle add = ... (int x, int y) -> (x+y) ...; assert(add.type().equals(intfn2)); MethodHandle twice = permuteArguments(add, intfn1, 0, 0); assert(twice.type().equals(intfn1)); assert((int)twice.invokeExact(21) == 42); * }</pre></blockquote> * <p> * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector * variable-arity method handle}, even if the original target method handle was. * @param target the method handle to invoke after arguments are reordered * @param newType the expected type of the new method handle * @param reorder an index array which controls the reordering * @return a method handle which delegates to the target after it * drops unused arguments and moves and/or duplicates the other arguments * @throws NullPointerException if any argument is null * @throws IllegalArgumentException if the index array length is not equal to * the arity of the target, or if any index array element * not a valid index for a parameter of {@code newType}, * or if two corresponding parameter types in * {@code target.type()} and {@code newType} are not identical, */ public static MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) { reorder = reorder.clone(); // get a private copy MethodType oldType = target.type(); permuteArgumentChecks(reorder, newType, oldType); // first detect dropped arguments and handle them separately int[] originalReorder = reorder; BoundMethodHandle result = target.rebind(); LambdaForm form = result.form; int newArity = newType.parameterCount(); // Normalize the reordering into a real permutation, // by removing duplicates and adding dropped elements. // This somewhat improves lambda form caching, as well // as simplifying the transform by breaking it up into steps. for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0;) { if (ddIdx > 0) { // We found a duplicated entry at reorder[ddIdx]. // Example: (x,y,z)->asList(x,y,z) // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1) // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0) // The starred element corresponds to the argument // deleted by the dupArgumentForm transform. int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos]; boolean killFirst = false; for (int val; (val = reorder[--dstPos]) != dupVal;) { // Set killFirst if the dup is larger than an intervening position. // This will remove at least one inversion from the permutation. if (dupVal > val) killFirst = true; } if (!killFirst) { srcPos = dstPos; dstPos = ddIdx; } form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos); assert (reorder[srcPos] == reorder[dstPos]); oldType = oldType.dropParameterTypes(dstPos, dstPos + 1); // contract the reordering by removing the element at dstPos int tailPos = dstPos + 1; System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos); reorder = Arrays.copyOf(reorder, reorder.length - 1); } else { int dropVal = ~ddIdx, insPos = 0; while (insPos < reorder.length && reorder[insPos] < dropVal) { // Find first element of reorder larger than dropVal. // This is where we will insert the dropVal. insPos += 1; } Class<?> ptype = newType.parameterType(dropVal); form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype)); oldType = oldType.insertParameterTypes(insPos, ptype); // expand the reordering by inserting an element at insPos int tailPos = insPos + 1; reorder = Arrays.copyOf(reorder, reorder.length + 1); System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos); reorder[insPos] = dropVal; } assert (permuteArgumentChecks(reorder, newType, oldType)); } assert (reorder.length == newArity); // a perfect permutation // Note: This may cache too many distinct LFs. Consider backing off to varargs code. form = form.editor().permuteArgumentsForm(1, reorder); if (newType == result.type() && form == result.internalForm()) return result; return result.copyWith(newType, form); } /** * Return an indication of any duplicate or omission in reorder. * If the reorder contains a duplicate entry, return the index of the second occurrence. * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder. * Otherwise, return zero. * If an element not in [0..newArity-1] is encountered, return reorder.length. */ private static int findFirstDupOrDrop(int[] reorder, int newArity) { final int BIT_LIMIT = 63; // max number of bits in bit mask if (newArity < BIT_LIMIT) { long mask = 0; for (int i = 0; i < reorder.length; i++) { int arg = reorder[i]; if (arg >= newArity) { return reorder.length; } long bit = 1L << arg; if ((mask & bit) != 0) { return i; // >0 indicates a dup } mask |= bit; } if (mask == (1L << newArity) - 1) { assert (Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity); return 0; } // find first zero long zeroBit = Long.lowestOneBit(~mask); int zeroPos = Long.numberOfTrailingZeros(zeroBit); assert (zeroPos <= newArity); if (zeroPos == newArity) { return 0; } return ~zeroPos; } else { // same algorithm, different bit set BitSet mask = new BitSet(newArity); for (int i = 0; i < reorder.length; i++) { int arg = reorder[i]; if (arg >= newArity) { return reorder.length; } if (mask.get(arg)) { return i; // >0 indicates a dup } mask.set(arg); } int zeroPos = mask.nextClearBit(0); assert (zeroPos <= newArity); if (zeroPos == newArity) { return 0; } return ~zeroPos; } } private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) { if (newType.returnType() != oldType.returnType()) throw newIllegalArgumentException("return types do not match", oldType, newType); if (reorder.length == oldType.parameterCount()) { int limit = newType.parameterCount(); boolean bad = false; for (int j = 0; j < reorder.length; j++) { int i = reorder[j]; if (i < 0 || i >= limit) { bad = true; break; } Class<?> src = newType.parameterType(i); Class<?> dst = oldType.parameterType(j); if (src != dst) throw newIllegalArgumentException("parameter types do not match after reorder", oldType, newType); } if (!bad) return true; } throw newIllegalArgumentException("bad reorder array: " + Arrays.toString(reorder)); } /** * Produces a method handle of the requested return type which returns the given * constant value every time it is invoked. * <p> * Before the method handle is returned, the passed-in value is converted to the requested type. * If the requested type is primitive, widening primitive conversions are attempted, * else reference conversions are attempted. * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}. * @param type the return type of the desired method handle * @param value the value to return * @return a method handle of the given return type and no arguments, which always returns the given value * @throws NullPointerException if the {@code type} argument is null * @throws ClassCastException if the value cannot be converted to the required return type * @throws IllegalArgumentException if the given type is {@code void.class} */ public static MethodHandle constant(Class<?> type, Object value) { if (type.isPrimitive()) { if (type == void.class) throw newIllegalArgumentException("void type"); Wrapper w = Wrapper.forPrimitiveType(type); value = w.convert(value, type); if (w.zero().equals(value)) return zero(w, type); return insertArguments(identity(type), 0, value); } else { if (value == null) return zero(Wrapper.OBJECT, type); return identity(type).bindTo(value); } } /** * Produces a method handle which returns its sole argument when invoked. * @param type the type of the sole parameter and return value of the desired method handle * @return a unary method handle which accepts and returns the given type * @throws NullPointerException if the argument is null * @throws IllegalArgumentException if the given type is {@code void.class} */ public static MethodHandle identity(Class<?> type) { Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT); int pos = btw.ordinal(); MethodHandle ident = IDENTITY_MHS[pos]; if (ident == null) { ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType())); } if (ident.type().returnType() == type) return ident; // something like identity(Foo.class); do not bother to intern these assert (btw == Wrapper.OBJECT); return makeIdentity(type); } /** * Produces a constant method handle of the requested return type which * returns the default value for that type every time it is invoked. * The resulting constant method handle will have no side effects. * <p>The returned method handle is equivalent to {@code empty(methodType(type))}. * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))}, * since {@code explicitCastArguments} converts {@code null} to default values. * @param type the expected return type of the desired method handle * @return a constant method handle that takes no arguments * and returns the default value of the given type (or void, if the type is void) * @throws NullPointerException if the argument is null * @see MethodHandles#constant * @see MethodHandles#empty * @see MethodHandles#explicitCastArguments * @since 9 */ public static MethodHandle zero(Class<?> type) { Objects.requireNonNull(type); return type.isPrimitive() ? zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type); } private static MethodHandle identityOrVoid(Class<?> type) { return type == void.class ? zero(type) : identity(type); } /** * Produces a method handle of the requested type which ignores any arguments, does nothing, * and returns a suitable default depending on the return type. * That is, it returns a zero primitive value, a {@code null}, or {@code void}. * <p>The returned method handle is equivalent to * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}. * * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as * {@code guardWithTest(pred, target, empty(target.type())}. * @param type the type of the desired method handle * @return a constant method handle of the given type, which returns a default value of the given return type * @throws NullPointerException if the argument is null * @see MethodHandles#zero * @see MethodHandles#constant * @since 9 */ public static MethodHandle empty(MethodType type) { Objects.requireNonNull(type); return dropArguments(zero(type.returnType()), 0, type.parameterList()); } private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT]; private static MethodHandle makeIdentity(Class<?> ptype) { MethodType mtype = methodType(ptype, ptype); LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype)); return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY); } private static MethodHandle zero(Wrapper btw, Class<?> rtype) { int pos = btw.ordinal(); MethodHandle zero = ZERO_MHS[pos]; if (zero == null) { zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType())); } if (zero.type().returnType() == rtype) return zero; assert (btw == Wrapper.OBJECT); return makeZero(rtype); } private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT]; private static MethodHandle makeZero(Class<?> rtype) { MethodType mtype = methodType(rtype); LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype)); return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO); } private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) { // Simulate a CAS, to avoid racy duplication of results. MethodHandle prev = cache[pos]; if (prev != null) return prev; return cache[pos] = value; } /** * Provides a target method handle with one or more <em>bound arguments</em> * in advance of the method handle's invocation. * The formal parameters to the target corresponding to the bound * arguments are called <em>bound parameters</em>. * Returns a new method handle which saves away the bound arguments. * When it is invoked, it receives arguments for any non-bound parameters, * binds the saved arguments to their corresponding parameters, * and calls the original target. * <p> * The type of the new method handle will drop the types for the bound * parameters from the original target type, since the new method handle * will no longer require those arguments to be supplied by its callers. * <p> * Each given argument object must match the corresponding bound parameter type. * If a bound parameter type is a primitive, the argument object * must be a wrapper, and will be unboxed to produce the primitive value. * <p> * The {@code pos} argument selects which parameters are to be bound. * It may range between zero and <i>N-L</i> (inclusively), * where <i>N</i> is the arity of the target method handle * and <i>L</i> is the length of the values array. * <p> * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector * variable-arity method handle}, even if the original target method handle was. * @param target the method handle to invoke after the argument is inserted * @param pos where to insert the argument (zero for the first) * @param values the series of arguments to insert * @return a method handle which inserts an additional argument, * before calling the original method handle * @throws NullPointerException if the target or the {@code values} array is null * @throws IllegalArgumentException if (@code pos) is less than {@code 0} or greater than * {@code N - L} where {@code N} is the arity of the target method handle and {@code L} * is the length of the values array. * @throws ClassCastException if an argument does not match the corresponding bound parameter * type. * @see MethodHandle#bindTo */ public static MethodHandle insertArguments(MethodHandle target, int pos, Object... values) { int insCount = values.length; Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos); if (insCount == 0) return target; BoundMethodHandle result = target.rebind(); for (int i = 0; i < insCount; i++) { Object value = values[i]; Class<?> ptype = ptypes[pos + i]; if (ptype.isPrimitive()) { result = insertArgumentPrimitive(result, pos, ptype, value); } else { value = ptype.cast(value); // throw CCE if needed result = result.bindArgumentL(pos, value); } } return result; } private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos, Class<?> ptype, Object value) { Wrapper w = Wrapper.forPrimitiveType(ptype); // perform unboxing and/or primitive conversion value = w.convert(value, ptype); switch (w) { case INT: return result.bindArgumentI(pos, (int) value); case LONG: return result.bindArgumentJ(pos, (long) value); case FLOAT: return result.bindArgumentF(pos, (float) value); case DOUBLE: return result.bindArgumentD(pos, (double) value); default: return result.bindArgumentI(pos, ValueConversions.widenSubword(value)); } } private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException { MethodType oldType = target.type(); int outargs = oldType.parameterCount(); int inargs = outargs - insCount; if (inargs < 0) throw newIllegalArgumentException("too many values to insert"); if (pos < 0 || pos > inargs) throw newIllegalArgumentException("no argument type to append"); return oldType.ptypes(); } /** * Produces a method handle which will discard some dummy arguments * before calling some other specified <i>target</i> method handle. * The type of the new method handle will be the same as the target's type, * except it will also include the dummy argument types, * at some given position. * <p> * The {@code pos} argument may range between zero and <i>N</i>, * where <i>N</i> is the arity of the target. * If {@code pos} is zero, the dummy arguments will precede * the target's real arguments; if {@code pos} is <i>N</i> * they will come after. * <p> * <b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); assertEquals("xy", (String) cat.invokeExact("x", "y")); MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class); MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2)); assertEquals(bigType, d0.type()); assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z")); * }</pre></blockquote> * <p> * This method is also equivalent to the following code: * <blockquote><pre> * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))} * </pre></blockquote> * @param target the method handle to invoke after the arguments are dropped * @param valueTypes the type(s) of the argument(s) to drop * @param pos position of first argument to drop (zero for the leftmost) * @return a method handle which drops arguments of the given types, * before calling the original method handle * @throws NullPointerException if the target is null, * or if the {@code valueTypes} list or any of its elements is null * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class}, * or if {@code pos} is negative or greater than the arity of the target, * or if the new method handle's type would have too many parameters */ public static MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) { return dropArguments0(target, pos, copyTypes(valueTypes.toArray())); } private static List<Class<?>> copyTypes(Object[] array) { return Arrays.asList(Arrays.copyOf(array, array.length, Class[].class)); } private static MethodHandle dropArguments0(MethodHandle target, int pos, List<Class<?>> valueTypes) { MethodType oldType = target.type(); // get NPE int dropped = dropArgumentChecks(oldType, pos, valueTypes); MethodType newType = oldType.insertParameterTypes(pos, valueTypes); if (dropped == 0) return target; BoundMethodHandle result = target.rebind(); LambdaForm lform = result.form; int insertFormArg = 1 + pos; for (Class<?> ptype : valueTypes) { lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype)); } result = result.copyWith(newType, lform); return result; } private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) { int dropped = valueTypes.size(); MethodType.checkSlotCount(dropped); int outargs = oldType.parameterCount(); int inargs = outargs + dropped; if (pos < 0 || pos > outargs) throw newIllegalArgumentException( "no argument type to remove" + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)); return dropped; } /** * Produces a method handle which will discard some dummy arguments * before calling some other specified <i>target</i> method handle. * The type of the new method handle will be the same as the target's type, * except it will also include the dummy argument types, * at some given position. * <p> * The {@code pos} argument may range between zero and <i>N</i>, * where <i>N</i> is the arity of the target. * If {@code pos} is zero, the dummy arguments will precede * the target's real arguments; if {@code pos} is <i>N</i> * they will come after. * @apiNote * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); assertEquals("xy", (String) cat.invokeExact("x", "y")); MethodHandle d0 = dropArguments(cat, 0, String.class); assertEquals("yz", (String) d0.invokeExact("x", "y", "z")); MethodHandle d1 = dropArguments(cat, 1, String.class); assertEquals("xz", (String) d1.invokeExact("x", "y", "z")); MethodHandle d2 = dropArguments(cat, 2, String.class); assertEquals("xy", (String) d2.invokeExact("x", "y", "z")); MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class); assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z")); * }</pre></blockquote> * <p> * This method is also equivalent to the following code: * <blockquote><pre> * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))} * </pre></blockquote> * @param target the method handle to invoke after the arguments are dropped * @param valueTypes the type(s) of the argument(s) to drop * @param pos position of first argument to drop (zero for the leftmost) * @return a method handle which drops arguments of the given types, * before calling the original method handle * @throws NullPointerException if the target is null, * or if the {@code valueTypes} array or any of its elements is null * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class}, * or if {@code pos} is negative or greater than the arity of the target, * or if the new method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> */ public static MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) { return dropArguments0(target, pos, copyTypes(valueTypes)); } // private version which allows caller some freedom with error handling private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos, boolean nullOnFailure) { newTypes = copyTypes(newTypes.toArray()); List<Class<?>> oldTypes = target.type().parameterList(); int match = oldTypes.size(); if (skip != 0) { if (skip < 0 || skip > match) { throw newIllegalArgumentException("illegal skip", skip, target); } oldTypes = oldTypes.subList(skip, match); match -= skip; } List<Class<?>> addTypes = newTypes; int add = addTypes.size(); if (pos != 0) { if (pos < 0 || pos > add) { throw newIllegalArgumentException("illegal pos", pos, newTypes); } addTypes = addTypes.subList(pos, add); add -= pos; assert (addTypes.size() == add); } // Do not add types which already match the existing arguments. if (match > add || !oldTypes.equals(addTypes.subList(0, match))) { if (nullOnFailure) { return null; } throw newIllegalArgumentException("argument lists do not match", oldTypes, newTypes); } addTypes = addTypes.subList(match, add); add -= match; assert (addTypes.size() == add); // newTypes: ( P*[pos], M*[match], A*[add] ) // target: ( S*[skip], M*[match] ) MethodHandle adapter = target; if (add > 0) { adapter = dropArguments0(adapter, skip + match, addTypes); } // adapter: (S*[skip], M*[match], A*[add] ) if (pos > 0) { adapter = dropArguments0(adapter, skip, newTypes.subList(0, pos)); } // adapter: (S*[skip], P*[pos], M*[match], A*[add] ) return adapter; } /** * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by * {@link #dropArguments(MethodHandle, int, Class[])}. * <p> * The resulting handle will have the same return type as the target handle. * <p> * In more formal terms, assume these two type lists:<ul> * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list, * {@code newTypes}. * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching * sub-list. * </ul> * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by * {@link #dropArguments(MethodHandle, int, Class[])}. * * @apiNote * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows: * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... ... MethodHandle h0 = constant(boolean.class, true); MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class); MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList()); if (h1.type().parameterCount() < h2.type().parameterCount()) h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0); // lengthen h1 else h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0); // lengthen h2 MethodHandle h3 = guardWithTest(h0, h1, h2); assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c")); * }</pre></blockquote> * @param target the method handle to adapt * @param skip number of targets parameters to disregard (they will be unchanged) * @param newTypes the list of types to match {@code target}'s parameter type list to * @param pos place in {@code newTypes} where the non-skipped target parameters must occur * @return a possibly adapted method handle * @throws NullPointerException if either argument is null * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class}, * or if {@code skip} is negative or greater than the arity of the target, * or if {@code pos} is negative or greater than the newTypes list size, * or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position * {@code pos}. * @since 9 */ public static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) { Objects.requireNonNull(target); Objects.requireNonNull(newTypes); return dropArgumentsToMatch(target, skip, newTypes, pos, false); } /** * Adapts a target method handle by pre-processing * one or more of its arguments, each with its own unary filter function, * and then calling the target with each pre-processed argument * replaced by the result of its corresponding filter function. * <p> * The pre-processing is performed by one or more method handles, * specified in the elements of the {@code filters} array. * The first element of the filter array corresponds to the {@code pos} * argument of the target, and so on in sequence. * The filter functions are invoked in left to right order. * <p> * Null arguments in the array are treated as identity functions, * and the corresponding arguments left unchanged. * (If there are no non-null elements in the array, the original target is returned.) * Each filter is applied to the corresponding argument of the adapter. * <p> * If a filter {@code F} applies to the {@code N}th argument of * the target, then {@code F} must be a method handle which * takes exactly one argument. The type of {@code F}'s sole argument * replaces the corresponding argument type of the target * in the resulting adapted method handle. * The return type of {@code F} must be identical to the corresponding * parameter type of the target. * <p> * It is an error if there are elements of {@code filters} * (null or not) * which do not correspond to argument positions in the target. * <p><b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); MethodHandle upcase = lookup().findVirtual(String.class, "toUpperCase", methodType(String.class)); assertEquals("xy", (String) cat.invokeExact("x", "y")); MethodHandle f0 = filterArguments(cat, 0, upcase); assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy MethodHandle f1 = filterArguments(cat, 1, upcase); assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY MethodHandle f2 = filterArguments(cat, 0, upcase, upcase); assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY * }</pre></blockquote> * <p>Here is pseudocode for the resulting adapter. In the code, {@code T} * denotes the return type of both the {@code target} and resulting adapter. * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values * of the parameters and arguments that precede and follow the filter position * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and * values of the filtered parameters and arguments; they also represent the * return types of the {@code filter[i]} handles. The latter accept arguments * {@code v[i]} of type {@code V[i]}, which also appear in the signature of * the resulting adapter. * <blockquote><pre>{@code * T target(P... p, A[i]... a[i], B... b); * A[i] filter[i](V[i]); * T adapter(P... p, V[i]... v[i], B... b) { * return target(p..., filter[i](v[i])..., b...); * } * }</pre></blockquote> * <p> * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector * variable-arity method handle}, even if the original target method handle was. * * @param target the method handle to invoke after arguments are filtered * @param pos the position of the first argument to filter * @param filters method handles to call initially on filtered arguments * @return method handle which incorporates the specified argument filtering logic * @throws NullPointerException if the target is null * or if the {@code filters} array is null * @throws IllegalArgumentException if a non-null element of {@code filters} * does not match a corresponding argument type of target as described above, * or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()}, * or if the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> */ public static MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) { // In method types arguments start at index 0, while the LF // editor have the MH receiver at position 0 - adjust appropriately. final int MH_RECEIVER_OFFSET = 1; filterArgumentsCheckArity(target, pos, filters); MethodHandle adapter = target; // keep track of currently matched filters, as to optimize repeated filters int index = 0; int[] positions = new int[filters.length]; MethodHandle filter = null; // process filters in reverse order so that the invocation of // the resulting adapter will invoke the filters in left-to-right order for (int i = filters.length - 1; i >= 0; --i) { MethodHandle newFilter = filters[i]; if (newFilter == null) continue; // ignore null elements of filters // flush changes on update if (filter != newFilter) { if (filter != null) { if (index > 1) { adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index)); } else { adapter = filterArgument(adapter, positions[0] - 1, filter); } } filter = newFilter; index = 0; } filterArgumentChecks(target, pos + i, newFilter); positions[index++] = pos + i + MH_RECEIVER_OFFSET; } if (index > 1) { adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index)); } else if (index == 1) { adapter = filterArgument(adapter, positions[0] - 1, filter); } return adapter; } private static MethodHandle filterRepeatedArgument(MethodHandle adapter, MethodHandle filter, int[] positions) { MethodType targetType = adapter.type(); MethodType filterType = filter.type(); BoundMethodHandle result = adapter.rebind(); Class<?> newParamType = filterType.parameterType(0); Class<?>[] ptypes = targetType.ptypes().clone(); for (int pos : positions) { ptypes[pos - 1] = newParamType; } MethodType newType = MethodType.makeImpl(targetType.rtype(), ptypes, true); LambdaForm lform = result.editor().filterRepeatedArgumentForm(BasicType.basicType(newParamType), positions); return result.copyWithExtendL(newType, lform, filter); } /*non-public*/ static MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) { filterArgumentChecks(target, pos, filter); MethodType targetType = target.type(); MethodType filterType = filter.type(); BoundMethodHandle result = target.rebind(); Class<?> newParamType = filterType.parameterType(0); LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType)); MethodType newType = targetType.changeParameterType(pos, newParamType); result = result.copyWithExtendL(newType, lform, filter); return result; } private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) { MethodType targetType = target.type(); int maxPos = targetType.parameterCount(); if (pos + filters.length > maxPos) throw newIllegalArgumentException("too many filters"); } private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException { MethodType targetType = target.type(); MethodType filterType = filter.type(); if (filterType.parameterCount() != 1 || filterType.returnType() != targetType.parameterType(pos)) throw newIllegalArgumentException("target and filter types do not match", targetType, filterType); } /** * Adapts a target method handle by pre-processing * a sub-sequence of its arguments with a filter (another method handle). * The pre-processed arguments are replaced by the result (if any) of the * filter function. * The target is then called on the modified (usually shortened) argument list. * <p> * If the filter returns a value, the target must accept that value as * its argument in position {@code pos}, preceded and/or followed by * any arguments not passed to the filter. * If the filter returns void, the target must accept all arguments * not passed to the filter. * No arguments are reordered, and a result returned from the filter * replaces (in order) the whole subsequence of arguments originally * passed to the adapter. * <p> * The argument types (if any) of the filter * replace zero or one argument types of the target, at position {@code pos}, * in the resulting adapted method handle. * The return type of the filter (if any) must be identical to the * argument type of the target at position {@code pos}, and that target argument * is supplied by the return value of the filter. * <p> * In all cases, {@code pos} must be greater than or equal to zero, and * {@code pos} must also be less than or equal to the target's arity. * <p><b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle deepToString = publicLookup() .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); MethodHandle ts1 = deepToString.asCollector(String[].class, 1); assertEquals("[strange]", (String) ts1.invokeExact("strange")); MethodHandle ts2 = deepToString.asCollector(String[].class, 2); assertEquals("[up, down]", (String) ts2.invokeExact("up", "down")); MethodHandle ts3 = deepToString.asCollector(String[].class, 3); MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2); assertEquals("[top, [up, down], strange]", (String) ts3_ts2.invokeExact("top", "up", "down", "strange")); MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1); assertEquals("[top, [up, down], [strange]]", (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange")); MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3); assertEquals("[top, [[up, down, strange], charm], bottom]", (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom")); * }</pre></blockquote> * <p>Here is pseudocode for the resulting adapter. In the code, {@code T} * represents the return type of the {@code target} and resulting adapter. * {@code V}/{@code v} stand for the return type and value of the * {@code filter}, which are also found in the signature and arguments of * the {@code target}, respectively, unless {@code V} is {@code void}. * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types * and values preceding and following the collection position, {@code pos}, * in the {@code target}'s signature. They also turn up in the resulting * adapter's signature and arguments, where they surround * {@code B}/{@code b}, which represent the parameter types and arguments * to the {@code filter} (if any). * <blockquote><pre>{@code * T target(A...,V,C...); * V filter(B...); * T adapter(A... a,B... b,C... c) { * V v = filter(b...); * return target(a...,v,c...); * } * // and if the filter has no arguments: * T target2(A...,V,C...); * V filter2(); * T adapter2(A... a,C... c) { * V v = filter2(); * return target2(a...,v,c...); * } * // and if the filter has a void return: * T target3(A...,C...); * void filter3(B...); * T adapter3(A... a,B... b,C... c) { * filter3(b...); * return target3(a...,c...); * } * }</pre></blockquote> * <p> * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to * one which first "folds" the affected arguments, and then drops them, in separate * steps as follows: * <blockquote><pre>{@code * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2 * mh = MethodHandles.foldArguments(mh, coll); //step 1 * }</pre></blockquote> * If the target method handle consumes no arguments besides than the result * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)} * is equivalent to {@code filterReturnValue(coll, mh)}. * If the filter method handle {@code coll} consumes one argument and produces * a non-void result, then {@code collectArguments(mh, N, coll)} * is equivalent to {@code filterArguments(mh, N, coll)}. * Other equivalences are possible but would require argument permutation. * <p> * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector * variable-arity method handle}, even if the original target method handle was. * * @param target the method handle to invoke after filtering the subsequence of arguments * @param pos the position of the first adapter argument to pass to the filter, * and/or the target argument which receives the result of the filter * @param filter method handle to call on the subsequence of arguments * @return method handle which incorporates the specified argument subsequence filtering logic * @throws NullPointerException if either argument is null * @throws IllegalArgumentException if the return type of {@code filter} * is non-void and is not the same as the {@code pos} argument of the target, * or if {@code pos} is not between 0 and the target's arity, inclusive, * or if the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> * @see MethodHandles#foldArguments * @see MethodHandles#filterArguments * @see MethodHandles#filterReturnValue */ public static MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) { MethodType newType = collectArgumentsChecks(target, pos, filter); MethodType collectorType = filter.type(); BoundMethodHandle result = target.rebind(); LambdaForm lform; if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) { lform = result.editor().collectArgumentArrayForm(1 + pos, filter); if (lform != null) { return result.copyWith(newType, lform); } } lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType()); return result.copyWithExtendL(newType, lform, filter); } private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException { MethodType targetType = target.type(); MethodType filterType = filter.type(); Class<?> rtype = filterType.returnType(); List<Class<?>> filterArgs = filterType.parameterList(); if (rtype == void.class) { return targetType.insertParameterTypes(pos, filterArgs); } if (rtype != targetType.parameterType(pos)) { throw newIllegalArgumentException("target and filter types do not match", targetType, filterType); } return targetType.dropParameterTypes(pos, pos + 1).insertParameterTypes(pos, filterArgs); } /** * Adapts a target method handle by post-processing * its return value (if any) with a filter (another method handle). * The result of the filter is returned from the adapter. * <p> * If the target returns a value, the filter must accept that value as * its only argument. * If the target returns void, the filter must accept no arguments. * <p> * The return type of the filter * replaces the return type of the target * in the resulting adapted method handle. * The argument type of the filter (if any) must be identical to the * return type of the target. * <p><b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); MethodHandle length = lookup().findVirtual(String.class, "length", methodType(int.class)); System.out.println((String) cat.invokeExact("x", "y")); // xy MethodHandle f0 = filterReturnValue(cat, length); System.out.println((int) f0.invokeExact("x", "y")); // 2 * }</pre></blockquote> * <p>Here is pseudocode for the resulting adapter. In the code, * {@code T}/{@code t} represent the result type and value of the * {@code target}; {@code V}, the result type of the {@code filter}; and * {@code A}/{@code a}, the types and values of the parameters and arguments * of the {@code target} as well as the resulting adapter. * <blockquote><pre>{@code * T target(A...); * V filter(T); * V adapter(A... a) { * T t = target(a...); * return filter(t); * } * // and if the target has a void return: * void target2(A...); * V filter2(); * V adapter2(A... a) { * target2(a...); * return filter2(); * } * // and if the filter has a void return: * T target3(A...); * void filter3(V); * void adapter3(A... a) { * T t = target3(a...); * filter3(t); * } * }</pre></blockquote> * <p> * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector * variable-arity method handle}, even if the original target method handle was. * @param target the method handle to invoke before filtering the return value * @param filter method handle to call on the return value * @return method handle which incorporates the specified return value filtering logic * @throws NullPointerException if either argument is null * @throws IllegalArgumentException if the argument list of {@code filter} * does not match the return type of target as described above */ public static MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) { MethodType targetType = target.type(); MethodType filterType = filter.type(); filterReturnValueChecks(targetType, filterType); BoundMethodHandle result = target.rebind(); BasicType rtype = BasicType.basicType(filterType.returnType()); LambdaForm lform = result.editor().filterReturnForm(rtype, false); MethodType newType = targetType.changeReturnType(filterType.returnType()); result = result.copyWithExtendL(newType, lform, filter); return result; } private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException { Class<?> rtype = targetType.returnType(); int filterValues = filterType.parameterCount(); if (filterValues == 0 ? (rtype != void.class) : (rtype != filterType.parameterType(0) || filterValues != 1)) throw newIllegalArgumentException("target and filter types do not match", targetType, filterType); } /** * Adapts a target method handle by pre-processing * some of its arguments, and then calling the target with * the result of the pre-processing, inserted into the original * sequence of arguments. * <p> * The pre-processing is performed by {@code combiner}, a second method handle. * Of the arguments passed to the adapter, the first {@code N} arguments * are copied to the combiner, which is then called. * (Here, {@code N} is defined as the parameter count of the combiner.) * After this, control passes to the target, with any result * from the combiner inserted before the original {@code N} incoming * arguments. * <p> * If the combiner returns a value, the first parameter type of the target * must be identical with the return type of the combiner, and the next * {@code N} parameter types of the target must exactly match the parameters * of the combiner. * <p> * If the combiner has a void return, no result will be inserted, * and the first {@code N} parameter types of the target * must exactly match the parameters of the combiner. * <p> * The resulting adapter is the same type as the target, except that the * first parameter type is dropped, * if it corresponds to the result of the combiner. * <p> * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments * that either the combiner or the target does not wish to receive. * If some of the incoming arguments are destined only for the combiner, * consider using {@link MethodHandle#asCollector asCollector} instead, since those * arguments will not need to be live on the stack on entry to the * target.) * <p><b>Example:</b> * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class, "println", methodType(void.class, String.class)) .bindTo(System.out); MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); assertEquals("boojum", (String) cat.invokeExact("boo", "jum")); MethodHandle catTrace = foldArguments(cat, trace); // also prints "boo": assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum")); * }</pre></blockquote> * <p>Here is pseudocode for the resulting adapter. In the code, {@code T} * represents the result type of the {@code target} and resulting adapter. * {@code V}/{@code v} represent the type and value of the parameter and argument * of {@code target} that precedes the folding position; {@code V} also is * the result type of the {@code combiner}. {@code A}/{@code a} denote the * types and values of the {@code N} parameters and arguments at the folding * position. {@code B}/{@code b} represent the types and values of the * {@code target} parameters and arguments that follow the folded parameters * and arguments. * <blockquote><pre>{@code * // there are N arguments in A... * T target(V, A[N]..., B...); * V combiner(A...); * T adapter(A... a, B... b) { * V v = combiner(a...); * return target(v, a..., b...); * } * // and if the combiner has a void return: * T target2(A[N]..., B...); * void combiner2(A...); * T adapter2(A... a, B... b) { * combiner2(a...); * return target2(a..., b...); * } * }</pre></blockquote> * <p> * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector * variable-arity method handle}, even if the original target method handle was. * @param target the method handle to invoke after arguments are combined * @param combiner method handle to call initially on the incoming arguments * @return method handle which incorporates the specified argument folding logic * @throws NullPointerException if either argument is null * @throws IllegalArgumentException if {@code combiner}'s return type * is non-void and not the same as the first argument type of * the target, or if the initial {@code N} argument types * of the target * (skipping one matching the {@code combiner}'s return type) * are not identical with the argument types of {@code combiner} */ public static MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) { return foldArguments(target, 0, combiner); } /** * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just * before the folded arguments. * <p> * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position * 0. * * @apiNote Example: * <blockquote><pre>{@code import static java.lang.invoke.MethodHandles.*; import static java.lang.invoke.MethodType.*; ... MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class, "println", methodType(void.class, String.class)) .bindTo(System.out); MethodHandle cat = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class)); assertEquals("boojum", (String) cat.invokeExact("boo", "jum")); MethodHandle catTrace = foldArguments(cat, 1, trace); // also prints "jum": assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum")); * }</pre></blockquote> * <p>Here is pseudocode for the resulting adapter. In the code, {@code T} * represents the result type of the {@code target} and resulting adapter. * {@code V}/{@code v} represent the type and value of the parameter and argument * of {@code target} that precedes the folding position; {@code V} also is * the result type of the {@code combiner}. {@code A}/{@code a} denote the * types and values of the {@code N} parameters and arguments at the folding * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types * and values of the {@code target} parameters and arguments that precede and * follow the folded parameters and arguments starting at {@code pos}, * respectively. * <blockquote><pre>{@code * // there are N arguments in A... * T target(Z..., V, A[N]..., B...); * V combiner(A...); * T adapter(Z... z, A... a, B... b) { * V v = combiner(a...); * return target(z..., v, a..., b...); * } * // and if the combiner has a void return: * T target2(Z..., A[N]..., B...); * void combiner2(A...); * T adapter2(Z... z, A... a, B... b) { * combiner2(a...); * return target2(z..., a..., b...); * } * }</pre></blockquote> * <p> * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector * variable-arity method handle}, even if the original target method handle was. * * @param target the method handle to invoke after arguments are combined * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code * 0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}. * @param combiner method handle to call initially on the incoming arguments * @return method handle which incorporates the specified argument folding logic * @throws NullPointerException if either argument is null * @throws IllegalArgumentException if either of the following two conditions holds: * (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position * {@code pos} of the target signature; * (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching * the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}. * * @see #foldArguments(MethodHandle, MethodHandle) * @since 9 */ public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) { MethodType targetType = target.type(); MethodType combinerType = combiner.type(); Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType); BoundMethodHandle result = target.rebind(); boolean dropResult = rtype == void.class; LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType()); MethodType newType = targetType; if (!dropResult) { newType = newType.dropParameterTypes(pos, pos + 1); } result = result.copyWithExtendL(newType, lform, combiner); return result; } private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) { int foldArgs = combinerType.parameterCount(); Class<?> rtype = combinerType.returnType(); int foldVals = rtype == void.class ? 0 : 1; int afterInsertPos = foldPos + foldVals; boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs); if (ok) { for (int i = 0; i < foldArgs; i++) { if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) { ok = false; break; } } } if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos)) ok = false; if (!ok) throw misMatchedTypes("target and combiner types", targetType, combinerType); return rtype; } /** * Adapts a target method handle by pre-processing some of its arguments, then calling the target with the result * of the pre-processing replacing the argument at the given position. * * @param target the method handle to invoke after arguments are combined * @param position the position at which to start folding and at which to insert the folding result; if this is {@code * 0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}. * @param combiner method handle to call initially on the incoming arguments * @param argPositions indexes of the target to pick arguments sent to the combiner from * @return method handle which incorporates the specified argument folding logic * @throws NullPointerException if either argument is null * @throws IllegalArgumentException if either of the following two conditions holds: * (1) {@code combiner}'s return type is not the same as the argument type at position * {@code pos} of the target signature; * (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature are * not identical with the argument types of {@code combiner}. */ /*non-public*/ static MethodHandle filterArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int... argPositions) { return argumentsWithCombiner(true, target, position, combiner, argPositions); } /** * Adapts a target method handle by pre-processing some of its arguments, calling the target with the result of * the pre-processing inserted into the original sequence of arguments at the given position. * * @param target the method handle to invoke after arguments are combined * @param position the position at which to start folding and at which to insert the folding result; if this is {@code * 0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}. * @param combiner method handle to call initially on the incoming arguments * @param argPositions indexes of the target to pick arguments sent to the combiner from * @return method handle which incorporates the specified argument folding logic * @throws NullPointerException if either argument is null * @throws IllegalArgumentException if either of the following two conditions holds: * (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position * {@code pos} of the target signature; * (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature * (skipping {@code position} where the {@code combiner}'s return will be folded in) are not identical * with the argument types of {@code combiner}. */ /*non-public*/ static MethodHandle foldArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int... argPositions) { return argumentsWithCombiner(false, target, position, combiner, argPositions); } private static MethodHandle argumentsWithCombiner(boolean filter, MethodHandle target, int position, MethodHandle combiner, int... argPositions) { MethodType targetType = target.type(); MethodType combinerType = combiner.type(); Class<?> rtype = argumentsWithCombinerChecks(position, filter, targetType, combinerType, argPositions); BoundMethodHandle result = target.rebind(); MethodType newType = targetType; LambdaForm lform; if (filter) { lform = result.editor().filterArgumentsForm(1 + position, combinerType.basicType(), argPositions); } else { boolean dropResult = rtype == void.class; lform = result.editor().foldArgumentsForm(1 + position, dropResult, combinerType.basicType(), argPositions); if (!dropResult) { newType = newType.dropParameterTypes(position, position + 1); } } result = result.copyWithExtendL(newType, lform, combiner); return result; } private static Class<?> argumentsWithCombinerChecks(int position, boolean filter, MethodType targetType, MethodType combinerType, int... argPos) { int combinerArgs = combinerType.parameterCount(); if (argPos.length != combinerArgs) { throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length); } Class<?> rtype = combinerType.returnType(); for (int i = 0; i < combinerArgs; i++) { int arg = argPos[i]; if (arg < 0 || arg > targetType.parameterCount()) { throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg); } if (combinerType.parameterType(i) != targetType.parameterType(arg)) { throw newIllegalArgumentException("target argument type at position " + arg + " must match combiner argument type at index " + i + ": " + targetType + " -> " + combinerType + ", map: " + Arrays.toString(argPos)); } } if (filter && combinerType.returnType() != targetType.parameterType(position)) { throw misMatchedTypes("target and combiner types", targetType, combinerType); } return rtype; } /** * Makes a method handle which adapts a target method handle, * by guarding it with a test, a boolean-valued method handle. * If the guard fails, a fallback handle is called instead. * All three method handles must have the same corresponding * argument and return types, except that the return type * of the test must be boolean, and the test is allowed * to have fewer arguments than the other two method handles. * <p> * Here is pseudocode for the resulting adapter. In the code, {@code T} * represents the uniform result type of the three involved handles; * {@code A}/{@code a}, the types and values of the {@code target} * parameters and arguments that are consumed by the {@code test}; and * {@code B}/{@code b}, those types and values of the {@code target} * parameters and arguments that are not consumed by the {@code test}. * <blockquote><pre>{@code * boolean test(A...); * T target(A...,B...); * T fallback(A...,B...); * T adapter(A... a,B... b) { * if (test(a...)) * return target(a..., b...); * else * return fallback(a..., b...); * } * }</pre></blockquote> * Note that the test arguments ({@code a...} in the pseudocode) cannot * be modified by execution of the test, and so are passed unchanged * from the caller to the target or fallback as appropriate. * @param test method handle used for test, must return boolean * @param target method handle to call if test passes * @param fallback method handle to call if test fails * @return method handle which incorporates the specified if/then/else logic * @throws NullPointerException if any argument is null * @throws IllegalArgumentException if {@code test} does not return boolean, * or if all three method types do not match (with the return * type of {@code test} changed to match that of the target). */ public static MethodHandle guardWithTest(MethodHandle test, MethodHandle target, MethodHandle fallback) { MethodType gtype = test.type(); MethodType ttype = target.type(); MethodType ftype = fallback.type(); if (!ttype.equals(ftype)) throw misMatchedTypes("target and fallback types", ttype, ftype); if (gtype.returnType() != boolean.class) throw newIllegalArgumentException("guard type is not a predicate " + gtype); List<Class<?>> targs = ttype.parameterList(); test = dropArgumentsToMatch(test, 0, targs, 0, true); if (test == null) { throw misMatchedTypes("target and test types", ttype, gtype); } return MethodHandleImpl.makeGuardWithTest(test, target, fallback); } static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) { return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2); } /** * Makes a method handle which adapts a target method handle, * by running it inside an exception handler. * If the target returns normally, the adapter returns that value. * If an exception matching the specified type is thrown, the fallback * handle is called instead on the exception, plus the original arguments. * <p> * The target and handler must have the same corresponding * argument and return types, except that handler may omit trailing arguments * (similarly to the predicate in {@link #guardWithTest guardWithTest}). * Also, the handler must have an extra leading parameter of {@code exType} or a supertype. * <p> * Here is pseudocode for the resulting adapter. In the code, {@code T} * represents the return type of the {@code target} and {@code handler}, * and correspondingly that of the resulting adapter; {@code A}/{@code a}, * the types and values of arguments to the resulting handle consumed by * {@code handler}; and {@code B}/{@code b}, those of arguments to the * resulting handle discarded by {@code handler}. * <blockquote><pre>{@code * T target(A..., B...); * T handler(ExType, A...); * T adapter(A... a, B... b) { * try { * return target(a..., b...); * } catch (ExType ex) { * return handler(ex, a...); * } * } * }</pre></blockquote> * Note that the saved arguments ({@code a...} in the pseudocode) cannot * be modified by execution of the target, and so are passed unchanged * from the caller to the handler, if the handler is invoked. * <p> * The target and handler must return the same type, even if the handler * always throws. (This might happen, for instance, because the handler * is simulating a {@code finally} clause). * To create such a throwing handler, compose the handler creation logic * with {@link #throwException throwException}, * in order to create a method handle of the correct return type. * @param target method handle to call * @param exType the type of exception which the handler will catch * @param handler method handle to call if a matching exception is thrown * @return method handle which incorporates the specified try/catch logic * @throws NullPointerException if any argument is null * @throws IllegalArgumentException if {@code handler} does not accept * the given exception type, or if the method handle types do * not match in their return types and their * corresponding parameters * @see MethodHandles#tryFinally(MethodHandle, MethodHandle) */ public static MethodHandle catchException(MethodHandle target, Class<? extends Throwable> exType, MethodHandle handler) { MethodType ttype = target.type(); MethodType htype = handler.type(); if (!Throwable.class.isAssignableFrom(exType)) throw new ClassCastException(exType.getName()); if (htype.parameterCount() < 1 || !htype.parameterType(0).isAssignableFrom(exType)) throw newIllegalArgumentException("handler does not accept exception type " + exType); if (htype.returnType() != ttype.returnType()) throw misMatchedTypes("target and handler return types", ttype, htype); handler = dropArgumentsToMatch(handler, 1, ttype.parameterList(), 0, true); if (handler == null) { throw misMatchedTypes("target and handler types", ttype, htype); } return MethodHandleImpl.makeGuardWithCatch(target, exType, handler); } /** * Produces a method handle which will throw exceptions of the given {@code exType}. * The method handle will accept a single argument of {@code exType}, * and immediately throw it as an exception. * The method type will nominally specify a return of {@code returnType}. * The return type may be anything convenient: It doesn't matter to the * method handle's behavior, since it will never return normally. * @param returnType the return type of the desired method handle * @param exType the parameter type of the desired method handle * @return method handle which can throw the given exceptions * @throws NullPointerException if either argument is null */ public static MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) { if (!Throwable.class.isAssignableFrom(exType)) throw new ClassCastException(exType.getName()); return MethodHandleImpl.throwException(methodType(returnType, exType)); } /** * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and * delivers the loop's result, which is the return value of the resulting handle. * <p> * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in * terms of method handles, each clause will specify up to four independent actions:<ul> * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}. * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}. * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit. * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value. * </ul> * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}. * The values themselves will be {@code (v...)}. When we speak of "parameter lists", we will usually * be referring to types, but in some contexts (describing execution) the lists will be of actual values. * <p> * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in * this case. See below for a detailed description. * <p> * <em>Parameters optional everywhere:</em> * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}. * As an exception, the init functions cannot take any {@code v} parameters, * because those values are not yet computed when the init functions are executed. * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take. * In fact, any clause function may take no arguments at all. * <p> * <em>Loop parameters:</em> * A clause function may take all the iteration variable values it is entitled to, in which case * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>, * with their types and values notated as {@code (A...)} and {@code (a...)}. * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed. * (Since init functions do not accept iteration variables {@code v}, any parameter to an * init function is automatically a loop parameter {@code a}.) * As with iteration variables, clause functions are allowed but not required to accept loop parameters. * These loop parameters act as loop-invariant values visible across the whole loop. * <p> * <em>Parameters visible everywhere:</em> * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full * list {@code (v... a...)} of current iteration variable values and incoming loop parameters. * The init functions can observe initial pre-loop state, in the form {@code (a...)}. * Most clause functions will not need all of this information, but they will be formally connected to it * as if by {@link #dropArguments}. * <a id="astar"></a> * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}). * In that notation, the general form of an init function parameter list * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}. * <p> * <em>Checking clause structure:</em> * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must" * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not * met by the inputs to the loop combinator. * <p> * <em>Effectively identical sequences:</em> * <a id="effid"></a> * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B} * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}. * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical" * as a whole if the set contains a longest list, and all members of the set are effectively identical to * that longest list. * For example, any set of type sequences of the form {@code (V*)} is effectively identical, * and the same is true if more sequences of the form {@code (V... A*)} are added. * <p> * <em>Step 0: Determine clause structure.</em><ol type="a"> * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element. * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements. * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length * four. Padding takes place by appending elements to the array. * <li>Clauses with all {@code null}s are disregarded. * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini". * </ol> * <p> * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a"> * <li>The iteration variable type for each clause is determined using the clause's init and step return types. * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's * iteration variable type. * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}. * <li>This list of types is called the "iteration variable types" ({@code (V...)}). * </ol> * <p> * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul> * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}). * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types. * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.) * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types. * (These types will be checked in step 2, along with all the clause function types.) * <li>Omitted clause functions are ignored. (Equivalently, they are deemed to have empty parameter lists.) * <li>All of the collected parameter lists must be effectively identical. * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}). * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence. * <li>The combined list consisting of iteration variable types followed by the external parameter types is called * the "internal parameter list". * </ul> * <p> * <em>Step 1C: Determine loop return type.</em><ol type="a"> * <li>Examine fini function return types, disregarding omitted fini functions. * <li>If there are no fini functions, the loop return type is {@code void}. * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return * type. * </ol> * <p> * <em>Step 1D: Check other types.</em><ol type="a"> * <li>There must be at least one non-omitted pred function. * <li>Every non-omitted pred function must have a {@code boolean} return type. * </ol> * <p> * <em>Step 2: Determine parameter lists.</em><ol type="a"> * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}. * <li>The parameter list for init functions will be adjusted to the external parameter list. * (Note that their parameter lists are already effectively identical to this list.) * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be * effectively identical to the internal parameter list {@code (V... A...)}. * </ol> * <p> * <em>Step 3: Fill in omitted functions.</em><ol type="a"> * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable * type. * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void} * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.) * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far * as this clause is concerned. Note that in such cases the corresponding fini function is unreachable.) * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the * loop return type. * </ol> * <p> * <em>Step 4: Fill in missing parameter types.</em><ol type="a"> * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)}, * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list. * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list, * pad out the end of the list. * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}. * </ol> * <p> * <em>Final observations.</em><ol type="a"> * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments. * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have. * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have. * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of * (non-{@code void}) iteration variables {@code V} followed by loop parameters. * <li>Each pair of init and step functions agrees in their return type {@code V}. * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables. * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters. * </ol> * <p> * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property: * <ul> * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}. * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters. * (Only one {@code Pn} has to be non-{@code null}.) * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}. * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types. * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}. * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}. * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine * the resulting loop handle's parameter types {@code (A...)}. * </ul> * In this example, the loop handle parameters {@code (A...)} were derived from the step functions, * which is natural if most of the loop computation happens in the steps. For some loops, * the burden of computation might be heaviest in the pred functions, and so the pred functions * might need to accept the loop parameter values. For loops with complex exit logic, the fini * functions might need to accept loop parameters, and likewise for loops with complex entry logic, * where the init functions will need the extra parameters. For such reasons, the rules for * determining these parameters are as symmetric as possible, across all clause parts. * In general, the loop parameters function as common invariant values across the whole * loop, while the iteration variables function as common variant values, or (if there is * no step function) as internal loop invariant temporaries. * <p> * <em>Loop execution.</em><ol type="a"> * <li>When the loop is called, the loop input values are saved in locals, to be passed to * every clause function. These locals are loop invariant. * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)}) * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals. * These locals will be loop varying (unless their steps behave as identity functions, as noted above). * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)} * (in argument order). * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function * returns {@code false}. * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the * sequence {@code (v...)} of loop variables. * The updated value is immediately visible to all subsequent function calls. * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value * (of type {@code R}) is returned from the loop as a whole. * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit * except by throwing an exception. * </ol> * <p> * <em>Usage tips.</em> * <ul> * <li>Although each step function will receive the current values of <em>all</em> the loop variables, * sometimes a step function only needs to observe the current value of its own variable. * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}. * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}. * <li>Loop variables are not required to vary; they can be loop invariant. A clause can create * a loop invariant by a suitable init function with no step, pred, or fini function. This may be * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable. * <li>If some of the clause functions are virtual methods on an instance, the instance * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause * like {@code new MethodHandle[]{identity(ObjType.class)}}. In that case, the instance reference * will be the first iteration variable value, and it will be easy to use virtual * methods as clause parts, since all of them will take a leading instance reference matching that value. * </ul> * <p> * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop; * and {@code R} is the common result type of all finalizers as well as of the resulting loop. * <blockquote><pre>{@code * V... init...(A...); * boolean pred...(V..., A...); * V... step...(V..., A...); * R fini...(V..., A...); * R loop(A... a) { * V... v... = init...(a...); * for (;;) { * for ((v, p, s, f) in (v..., pred..., step..., fini...)) { * v = s(v..., a...); * if (!p(v..., a...)) { * return f(v..., a...); * } * } * } * } * }</pre></blockquote> * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded * to their full length, even though individual clause functions may neglect to take them all. * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}. * * @apiNote Example: * <blockquote><pre>{@code * // iterative implementation of the factorial function as a loop handle * static int one(int k) { return 1; } * static int inc(int i, int acc, int k) { return i + 1; } * static int mult(int i, int acc, int k) { return i * acc; } * static boolean pred(int i, int acc, int k) { return i < k; } * static int fin(int i, int acc, int k) { return acc; } * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods * // null initializer for counter, should initialize to 0 * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc}; * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin}; * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause); * assertEquals(120, loop.invoke(5)); * }</pre></blockquote> * The same example, dropping arguments and using combinators: * <blockquote><pre>{@code * // simplified implementation of the factorial function as a loop handle * static int inc(int i) { return i + 1; } // drop acc, k * static int mult(int i, int acc) { return i * acc; } //drop k * static boolean cmp(int i, int k) { return i < k; } * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods * // null initializer for counter, should initialize to 0 * MethodHandle MH_one = MethodHandles.constant(int.class, 1); * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc}; * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin}; * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause); * assertEquals(720, loop.invoke(6)); * }</pre></blockquote> * A similar example, using a helper object to hold a loop parameter: * <blockquote><pre>{@code * // instance-based implementation of the factorial function as a loop handle * static class FacLoop { * final int k; * FacLoop(int k) { this.k = k; } * int inc(int i) { return i + 1; } * int mult(int i, int acc) { return i * acc; } * boolean pred(int i) { return i < k; } * int fin(int i, int acc) { return acc; } * } * // assume MH_FacLoop is a handle to the constructor * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods * // null initializer for counter, should initialize to 0 * MethodHandle MH_one = MethodHandles.constant(int.class, 1); * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop}; * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc}; * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin}; * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause); * assertEquals(5040, loop.invoke(7)); * }</pre></blockquote> * * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above. * * @return a method handle embodying the looping behavior as defined by the arguments. * * @throws IllegalArgumentException in case any of the constraints described above is violated. * * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle) * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle) * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle) * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle) * @since 9 */ public static MethodHandle loop(MethodHandle[]... clauses) { // Step 0: determine clause structure. loopChecks0(clauses); List<MethodHandle> init = new ArrayList<>(); List<MethodHandle> step = new ArrayList<>(); List<MethodHandle> pred = new ArrayList<>(); List<MethodHandle> fini = new ArrayList<>(); Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> { init.add(clause[0]); // all clauses have at least length 1 step.add(clause.length <= 1 ? null : clause[1]); pred.add(clause.length <= 2 ? null : clause[2]); fini.add(clause.length <= 3 ? null : clause[3]); }); assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1; final int nclauses = init.size(); // Step 1A: determine iteration variables (V...). final List<Class<?>> iterationVariableTypes = new ArrayList<>(); for (int i = 0; i < nclauses; ++i) { MethodHandle in = init.get(i); MethodHandle st = step.get(i); if (in == null && st == null) { iterationVariableTypes.add(void.class); } else if (in != null && st != null) { loopChecks1a(i, in, st); iterationVariableTypes.add(in.type().returnType()); } else { iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType()); } } final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class) .collect(Collectors.toList()); // Step 1B: determine loop parameters (A...). final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size()); loopChecks1b(init, commonSuffix); // Step 1C: determine loop return type. // Step 1D: check other types. // local variable required here; see JDK-8223553 Stream<Class<?>> cstream = fini.stream().filter(Objects::nonNull).map(MethodHandle::type) .map(MethodType::returnType); final Class<?> loopReturnType = cstream.findFirst().orElse(void.class); loopChecks1cd(pred, fini, loopReturnType); // Step 2: determine parameter lists. final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix); commonParameterSequence.addAll(commonSuffix); loopChecks2(step, pred, fini, commonParameterSequence); // Step 3: fill in omitted functions. for (int i = 0; i < nclauses; ++i) { Class<?> t = iterationVariableTypes.get(i); if (init.get(i) == null) { init.set(i, empty(methodType(t, commonSuffix))); } if (step.get(i) == null) { step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i)); } if (pred.get(i) == null) { pred.set(i, dropArguments0(constant(boolean.class, true), 0, commonParameterSequence)); } if (fini.get(i) == null) { fini.set(i, empty(methodType(t, commonParameterSequence))); } } // Step 4: fill in missing parameter types. // Also convert all handles to fixed-arity handles. List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix)); List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence)); List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence)); List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence)); assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList) .allMatch(pl -> pl.equals(commonSuffix)); assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type) .map(MethodType::parameterList).allMatch(pl -> pl.equals(commonParameterSequence)); return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini); } private static void loopChecks0(MethodHandle[][] clauses) { if (clauses == null || clauses.length == 0) { throw newIllegalArgumentException("null or no clauses passed"); } if (Stream.of(clauses).anyMatch(Objects::isNull)) { throw newIllegalArgumentException("null clauses are not allowed"); } if (Stream.of(clauses).anyMatch(c -> c.length > 4)) { throw newIllegalArgumentException( "All loop clauses must be represented as MethodHandle arrays with at most 4 elements."); } } private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) { if (in.type().returnType() != st.type().returnType()) { throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(), st.type().returnType()); } } private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) { final List<Class<?>> empty = List.of(); final List<Class<?>> longest = mhs.filter(Objects::nonNull). // take only those that can contribute to a common suffix because they are longer than the prefix map(MethodHandle::type).filter(t -> t.parameterCount() > skipSize).map(MethodType::parameterList) .reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty); return longest.size() == 0 ? empty : longest.subList(skipSize, longest.size()); } private static List<Class<?>> longestParameterList(List<List<Class<?>>> lists) { final List<Class<?>> empty = List.of(); return lists.stream().reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty); } private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) { final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize); final List<Class<?>> longest2 = longestParameterList(init.stream(), 0); return longestParameterList(Arrays.asList(longest1, longest2)); } private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) { if (init.stream().filter(Objects::nonNull).map(MethodHandle::type) .anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) { throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init + " (common suffix: " + commonSuffix + ")"); } } private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) { if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType) .anyMatch(t -> t != loopReturnType)) { throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " + loopReturnType + ")"); } if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) { throw newIllegalArgumentException("no predicate found", pred); } if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType) .anyMatch(t -> t != boolean.class)) { throw newIllegalArgumentException("predicates must have boolean return type", pred); } } private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) { if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type) .anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) { throw newIllegalArgumentException( "found non-effectively identical parameter type lists:\nstep: " + step + "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")"); } } private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) { return hs.stream().map(h -> { int pc = h.type().parameterCount(); int tpsize = targetParams.size(); return pc < tpsize ? dropArguments0(h, pc, targetParams.subList(pc, tpsize)) : h; }).collect(Collectors.toList()); } private static List<MethodHandle> fixArities(List<MethodHandle> hs) { return hs.stream().map(MethodHandle::asFixedArity).collect(Collectors.toList()); } /** * Constructs a {@code while} loop from an initializer, a body, and a predicate. * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}. * <p> * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate * evaluates to {@code true}). * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case). * <p> * The {@code init} handle describes the initial value of an additional optional loop-local variable. * In each iteration, this loop-local variable, if present, will be passed to the {@code body} * and updated with the value returned from its invocation. The result of loop execution will be * the final value of the additional loop-local variable (if present). * <p> * The following rules hold for these argument handles:<ul> * <li>The {@code body} handle must not be {@code null}; its type must be of the form * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}. * (In the {@code void} case, we assign the type {@code void} to the name {@code V}, * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V} * is quietly dropped from the parameter list, leaving {@code (A...)V}.) * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>. * It will constrain the parameter lists of the other loop parts. * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter * list {@code (A...)} is called the <em>external parameter list</em>. * <li>The body return type {@code V}, if non-{@code void}, determines the type of an * additional state variable of the loop. * The body must both accept and return a value of this type {@code V}. * <li>If {@code init} is non-{@code null}, it must have return type {@code V}. * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be * <a href="MethodHandles.html#effid">effectively identical</a> * to the external parameter list {@code (A...)}. * <li>If {@code init} is {@code null}, the loop variable will be initialized to its * {@linkplain #empty default value}. * <li>The {@code pred} handle must not be {@code null}. It must have {@code boolean} as its return type. * Its parameter list (either empty or of the form {@code (V A*)}) must be * effectively identical to the internal parameter list. * </ul> * <p> * The resulting loop handle's result type and parameter signature are determined as follows:<ul> * <li>The loop handle's result type is the result type {@code V} of the body. * <li>The loop handle's parameter types are the types {@code (A...)}, * from the external parameter list. * </ul> * <p> * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument * passed to the loop. * <blockquote><pre>{@code * V init(A...); * boolean pred(V, A...); * V body(V, A...); * V whileLoop(A... a...) { * V v = init(a...); * while (pred(v, a...)) { * v = body(v, a...); * } * return v; * } * }</pre></blockquote> * * @apiNote Example: * <blockquote><pre>{@code * // implement the zip function for lists as a loop handle * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); } * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); } * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) { * zip.add(a.next()); * zip.add(b.next()); * return zip; * } * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep); * List<String> a = Arrays.asList("a", "b", "c", "d"); * List<String> b = Arrays.asList("e", "f", "g", "h"); * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h"); * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator())); * }</pre></blockquote> * * * @apiNote The implementation of this method can be expressed as follows: * <blockquote><pre>{@code * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) { * MethodHandle fini = (body.type().returnType() == void.class * ? null : identity(body.type().returnType())); * MethodHandle[] * checkExit = { null, null, pred, fini }, * varBody = { init, body }; * return loop(checkExit, varBody); * } * }</pre></blockquote> * * @param init optional initializer, providing the initial value of the loop variable. * May be {@code null}, implying a default initial value. See above for other constraints. * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See * above for other constraints. * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type. * See above for other constraints. * * @return a method handle implementing the {@code while} loop as described by the arguments. * @throws IllegalArgumentException if the rules for the arguments are violated. * @throws NullPointerException if {@code pred} or {@code body} are {@code null}. * * @see #loop(MethodHandle[][]) * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle) * @since 9 */ public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) { whileLoopChecks(init, pred, body); MethodHandle fini = identityOrVoid(body.type().returnType()); MethodHandle[] checkExit = { null, null, pred, fini }; MethodHandle[] varBody = { init, body }; return loop(checkExit, varBody); } /** * Constructs a {@code do-while} loop from an initializer, a body, and a predicate. * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}. * <p> * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this * method will, in each iteration, first execute its body and then evaluate the predicate. * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body. * <p> * The {@code init} handle describes the initial value of an additional optional loop-local variable. * In each iteration, this loop-local variable, if present, will be passed to the {@code body} * and updated with the value returned from its invocation. The result of loop execution will be * the final value of the additional loop-local variable (if present). * <p> * The following rules hold for these argument handles:<ul> * <li>The {@code body} handle must not be {@code null}; its type must be of the form * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}. * (In the {@code void} case, we assign the type {@code void} to the name {@code V}, * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V} * is quietly dropped from the parameter list, leaving {@code (A...)V}.) * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>. * It will constrain the parameter lists of the other loop parts. * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter * list {@code (A...)} is called the <em>external parameter list</em>. * <li>The body return type {@code V}, if non-{@code void}, determines the type of an * additional state variable of the loop. * The body must both accept and return a value of this type {@code V}. * <li>If {@code init} is non-{@code null}, it must have return type {@code V}. * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be * <a href="MethodHandles.html#effid">effectively identical</a> * to the external parameter list {@code (A...)}. * <li>If {@code init} is {@code null}, the loop variable will be initialized to its * {@linkplain #empty default value}. * <li>The {@code pred} handle must not be {@code null}. It must have {@code boolean} as its return type. * Its parameter list (either empty or of the form {@code (V A*)}) must be * effectively identical to the internal parameter list. * </ul> * <p> * The resulting loop handle's result type and parameter signature are determined as follows:<ul> * <li>The loop handle's result type is the result type {@code V} of the body. * <li>The loop handle's parameter types are the types {@code (A...)}, * from the external parameter list. * </ul> * <p> * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument * passed to the loop. * <blockquote><pre>{@code * V init(A...); * boolean pred(V, A...); * V body(V, A...); * V doWhileLoop(A... a...) { * V v = init(a...); * do { * v = body(v, a...); * } while (pred(v, a...)); * return v; * } * }</pre></blockquote> * * @apiNote Example: * <blockquote><pre>{@code * // int i = 0; while (i < limit) { ++i; } return i; => limit * static int zero(int limit) { return 0; } * static int step(int i, int limit) { return i + 1; } * static boolean pred(int i, int limit) { return i < limit; } * // assume MH_zero, MH_step, and MH_pred are handles to the above methods * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred); * assertEquals(23, loop.invoke(23)); * }</pre></blockquote> * * * @apiNote The implementation of this method can be expressed as follows: * <blockquote><pre>{@code * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) { * MethodHandle fini = (body.type().returnType() == void.class * ? null : identity(body.type().returnType())); * MethodHandle[] clause = { init, body, pred, fini }; * return loop(clause); * } * }</pre></blockquote> * * @param init optional initializer, providing the initial value of the loop variable. * May be {@code null}, implying a default initial value. See above for other constraints. * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type. * See above for other constraints. * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See * above for other constraints. * * @return a method handle implementing the {@code while} loop as described by the arguments. * @throws IllegalArgumentException if the rules for the arguments are violated. * @throws NullPointerException if {@code pred} or {@code body} are {@code null}. * * @see #loop(MethodHandle[][]) * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle) * @since 9 */ public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) { whileLoopChecks(init, pred, body); MethodHandle fini = identityOrVoid(body.type().returnType()); MethodHandle[] clause = { init, body, pred, fini }; return loop(clause); } private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) { Objects.requireNonNull(pred); Objects.requireNonNull(body); MethodType bodyType = body.type(); Class<?> returnType = bodyType.returnType(); List<Class<?>> innerList = bodyType.parameterList(); List<Class<?>> outerList = innerList; if (returnType == void.class) { // OK } else if (innerList.size() == 0 || innerList.get(0) != returnType) { // leading V argument missing => error MethodType expected = bodyType.insertParameterTypes(0, returnType); throw misMatchedTypes("body function", bodyType, expected); } else { outerList = innerList.subList(1, innerList.size()); } MethodType predType = pred.type(); if (predType.returnType() != boolean.class || !predType.effectivelyIdenticalParameters(0, innerList)) { throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList)); } if (init != null) { MethodType initType = init.type(); if (initType.returnType() != returnType || !initType.effectivelyIdenticalParameters(0, outerList)) { throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList)); } } } /** * Constructs a loop that runs a given number of iterations. * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}. * <p> * The number of iterations is determined by the {@code iterations} handle evaluation result. * The loop counter {@code i} is an extra loop iteration variable of type {@code int}. * It will be initialized to 0 and incremented by 1 in each iteration. * <p> * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable * of that type is also present. This variable is initialized using the optional {@code init} handle, * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}. * <p> * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle. * A non-{@code void} value returned from the body (of type {@code V}) updates the leading * iteration variable. * The result of the loop handle execution will be the final {@code V} value of that variable * (or {@code void} if there is no {@code V} variable). * <p> * The following rules hold for the argument handles:<ul> * <li>The {@code iterations} handle must not be {@code null}, and must return * the type {@code int}, referred to here as {@code I} in parameter type lists. * <li>The {@code body} handle must not be {@code null}; its type must be of the form * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}. * (In the {@code void} case, we assign the type {@code void} to the name {@code V}, * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V} * is quietly dropped from the parameter list, leaving {@code (I A...)V}.) * <li>The parameter list {@code (V I A...)} of the body contributes to a list * of types called the <em>internal parameter list</em>. * It will constrain the parameter lists of the other loop parts. * <li>As a special case, if the body contributes only {@code V} and {@code I} types, * with no additional {@code A} types, then the internal parameter list is extended by * the argument types {@code A...} of the {@code iterations} handle. * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter * list {@code (A...)} is called the <em>external parameter list</em>. * <li>The body return type {@code V}, if non-{@code void}, determines the type of an * additional state variable of the loop. * The body must both accept a leading parameter and return a value of this type {@code V}. * <li>If {@code init} is non-{@code null}, it must have return type {@code V}. * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be * <a href="MethodHandles.html#effid">effectively identical</a> * to the external parameter list {@code (A...)}. * <li>If {@code init} is {@code null}, the loop variable will be initialized to its * {@linkplain #empty default value}. * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be * effectively identical to the external parameter list {@code (A...)}. * </ul> * <p> * The resulting loop handle's result type and parameter signature are determined as follows:<ul> * <li>The loop handle's result type is the result type {@code V} of the body. * <li>The loop handle's parameter types are the types {@code (A...)}, * from the external parameter list. * </ul> * <p> * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent * arguments passed to the loop. * <blockquote><pre>{@code * int iterations(A...); * V init(A...); * V body(V, int, A...); * V countedLoop(A... a...) { * int end = iterations(a...); * V v = init(a...); * for (int i = 0; i < end; ++i) { * v = body(v, i, a...); * } * return v; * } * }</pre></blockquote> * * @apiNote Example with a fully conformant body method: * <blockquote><pre>{@code * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s; * // => a variation on a well known theme * static String step(String v, int counter, String init) { return "na " + v; } * // assume MH_step is a handle to the method above * MethodHandle fit13 = MethodHandles.constant(int.class, 13); * MethodHandle start = MethodHandles.identity(String.class); * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step); * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!")); * }</pre></blockquote> * * @apiNote Example with the simplest possible body method type, * and passing the number of iterations to the loop invocation: * <blockquote><pre>{@code * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s; * // => a variation on a well known theme * static String step(String v, int counter ) { return "na " + v; } * // assume MH_step is a handle to the method above * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class); * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class); * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step); // (v, i) -> "na " + v * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!")); * }</pre></blockquote> * * @apiNote Example that treats the number of iterations, string to append to, and string to append * as loop parameters: * <blockquote><pre>{@code * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s; * // => a variation on a well known theme * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; } * // assume MH_step is a handle to the method above * MethodHandle count = MethodHandles.identity(int.class); * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class); * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step); // (v, i, _, pre, _) -> pre + " " + v * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!")); * }</pre></blockquote> * * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)} * to enforce a loop type: * <blockquote><pre>{@code * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s; * // => a variation on a well known theme * static String step(String v, int counter, String pre) { return pre + " " + v; } * // assume MH_step is a handle to the method above * MethodType loopType = methodType(String.class, String.class, int.class, String.class); * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class), 0, loopType.parameterList(), 1); * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2); * MethodHandle body = MethodHandles.dropArgumentsToMatch(MH_step, 2, loopType.parameterList(), 0); * MethodHandle loop = MethodHandles.countedLoop(count, start, body); // (v, i, pre, _, _) -> pre + " " + v * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!")); * }</pre></blockquote> * * @apiNote The implementation of this method can be expressed as follows: * <blockquote><pre>{@code * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) { * return countedLoop(empty(iterations.type()), iterations, init, body); * } * }</pre></blockquote> * * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's * result type must be {@code int}. See above for other constraints. * @param init optional initializer, providing the initial value of the loop variable. * May be {@code null}, implying a default initial value. See above for other constraints. * @param body body of the loop, which may not be {@code null}. * It controls the loop parameters and result type in the standard case (see above for details). * It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter), * and may accept any number of additional types. * See above for other constraints. * * @return a method handle representing the loop. * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}. * @throws IllegalArgumentException if any argument violates the rules formulated above. * * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle) * @since 9 */ public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) { return countedLoop(empty(iterations.type()), iterations, init, body); } /** * Constructs a loop that counts over a range of numbers. * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}. * <p> * The loop counter {@code i} is a loop iteration variable of type {@code int}. * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive) * values of the loop counter. * The loop counter will be initialized to the {@code int} value returned from the evaluation of the * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1. * <p> * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable * of that type is also present. This variable is initialized using the optional {@code init} handle, * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}. * <p> * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle. * A non-{@code void} value returned from the body (of type {@code V}) updates the leading * iteration variable. * The result of the loop handle execution will be the final {@code V} value of that variable * (or {@code void} if there is no {@code V} variable). * <p> * The following rules hold for the argument handles:<ul> * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return * the common type {@code int}, referred to here as {@code I} in parameter type lists. * <li>The {@code body} handle must not be {@code null}; its type must be of the form * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}. * (In the {@code void} case, we assign the type {@code void} to the name {@code V}, * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V} * is quietly dropped from the parameter list, leaving {@code (I A...)V}.) * <li>The parameter list {@code (V I A...)} of the body contributes to a list * of types called the <em>internal parameter list</em>. * It will constrain the parameter lists of the other loop parts. * <li>As a special case, if the body contributes only {@code V} and {@code I} types, * with no additional {@code A} types, then the internal parameter list is extended by * the argument types {@code A...} of the {@code end} handle. * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter * list {@code (A...)} is called the <em>external parameter list</em>. * <li>The body return type {@code V}, if non-{@code void}, determines the type of an * additional state variable of the loop. * The body must both accept a leading parameter and return a value of this type {@code V}. * <li>If {@code init} is non-{@code null}, it must have return type {@code V}. * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be * <a href="MethodHandles.html#effid">effectively identical</a> * to the external parameter list {@code (A...)}. * <li>If {@code init} is {@code null}, the loop variable will be initialized to its * {@linkplain #empty default value}. * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be * effectively identical to the external parameter list {@code (A...)}. * <li>Likewise, the parameter list of {@code end} must be effectively identical * to the external parameter list. * </ul> * <p> * The resulting loop handle's result type and parameter signature are determined as follows:<ul> * <li>The loop handle's result type is the result type {@code V} of the body. * <li>The loop handle's parameter types are the types {@code (A...)}, * from the external parameter list. * </ul> * <p> * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent * arguments passed to the loop. * <blockquote><pre>{@code * int start(A...); * int end(A...); * V init(A...); * V body(V, int, A...); * V countedLoop(A... a...) { * int e = end(a...); * int s = start(a...); * V v = init(a...); * for (int i = s; i < e; ++i) { * v = body(v, i, a...); * } * return v; * } * }</pre></blockquote> * * @apiNote The implementation of this method can be expressed as follows: * <blockquote><pre>{@code * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) { * MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class); * // assume MH_increment and MH_predicate are handles to implementation-internal methods with * // the following semantics: * // MH_increment: (int limit, int counter) -> counter + 1 * // MH_predicate: (int limit, int counter) -> counter < limit * Class<?> counterType = start.type().returnType(); // int * Class<?> returnType = body.type().returnType(); * MethodHandle incr = MH_increment, pred = MH_predicate, retv = null; * if (returnType != void.class) { // ignore the V variable * incr = dropArguments(incr, 1, returnType); // (limit, v, i) => (limit, i) * pred = dropArguments(pred, 1, returnType); // ditto * retv = dropArguments(identity(returnType), 0, counterType); // ignore limit * } * body = dropArguments(body, 0, counterType); // ignore the limit variable * MethodHandle[] * loopLimit = { end, null, pred, retv }, // limit = end(); i < limit || return v * bodyClause = { init, body }, // v = init(); v = body(v, i) * indexVar = { start, incr }; // i = start(); i = i + 1 * return loop(loopLimit, bodyClause, indexVar); * } * }</pre></blockquote> * * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}. * See above for other constraints. * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to * {@code end-1}). The result type must be {@code int}. See above for other constraints. * @param init optional initializer, providing the initial value of the loop variable. * May be {@code null}, implying a default initial value. See above for other constraints. * @param body body of the loop, which may not be {@code null}. * It controls the loop parameters and result type in the standard case (see above for details). * It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter), * and may accept any number of additional types. * See above for other constraints. * * @return a method handle representing the loop. * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}. * @throws IllegalArgumentException if any argument violates the rules formulated above. * * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle) * @since 9 */ public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) { countedLoopChecks(start, end, init, body); Class<?> counterType = start.type().returnType(); // int, but who's counting? Class<?> limitType = end.type().returnType(); // yes, int again Class<?> returnType = body.type().returnType(); MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep); MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred); MethodHandle retv = null; if (returnType != void.class) { incr = dropArguments(incr, 1, returnType); // (limit, v, i) => (limit, i) pred = dropArguments(pred, 1, returnType); // ditto retv = dropArguments(identity(returnType), 0, counterType); } body = dropArguments(body, 0, counterType); // ignore the limit variable MethodHandle[] loopLimit = { end, null, pred, retv }, // limit = end(); i < limit || return v bodyClause = { init, body }, // v = init(); v = body(v, i) indexVar = { start, incr }; // i = start(); i = i + 1 return loop(loopLimit, bodyClause, indexVar); } private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) { Objects.requireNonNull(start); Objects.requireNonNull(end); Objects.requireNonNull(body); Class<?> counterType = start.type().returnType(); if (counterType != int.class) { MethodType expected = start.type().changeReturnType(int.class); throw misMatchedTypes("start function", start.type(), expected); } else if (end.type().returnType() != counterType) { MethodType expected = end.type().changeReturnType(counterType); throw misMatchedTypes("end function", end.type(), expected); } MethodType bodyType = body.type(); Class<?> returnType = bodyType.returnType(); List<Class<?>> innerList = bodyType.parameterList(); // strip leading V value if present int vsize = (returnType == void.class ? 0 : 1); if (vsize != 0 && (innerList.size() == 0 || innerList.get(0) != returnType)) { // argument list has no "V" => error MethodType expected = bodyType.insertParameterTypes(0, returnType); throw misMatchedTypes("body function", bodyType, expected); } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) { // missing I type => error MethodType expected = bodyType.insertParameterTypes(vsize, counterType); throw misMatchedTypes("body function", bodyType, expected); } List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size()); if (outerList.isEmpty()) { // special case; take lists from end handle outerList = end.type().parameterList(); innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList(); } MethodType expected = methodType(counterType, outerList); if (!start.type().effectivelyIdenticalParameters(0, outerList)) { throw misMatchedTypes("start parameter types", start.type(), expected); } if (end.type() != start.type() && !end.type().effectivelyIdenticalParameters(0, outerList)) { throw misMatchedTypes("end parameter types", end.type(), expected); } if (init != null) { MethodType initType = init.type(); if (initType.returnType() != returnType || !initType.effectivelyIdenticalParameters(0, outerList)) { throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList)); } } } /** * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}. * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}. * <p> * The iterator itself will be determined by the evaluation of the {@code iterator} handle. * Each value it produces will be stored in a loop iteration variable of type {@code T}. * <p> * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable * of that type is also present. This variable is initialized using the optional {@code init} handle, * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}. * <p> * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle. * A non-{@code void} value returned from the body (of type {@code V}) updates the leading * iteration variable. * The result of the loop handle execution will be the final {@code V} value of that variable * (or {@code void} if there is no {@code V} variable). * <p> * The following rules hold for the argument handles:<ul> * <li>The {@code body} handle must not be {@code null}; its type must be of the form * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}. * (In the {@code void} case, we assign the type {@code void} to the name {@code V}, * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V} * is quietly dropped from the parameter list, leaving {@code (T A...)V}.) * <li>The parameter list {@code (V T A...)} of the body contributes to a list * of types called the <em>internal parameter list</em>. * It will constrain the parameter lists of the other loop parts. * <li>As a special case, if the body contributes only {@code V} and {@code T} types, * with no additional {@code A} types, then the internal parameter list is extended by * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the * single type {@code Iterable} is added and constitutes the {@code A...} list. * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter * list {@code (A...)} is called the <em>external parameter list</em>. * <li>The body return type {@code V}, if non-{@code void}, determines the type of an * additional state variable of the loop. * The body must both accept a leading parameter and return a value of this type {@code V}. * <li>If {@code init} is non-{@code null}, it must have return type {@code V}. * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be * <a href="MethodHandles.html#effid">effectively identical</a> * to the external parameter list {@code (A...)}. * <li>If {@code init} is {@code null}, the loop variable will be initialized to its * {@linkplain #empty default value}. * <li>If the {@code iterator} handle is non-{@code null}, it must have the return * type {@code java.util.Iterator} or a subtype thereof. * The iterator it produces when the loop is executed will be assumed * to yield values which can be converted to type {@code T}. * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be * effectively identical to the external parameter list {@code (A...)}. * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves * like {@link java.lang.Iterable#iterator()}. In that case, the internal parameter list * {@code (V T A...)} must have at least one {@code A} type, and the default iterator * handle parameter is adjusted to accept the leading {@code A} type, as if by * the {@link MethodHandle#asType asType} conversion method. * The leading {@code A} type must be {@code Iterable} or a subtype thereof. * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}. * </ul> * <p> * The type {@code T} may be either a primitive or reference. * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator}, * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object} * as if by the {@link MethodHandle#asType asType} conversion method. * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}. * <p> * The resulting loop handle's result type and parameter signature are determined as follows:<ul> * <li>The loop handle's result type is the result type {@code V} of the body. * <li>The loop handle's parameter types are the types {@code (A...)}, * from the external parameter list. * </ul> * <p> * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop. * <blockquote><pre>{@code * Iterator<T> iterator(A...); // defaults to Iterable::iterator * V init(A...); * V body(V,T,A...); * V iteratedLoop(A... a...) { * Iterator<T> it = iterator(a...); * V v = init(a...); * while (it.hasNext()) { * T t = it.next(); * v = body(v, t, a...); * } * return v; * } * }</pre></blockquote> * * @apiNote Example: * <blockquote><pre>{@code * // get an iterator from a list * static List<String> reverseStep(List<String> r, String e) { * r.add(0, e); * return r; * } * static List<String> newArrayList() { return new ArrayList<>(); } * // assume MH_reverseStep and MH_newArrayList are handles to the above methods * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep); * List<String> list = Arrays.asList("a", "b", "c", "d", "e"); * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a"); * assertEquals(reversedList, (List<String>) loop.invoke(list)); * }</pre></blockquote> * * @apiNote The implementation of this method can be expressed approximately as follows: * <blockquote><pre>{@code * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) { * // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable * Class<?> returnType = body.type().returnType(); * Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1); * MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype)); * MethodHandle retv = null, step = body, startIter = iterator; * if (returnType != void.class) { * // the simple thing first: in (I V A...), drop the I to get V * retv = dropArguments(identity(returnType), 0, Iterator.class); * // body type signature (V T A...), internal loop types (I V A...) * step = swapArguments(body, 0, 1); // swap V <-> T * } * if (startIter == null) startIter = MH_getIter; * MethodHandle[] * iterVar = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext()) * bodyClause = { init, filterArguments(step, 0, nextVal) }; // v = body(v, t, a) * return loop(iterVar, bodyClause); * } * }</pre></blockquote> * * @param iterator an optional handle to return the iterator to start the loop. * If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype. * See above for other constraints. * @param init optional initializer, providing the initial value of the loop variable. * May be {@code null}, implying a default initial value. See above for other constraints. * @param body body of the loop, which may not be {@code null}. * It controls the loop parameters and result type in the standard case (see above for details). * It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values), * and may accept any number of additional types. * See above for other constraints. * * @return a method handle embodying the iteration loop functionality. * @throws NullPointerException if the {@code body} handle is {@code null}. * @throws IllegalArgumentException if any argument violates the above requirements. * * @since 9 */ public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) { Class<?> iterableType = iteratedLoopChecks(iterator, init, body); Class<?> returnType = body.type().returnType(); MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred); MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext); MethodHandle startIter; MethodHandle nextVal; { MethodType iteratorType; if (iterator == null) { // derive argument type from body, if available, else use Iterable startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator); iteratorType = startIter.type().changeParameterType(0, iterableType); } else { // force return type to the internal iterator class iteratorType = iterator.type().changeReturnType(Iterator.class); startIter = iterator; } Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1); MethodType nextValType = nextRaw.type().changeReturnType(ttype); // perform the asType transforms under an exception transformer, as per spec.: try { startIter = startIter.asType(iteratorType); nextVal = nextRaw.asType(nextValType); } catch (WrongMethodTypeException ex) { throw new IllegalArgumentException(ex); } } MethodHandle retv = null, step = body; if (returnType != void.class) { // the simple thing first: in (I V A...), drop the I to get V retv = dropArguments(identity(returnType), 0, Iterator.class); // body type signature (V T A...), internal loop types (I V A...) step = swapArguments(body, 0, 1); // swap V <-> T } MethodHandle[] iterVar = { startIter, null, hasNext, retv }, bodyClause = { init, filterArgument(step, 0, nextVal) }; return loop(iterVar, bodyClause); } private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) { Objects.requireNonNull(body); MethodType bodyType = body.type(); Class<?> returnType = bodyType.returnType(); List<Class<?>> internalParamList = bodyType.parameterList(); // strip leading V value if present int vsize = (returnType == void.class ? 0 : 1); if (vsize != 0 && (internalParamList.size() == 0 || internalParamList.get(0) != returnType)) { // argument list has no "V" => error MethodType expected = bodyType.insertParameterTypes(0, returnType); throw misMatchedTypes("body function", bodyType, expected); } else if (internalParamList.size() <= vsize) { // missing T type => error MethodType expected = bodyType.insertParameterTypes(vsize, Object.class); throw misMatchedTypes("body function", bodyType, expected); } List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size()); Class<?> iterableType = null; if (iterator != null) { // special case; if the body handle only declares V and T then // the external parameter list is obtained from iterator handle if (externalParamList.isEmpty()) { externalParamList = iterator.type().parameterList(); } MethodType itype = iterator.type(); if (!Iterator.class.isAssignableFrom(itype.returnType())) { throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type"); } if (!itype.effectivelyIdenticalParameters(0, externalParamList)) { MethodType expected = methodType(itype.returnType(), externalParamList); throw misMatchedTypes("iterator parameters", itype, expected); } } else { if (externalParamList.isEmpty()) { // special case; if the iterator handle is null and the body handle // only declares V and T then the external parameter list consists // of Iterable externalParamList = Arrays.asList(Iterable.class); iterableType = Iterable.class; } else { // special case; if the iterator handle is null and the external // parameter list is not empty then the first parameter must be // assignable to Iterable iterableType = externalParamList.get(0); if (!Iterable.class.isAssignableFrom(iterableType)) { throw newIllegalArgumentException( "inferred first loop argument must inherit from Iterable: " + iterableType); } } } if (init != null) { MethodType initType = init.type(); if (initType.returnType() != returnType || !initType.effectivelyIdenticalParameters(0, externalParamList)) { throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList)); } } return iterableType; // help the caller a bit } /*non-public*/ static MethodHandle swapArguments(MethodHandle mh, int i, int j) { // there should be a better way to uncross my wires int arity = mh.type().parameterCount(); int[] order = new int[arity]; for (int k = 0; k < arity; k++) order[k] = k; order[i] = j; order[j] = i; Class<?>[] types = mh.type().parameterArray(); Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti; MethodType swapType = methodType(mh.type().returnType(), types); return permuteArguments(mh, swapType, order); } /** * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block. * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The * exception will be rethrown, unless {@code cleanup} handle throws an exception first. The * value returned from the {@code cleanup} handle's execution will be the result of the execution of the * {@code try-finally} handle. * <p> * The {@code cleanup} handle will be passed one or two additional leading arguments. * The first is the exception thrown during the * execution of the {@code target} handle, or {@code null} if no exception was thrown. * The second is the result of the execution of the {@code target} handle, or, if it throws an exception, * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder. * The second argument is not present if the {@code target} handle has a {@code void} return type. * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.) * <p> * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or * two extra leading parameters:<ul> * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry * the result from the execution of the {@code target} handle. * This parameter is not present if the {@code target} returns {@code void}. * </ul> * <p> * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by * the cleanup. * <blockquote><pre>{@code * V target(A..., B...); * V cleanup(Throwable, V, A...); * V adapter(A... a, B... b) { * V result = (zero value for V); * Throwable throwable = null; * try { * result = target(a..., b...); * } catch (Throwable t) { * throwable = t; * throw t; * } finally { * result = cleanup(throwable, result, a...); * } * return result; * } * }</pre></blockquote> * <p> * Note that the saved arguments ({@code a...} in the pseudocode) cannot * be modified by execution of the target, and so are passed unchanged * from the caller to the cleanup, if it is invoked. * <p> * The target and cleanup must return the same type, even if the cleanup * always throws. * To create such a throwing cleanup, compose the cleanup logic * with {@link #throwException throwException}, * in order to create a method handle of the correct return type. * <p> * Note that {@code tryFinally} never converts exceptions into normal returns. * In rare cases where exceptions must be converted in that way, first wrap * the target with {@link #catchException(MethodHandle, Class, MethodHandle)} * to capture an outgoing exception, and then wrap with {@code tryFinally}. * <p> * It is recommended that the first parameter type of {@code cleanup} be * declared {@code Throwable} rather than a narrower subtype. This ensures * {@code cleanup} will always be invoked with whatever exception that * {@code target} throws. Declaring a narrower type may result in a * {@code ClassCastException} being thrown by the {@code try-finally} * handle if the type of the exception thrown by {@code target} is not * assignable to the first parameter type of {@code cleanup}. Note that * various exception types of {@code VirtualMachineError}, * {@code LinkageError}, and {@code RuntimeException} can in principle be * thrown by almost any kind of Java code, and a finally clause that * catches (say) only {@code IOException} would mask any of the others * behind a {@code ClassCastException}. * * @param target the handle whose execution is to be wrapped in a {@code try} block. * @param cleanup the handle that is invoked in the finally block. * * @return a method handle embodying the {@code try-finally} block composed of the two arguments. * @throws NullPointerException if any argument is null * @throws IllegalArgumentException if {@code cleanup} does not accept * the required leading arguments, or if the method handle types do * not match in their return types and their * corresponding trailing parameters * * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle) * @since 9 */ public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) { List<Class<?>> targetParamTypes = target.type().parameterList(); Class<?> rtype = target.type().returnType(); tryFinallyChecks(target, cleanup); // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments. // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the // target parameter list. cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0); // Ensure that the intrinsic type checks the instance thrown by the // target against the first parameter of cleanup cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class)); // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case. return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes); } private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) { Class<?> rtype = target.type().returnType(); if (rtype != cleanup.type().returnType()) { throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype); } MethodType cleanupType = cleanup.type(); if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) { throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class); } if (rtype != void.class && cleanupType.parameterType(1) != rtype) { throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype); } // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the // target parameter list. int cleanupArgIndex = rtype == void.class ? 1 : 2; if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) { throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix", cleanup.type(), target.type()); } } }