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.HotSpotIntrinsicCandidate; import java.lang.constant.ClassDesc; import java.lang.constant.Constable; import java.lang.constant.ConstantDesc; import java.lang.constant.DirectMethodHandleDesc; import java.lang.constant.MethodHandleDesc; import java.lang.constant.MethodTypeDesc; import java.util.Arrays; import java.util.Objects; import java.util.Optional; import static java.lang.invoke.MethodHandleInfo.*; import static java.lang.invoke.MethodHandleStatics.*; import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP; /** * A method handle is a typed, directly executable reference to an underlying method, * constructor, field, or similar low-level operation, with optional * transformations of arguments or return values. * These transformations are quite general, and include such patterns as * {@linkplain #asType conversion}, * {@linkplain #bindTo insertion}, * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion}, * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}. * * <h2>Method handle contents</h2> * Method handles are dynamically and strongly typed according to their parameter and return types. * They are not distinguished by the name or the defining class of their underlying methods. * A method handle must be invoked using a symbolic type descriptor which matches * the method handle's own {@linkplain #type() type descriptor}. * <p> * Every method handle reports its type descriptor via the {@link #type() type} accessor. * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object, * whose structure is a series of classes, one of which is * the return type of the method (or {@code void.class} if none). * <p> * A method handle's type controls the types of invocations it accepts, * and the kinds of transformations that apply to it. * <p> * A method handle contains a pair of special invoker methods * called {@link #invokeExact invokeExact} and {@link #invoke invoke}. * Both invoker methods provide direct access to the method handle's * underlying method, constructor, field, or other operation, * as modified by transformations of arguments and return values. * Both invokers accept calls which exactly match the method handle's own type. * The plain, inexact invoker also accepts a range of other call types. * <p> * Method handles are immutable and have no visible state. * Of course, they can be bound to underlying methods or data which exhibit state. * With respect to the Java Memory Model, any method handle will behave * as if all of its (internal) fields are final variables. This means that any method * handle made visible to the application will always be fully formed. * This is true even if the method handle is published through a shared * variable in a data race. * <p> * Method handles cannot be subclassed by the user. * Implementations may (or may not) create internal subclasses of {@code MethodHandle} * which may be visible via the {@link java.lang.Object#getClass Object.getClass} * operation. The programmer should not draw conclusions about a method handle * from its specific class, as the method handle class hierarchy (if any) * may change from time to time or across implementations from different vendors. * * <h2>Method handle compilation</h2> * A Java method call expression naming {@code invokeExact} or {@code invoke} * can invoke a method handle from Java source code. * From the viewpoint of source code, these methods can take any arguments * and their result can be cast to any return type. * Formally this is accomplished by giving the invoker methods * {@code Object} return types and variable arity {@code Object} arguments, * but they have an additional quality called <em>signature polymorphism</em> * which connects this freedom of invocation directly to the JVM execution stack. * <p> * As is usual with virtual methods, source-level calls to {@code invokeExact} * and {@code invoke} compile to an {@code invokevirtual} instruction. * More unusually, the compiler must record the actual argument types, * and may not perform method invocation conversions on the arguments. * Instead, it must generate instructions that push them on the stack according * to their own unconverted types. The method handle object itself is pushed on * the stack before the arguments. * The compiler then generates an {@code invokevirtual} instruction that invokes * the method handle with a symbolic type descriptor which describes the argument * and return types. * <p> * To issue a complete symbolic type descriptor, the compiler must also determine * the return type. This is based on a cast on the method invocation expression, * if there is one, or else {@code Object} if the invocation is an expression, * or else {@code void} if the invocation is a statement. * The cast may be to a primitive type (but not {@code void}). * <p> * As a corner case, an uncasted {@code null} argument is given * a symbolic type descriptor of {@code java.lang.Void}. * The ambiguity with the type {@code Void} is harmless, since there are no references of type * {@code Void} except the null reference. * * <h2>Method handle invocation</h2> * The first time an {@code invokevirtual} instruction is executed * it is linked by symbolically resolving the names in the instruction * and verifying that the method call is statically legal. * This also holds for calls to {@code invokeExact} and {@code invoke}. * In this case, the symbolic type descriptor emitted by the compiler is checked for * correct syntax, and names it contains are resolved. * Thus, an {@code invokevirtual} instruction which invokes * a method handle will always link, as long * as the symbolic type descriptor is syntactically well-formed * and the types exist. * <p> * When the {@code invokevirtual} is executed after linking, * the receiving method handle's type is first checked by the JVM * to ensure that it matches the symbolic type descriptor. * If the type match fails, it means that the method which the * caller is invoking is not present on the individual * method handle being invoked. * <p> * In the case of {@code invokeExact}, the type descriptor of the invocation * (after resolving symbolic type names) must exactly match the method type * of the receiving method handle. * In the case of plain, inexact {@code invoke}, the resolved type descriptor * must be a valid argument to the receiver's {@link #asType asType} method. * Thus, plain {@code invoke} is more permissive than {@code invokeExact}. * <p> * After type matching, a call to {@code invokeExact} directly * and immediately invoke the method handle's underlying method * (or other behavior, as the case may be). * <p> * A call to plain {@code invoke} works the same as a call to * {@code invokeExact}, if the symbolic type descriptor specified by the caller * exactly matches the method handle's own type. * If there is a type mismatch, {@code invoke} attempts * to adjust the type of the receiving method handle, * as if by a call to {@link #asType asType}, * to obtain an exactly invokable method handle {@code M2}. * This allows a more powerful negotiation of method type * between caller and callee. * <p> * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable, * and implementations are therefore not required to materialize it.) * * <h2>Invocation checking</h2> * In typical programs, method handle type matching will usually succeed. * But if a match fails, the JVM will throw a {@link WrongMethodTypeException}, * either directly (in the case of {@code invokeExact}) or indirectly as if * by a failed call to {@code asType} (in the case of {@code invoke}). * <p> * Thus, a method type mismatch which might show up as a linkage error * in a statically typed program can show up as * a dynamic {@code WrongMethodTypeException} * in a program which uses method handles. * <p> * Because method types contain "live" {@code Class} objects, * method type matching takes into account both type names and class loaders. * Thus, even if a method handle {@code M} is created in one * class loader {@code L1} and used in another {@code L2}, * method handle calls are type-safe, because the caller's symbolic type * descriptor, as resolved in {@code L2}, * is matched against the original callee method's symbolic type descriptor, * as resolved in {@code L1}. * The resolution in {@code L1} happens when {@code M} is created * and its type is assigned, while the resolution in {@code L2} happens * when the {@code invokevirtual} instruction is linked. * <p> * Apart from type descriptor checks, * a method handle's capability to call its underlying method is unrestricted. * If a method handle is formed on a non-public method by a class * that has access to that method, the resulting handle can be used * in any place by any caller who receives a reference to it. * <p> * Unlike with the Core Reflection API, where access is checked every time * a reflective method is invoked, * method handle access checking is performed * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>. * In the case of {@code ldc} (see below), access checking is performed as part of linking * the constant pool entry underlying the constant method handle. * <p> * Thus, handles to non-public methods, or to methods in non-public classes, * should generally be kept secret. * They should not be passed to untrusted code unless their use from * the untrusted code would be harmless. * * <h2>Method handle creation</h2> * Java code can create a method handle that directly accesses * any method, constructor, or field that is accessible to that code. * This is done via a reflective, capability-based API called * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}. * For example, a static method handle can be obtained * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}. * There are also conversion methods from Core Reflection API objects, * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. * <p> * Like classes and strings, method handles that correspond to accessible * fields, methods, and constructors can also be represented directly * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes. * A new type of constant pool entry, {@code CONSTANT_MethodHandle}, * refers directly to an associated {@code CONSTANT_Methodref}, * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref} * constant pool entry. * (For full details on method handle constants, * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.) * <p> * Method handles produced by lookups or constant loads from methods or * constructors with the variable arity modifier bit ({@code 0x0080}) * have a corresponding variable arity, as if they were defined with * the help of {@link #asVarargsCollector asVarargsCollector} * or {@link #withVarargs withVarargs}. * <p> * A method reference may refer either to a static or non-static method. * In the non-static case, the method handle type includes an explicit * receiver argument, prepended before any other arguments. * In the method handle's type, the initial receiver argument is typed * according to the class under which the method was initially requested. * (E.g., if a non-static method handle is obtained via {@code ldc}, * the type of the receiver is the class named in the constant pool entry.) * <p> * Method handle constants are subject to the same link-time access checks * their corresponding bytecode instructions, and the {@code ldc} instruction * will throw corresponding linkage errors if the bytecode behaviors would * throw such errors. * <p> * As a corollary of this, access to protected members is restricted * to receivers only of the accessing class, or one of its subclasses, * and the accessing class must in turn be a subclass (or package sibling) * of the protected member's defining class. * If a method reference refers to a protected non-static method or field * of a class outside the current package, the receiver argument will * be narrowed to the type of the accessing class. * <p> * When a method handle to a virtual method is invoked, the method is * always looked up in the receiver (that is, the first argument). * <p> * A non-virtual method handle to a specific virtual method implementation * can also be created. These do not perform virtual lookup based on * receiver type. Such a method handle simulates the effect of * an {@code invokespecial} instruction to the same method. * A non-virtual method handle can also be created to simulate the effect * of an {@code invokevirtual} or {@code invokeinterface} instruction on * a private method (as applicable). * * <h2>Usage examples</h2> * Here are some examples of usage: * <blockquote><pre>{@code Object x, y; String s; int i; MethodType mt; MethodHandle mh; MethodHandles.Lookup lookup = MethodHandles.lookup(); // mt is (char,char)String mt = MethodType.methodType(String.class, char.class, char.class); mh = lookup.findVirtual(String.class, "replace", mt); s = (String) mh.invokeExact("daddy",'d','n'); // invokeExact(Ljava/lang/String;CC)Ljava/lang/String; assertEquals(s, "nanny"); // weakly typed invocation (using MHs.invoke) s = (String) mh.invokeWithArguments("sappy", 'p', 'v'); assertEquals(s, "savvy"); // mt is (Object[])List mt = MethodType.methodType(java.util.List.class, Object[].class); mh = lookup.findStatic(java.util.Arrays.class, "asList", mt); assert(mh.isVarargsCollector()); x = mh.invoke("one", "two"); // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object; assertEquals(x, java.util.Arrays.asList("one","two")); // mt is (Object,Object,Object)Object mt = MethodType.genericMethodType(3); mh = mh.asType(mt); x = mh.invokeExact((Object)1, (Object)2, (Object)3); // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object; assertEquals(x, java.util.Arrays.asList(1,2,3)); // mt is ()int mt = MethodType.methodType(int.class); mh = lookup.findVirtual(java.util.List.class, "size", mt); i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3)); // invokeExact(Ljava/util/List;)I assert(i == 3); mt = MethodType.methodType(void.class, String.class); mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt); mh.invokeExact(System.out, "Hello, world."); // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V * }</pre></blockquote> * Each of the above calls to {@code invokeExact} or plain {@code invoke} * generates a single invokevirtual instruction with * the symbolic type descriptor indicated in the following comment. * In these examples, the helper method {@code assertEquals} is assumed to * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals} * on its arguments, and asserts that the result is true. * * <h2>Exceptions</h2> * The methods {@code invokeExact} and {@code invoke} are declared * to throw {@link java.lang.Throwable Throwable}, * which is to say that there is no static restriction on what a method handle * can throw. Since the JVM does not distinguish between checked * and unchecked exceptions (other than by their class, of course), * there is no particular effect on bytecode shape from ascribing * checked exceptions to method handle invocations. But in Java source * code, methods which perform method handle calls must either explicitly * throw {@code Throwable}, or else must catch all * throwables locally, rethrowing only those which are legal in the context, * and wrapping ones which are illegal. * * <h2><a id="sigpoly"></a>Signature polymorphism</h2> * The unusual compilation and linkage behavior of * {@code invokeExact} and plain {@code invoke} * is referenced by the term <em>signature polymorphism</em>. * As defined in the Java Language Specification, * a signature polymorphic method is one which can operate with * any of a wide range of call signatures and return types. * <p> * In source code, a call to a signature polymorphic method will * compile, regardless of the requested symbolic type descriptor. * As usual, the Java compiler emits an {@code invokevirtual} * instruction with the given symbolic type descriptor against the named method. * The unusual part is that the symbolic type descriptor is derived from * the actual argument and return types, not from the method declaration. * <p> * When the JVM processes bytecode containing signature polymorphic calls, * it will successfully link any such call, regardless of its symbolic type descriptor. * (In order to retain type safety, the JVM will guard such calls with suitable * dynamic type checks, as described elsewhere.) * <p> * Bytecode generators, including the compiler back end, are required to emit * untransformed symbolic type descriptors for these methods. * Tools which determine symbolic linkage are required to accept such * untransformed descriptors, without reporting linkage errors. * * <h2>Interoperation between method handles and the Core Reflection API</h2> * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API, * any class member represented by a Core Reflection API object * can be converted to a behaviorally equivalent method handle. * For example, a reflective {@link java.lang.reflect.Method Method} can * be converted to a method handle using * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. * The resulting method handles generally provide more direct and efficient * access to the underlying class members. * <p> * As a special case, * when the Core Reflection API is used to view the signature polymorphic * methods {@code invokeExact} or plain {@code invoke} in this class, * they appear as ordinary non-polymorphic methods. * Their reflective appearance, as viewed by * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod}, * is unaffected by their special status in this API. * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers} * will report exactly those modifier bits required for any similarly * declared method, including in this case {@code native} and {@code varargs} bits. * <p> * As with any reflected method, these methods (when reflected) may be * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}. * However, such reflective calls do not result in method handle invocations. * Such a call, if passed the required argument * (a single one, of type {@code Object[]}), will ignore the argument and * will throw an {@code UnsupportedOperationException}. * <p> * Since {@code invokevirtual} instructions can natively * invoke method handles under any symbolic type descriptor, this reflective view conflicts * with the normal presentation of these methods via bytecodes. * Thus, these two native methods, when reflectively viewed by * {@code Class.getDeclaredMethod}, may be regarded as placeholders only. * <p> * In order to obtain an invoker method for a particular type descriptor, * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker}, * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}. * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual} * API is also able to return a method handle * to call {@code invokeExact} or plain {@code invoke}, * for any specified type descriptor . * * <h2>Interoperation between method handles and Java generics</h2> * A method handle can be obtained on a method, constructor, or field * which is declared with Java generic types. * As with the Core Reflection API, the type of the method handle * will be constructed from the erasure of the source-level type. * When a method handle is invoked, the types of its arguments * or the return value cast type may be generic types or type instances. * If this occurs, the compiler will replace those * types by their erasures when it constructs the symbolic type descriptor * for the {@code invokevirtual} instruction. * <p> * Method handles do not represent * their function-like types in terms of Java parameterized (generic) types, * because there are three mismatches between function-like types and parameterized * Java types. * <ul> * <li>Method types range over all possible arities, * from no arguments to up to the <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments. * Generics are not variadic, and so cannot represent this.</li> * <li>Method types can specify arguments of primitive types, * which Java generic types cannot range over.</li> * <li>Higher order functions over method handles (combinators) are * often generic across a wide range of function types, including * those of multiple arities. It is impossible to represent such * genericity with a Java type parameter.</li> * </ul> * * <h2><a id="maxarity"></a>Arity limits</h2> * The JVM imposes on all methods and constructors of any kind an absolute * limit of 255 stacked arguments. This limit can appear more restrictive * in certain cases: * <ul> * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots. * <li>A non-static method consumes an extra argument for the object on which the method is called. * <li>A constructor consumes an extra argument for the object which is being constructed. * <li>Since a method handle’s {@code invoke} method (or other signature-polymorphic method) is non-virtual, * it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object. * </ul> * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments. * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it. * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}. * In particular, a method handle’s type must not have an arity of the exact maximum 255. * * @see MethodType * @see MethodHandles * @author John Rose, JSR 292 EG * @since 1.7 */ public abstract class MethodHandle implements Constable { /** * Internal marker interface which distinguishes (to the Java compiler) * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>. */ @java.lang.annotation.Target({ java.lang.annotation.ElementType.METHOD }) @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME) @interface PolymorphicSignature { } private final MethodType type; /*private*/ final LambdaForm form; // form is not private so that invokers can easily fetch it /*private*/ MethodHandle asTypeCache; // asTypeCache is not private so that invokers can easily fetch it /*non-public*/ byte customizationCount; // customizationCount should be accessible from invokers /** * Reports the type of this method handle. * Every invocation of this method handle via {@code invokeExact} must exactly match this type. * @return the method handle type */ public MethodType type() { return type; } /** * Package-private constructor for the method handle implementation hierarchy. * Method handle inheritance will be contained completely within * the {@code java.lang.invoke} package. */ // @param type type (permanently assigned) of the new method handle /*non-public*/ MethodHandle(MethodType type, LambdaForm form) { this.type = Objects.requireNonNull(type); this.form = Objects.requireNonNull(form).uncustomize(); this.form.prepare(); // TO DO: Try to delay this step until just before invocation. } /** * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match. * The symbolic type descriptor at the call site of {@code invokeExact} must * exactly match this method handle's {@link #type() type}. * No conversions are allowed on arguments or return values. * <p> * When this method is observed via the Core Reflection API, * it will appear as a single native method, taking an object array and returning an object. * If this native method is invoked directly via * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, * it will throw an {@code UnsupportedOperationException}. * @param args the signature-polymorphic parameter list, statically represented using varargs * @return the signature-polymorphic result, statically represented using {@code Object} * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call */ @HotSpotIntrinsicCandidate public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable; /** * Invokes the method handle, allowing any caller type descriptor, * and optionally performing conversions on arguments and return values. * <p> * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type() type}, * the call proceeds as if by {@link #invokeExact invokeExact}. * <p> * Otherwise, the call proceeds as if this method handle were first * adjusted by calling {@link #asType asType} to adjust this method handle * to the required type, and then the call proceeds as if by * {@link #invokeExact invokeExact} on the adjusted method handle. * <p> * There is no guarantee that the {@code asType} call is actually made. * If the JVM can predict the results of making the call, it may perform * adaptations directly on the caller's arguments, * and call the target method handle according to its own exact type. * <p> * The resolved type descriptor at the call site of {@code invoke} must * be a valid argument to the receivers {@code asType} method. * In particular, the caller must specify the same argument arity * as the callee's type, * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}. * <p> * When this method is observed via the Core Reflection API, * it will appear as a single native method, taking an object array and returning an object. * If this native method is invoked directly via * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI, * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}, * it will throw an {@code UnsupportedOperationException}. * @param args the signature-polymorphic parameter list, statically represented using varargs * @return the signature-polymorphic result, statically represented using {@code Object} * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call */ @HotSpotIntrinsicCandidate public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable; /** * Private method for trusted invocation of a method handle respecting simplified signatures. * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM. * <p> * The caller signature is restricted to the following basic types: * Object, int, long, float, double, and void return. * <p> * The caller is responsible for maintaining type correctness by ensuring * that the each outgoing argument value is a member of the range of the corresponding * callee argument type. * (The caller should therefore issue appropriate casts and integer narrowing * operations on outgoing argument values.) * The caller can assume that the incoming result value is part of the range * of the callee's return type. * @param args the signature-polymorphic parameter list, statically represented using varargs * @return the signature-polymorphic result, statically represented using {@code Object} */ @HotSpotIntrinsicCandidate /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable; /** * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}. * The caller signature is restricted to basic types as with {@code invokeBasic}. * The trailing (not leading) argument must be a MemberName. * @param args the signature-polymorphic parameter list, statically represented using varargs * @return the signature-polymorphic result, statically represented using {@code Object} */ @HotSpotIntrinsicCandidate /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable; /** * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}. * The caller signature is restricted to basic types as with {@code invokeBasic}. * The trailing (not leading) argument must be a MemberName. * @param args the signature-polymorphic parameter list, statically represented using varargs * @return the signature-polymorphic result, statically represented using {@code Object} */ @HotSpotIntrinsicCandidate /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable; /** * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}. * The caller signature is restricted to basic types as with {@code invokeBasic}. * The trailing (not leading) argument must be a MemberName. * @param args the signature-polymorphic parameter list, statically represented using varargs * @return the signature-polymorphic result, statically represented using {@code Object} */ @HotSpotIntrinsicCandidate /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable; /** * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}. * The caller signature is restricted to basic types as with {@code invokeBasic}. * The trailing (not leading) argument must be a MemberName. * @param args the signature-polymorphic parameter list, statically represented using varargs * @return the signature-polymorphic result, statically represented using {@code Object} */ @HotSpotIntrinsicCandidate /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable; /** * Performs a variable arity invocation, passing the arguments in the given array * to the method handle, as if via an inexact {@link #invoke invoke} from a call site * which mentions only the type {@code Object}, and whose actual argument count is the length * of the argument array. * <p> * Specifically, execution proceeds as if by the following steps, * although the methods are not guaranteed to be called if the JVM * can predict their effects. * <ul> * <li>Determine the length of the argument array as {@code N}. * For a null reference, {@code N=0}. </li> * <li>Collect the {@code N} elements of the array as a logical * argument list, each argument statically typed as an {@code Object}. </li> * <li>Determine, as {@code M}, the parameter count of the type of this * method handle. </li> * <li>Determine the general type {@code TN} of {@code N} arguments or * {@code M} arguments, if smaller than {@code N}, as * {@code TN=MethodType.genericMethodType(Math.min(N, M))}.</li> * <li>If {@code N} is greater than {@code M}, perform the following * checks and actions to shorten the logical argument list: <ul> * <li>Check that this method handle has variable arity with a * {@linkplain MethodType#lastParameterType trailing parameter} * of some array type {@code A[]}. If not, fail with a * {@code WrongMethodTypeException}. </li> * <li>Collect the trailing elements (there are {@code N-M+1} of them) * from the logical argument list into a single array of * type {@code A[]}, using {@code asType} conversions to * convert each trailing argument to type {@code A}. </li> * <li>If any of these conversions proves impossible, fail with either * a {@code ClassCastException} if any trailing element cannot be * cast to {@code A} or a {@code NullPointerException} if any * trailing element is {@code null} and {@code A} is not a reference * type. </li> * <li>Replace the logical arguments gathered into the array of * type {@code A[]} with the array itself, thus shortening * the argument list to length {@code M}. This final argument * retains the static type {@code A[]}.</li> * <li>Adjust the type {@code TN} by changing the {@code N}th * parameter type from {@code Object} to {@code A[]}. * </ul> * <li>Force the original target method handle {@code MH0} to the * required type, as {@code MH1 = MH0.asType(TN)}. </li> * <li>Spread the argument list into {@code N} separate arguments {@code A0, ...}. </li> * <li>Invoke the type-adjusted method handle on the unpacked arguments: * MH1.invokeExact(A0, ...). </li> * <li>Take the return value as an {@code Object} reference. </li> * </ul> * <p> * If the target method handle has variable arity, and the argument list is longer * than that arity, the excess arguments, starting at the position of the trailing * array argument, will be gathered (if possible, as if by {@code asType} conversions) * into an array of the appropriate type, and invocation will proceed on the * shortened argument list. * In this way, <em>jumbo argument lists</em> which would spread into more * than 254 slots can still be processed uniformly. * <p> * Unlike the {@link #invoke(Object...) generic} invocation mode, which can * "recycle" an array argument, passing it directly to the target method, * this invocation mode <em>always</em> creates a new array parameter, even * if the original array passed to {@code invokeWithArguments} would have * been acceptable as a direct argument to the target method. * Even if the number {@code M} of actual arguments is the arity {@code N}, * and the last argument is dynamically a suitable array of type {@code A[]}, * it will still be boxed into a new one-element array, since the call * site statically types the argument as {@code Object}, not an array type. * This is not a special rule for this method, but rather a regular effect * of the {@linkplain #asVarargsCollector rules for variable-arity invocation}. * <p> * Because of the action of the {@code asType} step, the following argument * conversions are applied as necessary: * <ul> * <li>reference casting * <li>unboxing * <li>widening primitive conversions * <li>variable arity conversion * </ul> * <p> * The result returned by the call is boxed if it is a primitive, * or forced to null if the return type is void. * <p> * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke}, * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI. * It can therefore be used as a bridge between native or reflective code and method handles. * @apiNote * This call is approximately equivalent to the following code: * <blockquote><pre>{@code * // for jumbo argument lists, adapt varargs explicitly: * int N = (arguments == null? 0: arguments.length); * int M = this.type.parameterCount(); * int MAX_SAFE = 127; // 127 longs require 254 slots, which is OK * if (N > MAX_SAFE && N > M && this.isVarargsCollector()) { * Class<?> arrayType = this.type().lastParameterType(); * Class<?> elemType = arrayType.getComponentType(); * if (elemType != null) { * Object args2 = Array.newInstance(elemType, M); * MethodHandle arraySetter = MethodHandles.arrayElementSetter(arrayType); * for (int i = 0; i < M; i++) { * arraySetter.invoke(args2, i, arguments[M-1 + i]); * } * arguments = Arrays.copyOf(arguments, M); * arguments[M-1] = args2; * return this.asFixedArity().invokeWithArguments(arguments); * } * } // done with explicit varargs processing * * // Handle fixed arity and non-jumbo variable arity invocation. * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0); * Object result = invoker.invokeExact(this, arguments); * }</pre></blockquote> * * @param arguments the arguments to pass to the target * @return the result returned by the target * @throws ClassCastException if an argument cannot be converted by reference casting * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments * @throws Throwable anything thrown by the target method invocation * @see MethodHandles#spreadInvoker */ public Object invokeWithArguments(Object... arguments) throws Throwable { // Note: Jumbo argument lists are handled in the variable-arity subclass. MethodType invocationType = MethodType.genericMethodType(arguments == null ? 0 : arguments.length); return invocationType.invokers().spreadInvoker(0).invokeExact(asType(invocationType), arguments); } /** * Performs a variable arity invocation, passing the arguments in the given list * to the method handle, as if via an inexact {@link #invoke invoke} from a call site * which mentions only the type {@code Object}, and whose actual argument count is the length * of the argument list. * <p> * This method is also equivalent to the following code: * <blockquote><pre>{@code * invokeWithArguments(arguments.toArray()) * }</pre></blockquote> * <p> * Jumbo-sized lists are acceptable if this method handle has variable arity. * See {@link #invokeWithArguments(Object[])} for details. * * @param arguments the arguments to pass to the target * @return the result returned by the target * @throws NullPointerException if {@code arguments} is a null reference * @throws ClassCastException if an argument cannot be converted by reference casting * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments * @throws Throwable anything thrown by the target method invocation */ public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable { return invokeWithArguments(arguments.toArray()); } /** * Produces an adapter method handle which adapts the type of the * current method handle to a new type. * 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 {@code this}. * <p> * The new method handle, when invoked, will perform the following * steps: * <ul> * <li>Convert the incoming argument list to match the original * method handle's argument list. * <li>Invoke the original method handle on the converted argument list. * <li>Convert any result returned by the original method handle * to the return type of new method handle. * </ul> * <p> * This method provides the crucial behavioral difference between * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}. * The two methods * perform the same steps when the caller's type descriptor exactly matches * the callee's, but when the types differ, plain {@link #invoke invoke} * also calls {@code asType} (or some internal equivalent) in order * to match up the caller's and callee's types. * <p> * If the current method is a variable arity method handle * argument list conversion may involve the conversion and collection * of several arguments into an array, as * {@linkplain #asVarargsCollector described elsewhere}. * In every other case, all conversions are applied <em>pairwise</em>, * which means that each argument or return value is converted to * exactly one argument or return value (or no return value). * The applied conversions are defined by consulting * the corresponding component types of the old and new * method handle types. * <p> * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types, * or old and new return types. Specifically, for some valid index {@code i}, let * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}. * Or else, going the other way for return values, let * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}. * If the types are the same, the new method handle makes no change * to the corresponding argument or return value (if any). * Otherwise, one of the following conversions is applied * if possible: * <ul> * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied. * (The types do not need to be related in any particular way. * This is because a dynamic value of null can convert to any reference type.) * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation * conversion (JLS 5.3) is applied, if one exists. * (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.) * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference, * a Java casting conversion (JLS 5.5) is applied if one exists. * (Specifically, the value is boxed from <em>T0</em> to its wrapper class, * which is then widened as needed to <em>T1</em>.) * <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 method invocation conversion (JLS 5.3) * on the primitive value. (These are the primitive widening conversions.) * <em>T0</em> must be a wrapper class or a supertype of one. * (In the case where <em>T0</em> is Object, these are the conversions * allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.) * The unboxing conversion must have a possibility of success, which means that * if <em>T0</em> is not itself a wrapper class, there must exist at least one * wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed * primitive value can be widened to <em>T1</em>. * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced. * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive, * a zero value is introduced. * </ul> * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types, * because neither corresponds specifically to the <em>dynamic type</em> of any * actual argument or return value.) * <p> * The method handle conversion cannot be made if any one of the required * pairwise conversions cannot be made. * <p> * At runtime, the conversions applied to reference arguments * or return values may require additional runtime checks which can fail. * An unboxing operation may fail because the original reference is null, * causing a {@link java.lang.NullPointerException NullPointerException}. * An unboxing operation or a reference cast may also fail on a reference * to an object of the wrong type, * causing a {@link java.lang.ClassCastException ClassCastException}. * Although an unboxing operation may accept several kinds of wrappers, * if none are available, a {@code ClassCastException} will be thrown. * * @param newType the expected type of the new method handle * @return a method handle which delegates to {@code this} after performing * any necessary argument conversions, and arranges for any * necessary return value conversions * @throws NullPointerException if {@code newType} is a null reference * @throws WrongMethodTypeException if the conversion cannot be made * @see MethodHandles#explicitCastArguments */ public MethodHandle asType(MethodType newType) { // Fast path alternative to a heavyweight {@code asType} call. // Return 'this' if the conversion will be a no-op. if (newType == type) { return this; } // Return 'this.asTypeCache' if the conversion is already memoized. MethodHandle atc = asTypeCached(newType); if (atc != null) { return atc; } return asTypeUncached(newType); } private MethodHandle asTypeCached(MethodType newType) { MethodHandle atc = asTypeCache; if (atc != null && newType == atc.type) { return atc; } return null; } /** Override this to change asType behavior. */ /*non-public*/ MethodHandle asTypeUncached(MethodType newType) { if (!type.isConvertibleTo(newType)) throw new WrongMethodTypeException("cannot convert " + this + " to " + newType); return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true); } /** * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument * and spreads its elements as positional arguments. * The new method handle adapts, as its <i>target</i>, * the current method handle. The type of the adapter will be * the same as the type of the target, except that the final * {@code arrayLength} parameters of the target's type are replaced * by a single array parameter of type {@code arrayType}. * <p> * If the array element type differs from any of the corresponding * argument types on the original target, * the original target is adapted to take the array elements directly, * as if by a call to {@link #asType asType}. * <p> * When called, the adapter replaces a trailing array argument * by the array's elements, each as its own argument to the target. * (The order of the arguments is preserved.) * They are converted pairwise by casting and/or unboxing * to the types of the trailing parameters of the target. * Finally the target is called. * What the target eventually returns is returned unchanged by the adapter. * <p> * Before calling the target, the adapter verifies that the array * contains exactly enough elements to provide a correct argument count * to the target method handle. * (The array may also be null when zero elements are required.) * <p> * When the adapter is called, the length of the supplied {@code array} * argument is queried as if by {@code array.length} or {@code arraylength} * bytecode. If the adapter accepts a zero-length trailing array argument, * the supplied {@code array} argument can either be a zero-length array or * {@code null}; otherwise, the adapter will throw a {@code NullPointerException} * if the array is {@code null} and throw an {@link IllegalArgumentException} * if the array does not have the correct number of elements. * <p> * Here are some simple examples of array-spreading method handles: * <blockquote><pre>{@code MethodHandle equals = publicLookup() .findVirtual(String.class, "equals", methodType(boolean.class, Object.class)); assert( (boolean) equals.invokeExact("me", (Object)"me")); assert(!(boolean) equals.invokeExact("me", (Object)"thee")); // spread both arguments from a 2-array: MethodHandle eq2 = equals.asSpreader(Object[].class, 2); assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" })); assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" })); // try to spread from anything but a 2-array: for (int n = 0; n <= 10; n++) { Object[] badArityArgs = (n == 2 ? new Object[0] : new Object[n]); try { assert((boolean) eq2.invokeExact(badArityArgs) && false); } catch (IllegalArgumentException ex) { } // OK } // spread both arguments from a String array: MethodHandle eq2s = equals.asSpreader(String[].class, 2); assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" })); assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" })); // spread second arguments from a 1-array: MethodHandle eq1 = equals.asSpreader(Object[].class, 1); assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" })); assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" })); // spread no arguments from a 0-array or null: MethodHandle eq0 = equals.asSpreader(Object[].class, 0); assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0])); assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null)); // asSpreader and asCollector are approximate inverses: for (int n = 0; n <= 2; n++) { for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) { MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n); assert( (boolean) equals2.invokeWithArguments("me", "me")); assert(!(boolean) equals2.invokeWithArguments("me", "thee")); } } MethodHandle caToString = publicLookup() .findStatic(Arrays.class, "toString", methodType(String.class, char[].class)); assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray())); MethodHandle caString3 = caToString.asCollector(char[].class, 3); assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C')); MethodHandle caToString2 = caString3.asSpreader(char[].class, 2); assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray())); * }</pre></blockquote> * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments * @param arrayLength the number of arguments to spread from an incoming array argument * @return a new method handle which spreads its final array argument, * before calling the original method handle * @throws NullPointerException if {@code arrayType} is a null reference * @throws IllegalArgumentException if {@code arrayType} is not an array type, * or if target does not have at least * {@code arrayLength} parameter types, * or if {@code arrayLength} is negative, * or if the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> * @throws WrongMethodTypeException if the implied {@code asType} call fails * @see #asCollector */ public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) { return asSpreader(type().parameterCount() - arrayLength, arrayType, arrayLength); } /** * Makes an <em>array-spreading</em> method handle, which accepts an array argument at a given position and spreads * its elements as positional arguments in place of the array. The new method handle adapts, as its <i>target</i>, * the current method handle. The type of the adapter will be the same as the type of the target, except that the * {@code arrayLength} parameters of the target's type, starting at the zero-based position {@code spreadArgPos}, * are replaced by a single array parameter of type {@code arrayType}. * <p> * This method behaves very much like {@link #asSpreader(Class, int)}, but accepts an additional {@code spreadArgPos} * argument to indicate at which position in the parameter list the spreading should take place. * * @apiNote Example: * <blockquote><pre>{@code MethodHandle compare = LOOKUP.findStatic(Objects.class, "compare", methodType(int.class, Object.class, Object.class, Comparator.class)); MethodHandle compare2FromArray = compare.asSpreader(0, Object[].class, 2); Object[] ints = new Object[]{3, 9, 7, 7}; Comparator<Integer> cmp = (a, b) -> a - b; assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 0, 2), cmp) < 0); assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 1, 3), cmp) > 0); assertTrue((int) compare2FromArray.invoke(Arrays.copyOfRange(ints, 2, 4), cmp) == 0); * }</pre></blockquote> * @param spreadArgPos the position (zero-based index) in the argument list at which spreading should start. * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments * @param arrayLength the number of arguments to spread from an incoming array argument * @return a new method handle which spreads an array argument at a given position, * before calling the original method handle * @throws NullPointerException if {@code arrayType} is a null reference * @throws IllegalArgumentException if {@code arrayType} is not an array type, * or if target does not have at least * {@code arrayLength} parameter types, * or if {@code arrayLength} is negative, * or if {@code spreadArgPos} has an illegal value (negative, or together with arrayLength exceeding the * number of arguments), * or if the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> * @throws WrongMethodTypeException if the implied {@code asType} call fails * * @see #asSpreader(Class, int) * @since 9 */ public MethodHandle asSpreader(int spreadArgPos, Class<?> arrayType, int arrayLength) { MethodType postSpreadType = asSpreaderChecks(arrayType, spreadArgPos, arrayLength); MethodHandle afterSpread = this.asType(postSpreadType); BoundMethodHandle mh = afterSpread.rebind(); LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength); MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, spreadArgPos + arrayLength, arrayType); return mh.copyWith(preSpreadType, lform); } /** * See if {@code asSpreader} can be validly called with the given arguments. * Return the type of the method handle call after spreading but before conversions. */ private MethodType asSpreaderChecks(Class<?> arrayType, int pos, int arrayLength) { spreadArrayChecks(arrayType, arrayLength); int nargs = type().parameterCount(); if (nargs < arrayLength || arrayLength < 0) throw newIllegalArgumentException("bad spread array length"); if (pos < 0 || pos + arrayLength > nargs) { throw newIllegalArgumentException("bad spread position"); } Class<?> arrayElement = arrayType.getComponentType(); MethodType mtype = type(); boolean match = true, fail = false; for (int i = pos; i < pos + arrayLength; i++) { Class<?> ptype = mtype.parameterType(i); if (ptype != arrayElement) { match = false; if (!MethodType.canConvert(arrayElement, ptype)) { fail = true; break; } } } if (match) return mtype; MethodType needType = mtype.asSpreaderType(arrayType, pos, arrayLength); if (!fail) return needType; // elicit an error: this.asType(needType); throw newInternalError("should not return"); } private void spreadArrayChecks(Class<?> arrayType, int arrayLength) { Class<?> arrayElement = arrayType.getComponentType(); if (arrayElement == null) throw newIllegalArgumentException("not an array type", arrayType); if ((arrayLength & 0x7F) != arrayLength) { if ((arrayLength & 0xFF) != arrayLength) throw newIllegalArgumentException("array length is not legal", arrayLength); assert (arrayLength >= 128); if (arrayElement == long.class || arrayElement == double.class) throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength); } } /** * Adapts this method handle to be {@linkplain #asVarargsCollector variable arity} * if the boolean flag is true, else {@linkplain #asFixedArity fixed arity}. * If the method handle is already of the proper arity mode, it is returned * unchanged. * @apiNote * <p>This method is sometimes useful when adapting a method handle that * may be variable arity, to ensure that the resulting adapter is also * variable arity if and only if the original handle was. For example, * this code changes the first argument of a handle {@code mh} to {@code int} without * disturbing its variable arity property: * {@code mh.asType(mh.type().changeParameterType(0,int.class)) * .withVarargs(mh.isVarargsCollector())} * <p> * This call is approximately equivalent to the following code: * <blockquote><pre>{@code * if (makeVarargs == isVarargsCollector()) * return this; * else if (makeVarargs) * return asVarargsCollector(type().lastParameterType()); * else * return asFixedArity(); * }</pre></blockquote> * @param makeVarargs true if the return method handle should have variable arity behavior * @return a method handle of the same type, with possibly adjusted variable arity behavior * @throws IllegalArgumentException if {@code makeVarargs} is true and * this method handle does not have a trailing array parameter * @since 9 * @see #asVarargsCollector * @see #asFixedArity */ public MethodHandle withVarargs(boolean makeVarargs) { assert (!isVarargsCollector()); // subclass responsibility if (makeVarargs) { return asVarargsCollector(type().lastParameterType()); } else { return this; } } /** * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing * positional arguments and collects them into an array argument. * The new method handle adapts, as its <i>target</i>, * the current method handle. The type of the adapter will be * the same as the type of the target, except that a single trailing * parameter (usually of type {@code arrayType}) is replaced by * {@code arrayLength} parameters whose type is element type of {@code arrayType}. * <p> * If the array type differs from the final argument type on the original target, * the original target is adapted to take the array type directly, * as if by a call to {@link #asType asType}. * <p> * When called, the adapter replaces its trailing {@code arrayLength} * arguments by a single new array of type {@code arrayType}, whose elements * comprise (in order) the replaced arguments. * Finally the target is called. * What the target eventually returns is returned unchanged by the adapter. * <p> * (The array may also be a shared constant when {@code arrayLength} is zero.) * <p> * (<em>Note:</em> The {@code arrayType} is often identical to the * {@linkplain MethodType#lastParameterType last parameter type} * of the original target. * It is an explicit argument for symmetry with {@code asSpreader}, and also * to allow the target to use a simple {@code Object} as its last parameter type.) * <p> * In order to create a collecting adapter which is not restricted to a particular * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} * or {@link #withVarargs withVarargs} instead. * <p> * Here are some examples of array-collecting method handles: * <blockquote><pre>{@code MethodHandle deepToString = publicLookup() .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); assertEquals("[won]", (String) deepToString.invokeExact(new Object[]{"won"})); MethodHandle ts1 = deepToString.asCollector(Object[].class, 1); assertEquals(methodType(String.class, Object.class), ts1.type()); //assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); //FAIL assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"})); // arrayType can be a subtype of Object[] MethodHandle ts2 = deepToString.asCollector(String[].class, 2); assertEquals(methodType(String.class, String.class, String.class), ts2.type()); assertEquals("[two, too]", (String) ts2.invokeExact("two", "too")); MethodHandle ts0 = deepToString.asCollector(Object[].class, 0); assertEquals("[]", (String) ts0.invokeExact()); // collectors can be nested, Lisp-style MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2); assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D"))); // arrayType can be any primitive array type MethodHandle bytesToString = publicLookup() .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class)) .asCollector(byte[].class, 3); assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3)); MethodHandle longsToString = publicLookup() .findStatic(Arrays.class, "toString", methodType(String.class, long[].class)) .asCollector(long[].class, 1); assertEquals("[123]", (String) longsToString.invokeExact((long)123)); * }</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 arrayType often {@code Object[]}, the type of the array argument which will collect the arguments * @param arrayLength the number of arguments to collect into a new array argument * @return a new method handle which collects some trailing argument * into an array, before calling the original method handle * @throws NullPointerException if {@code arrayType} is a null reference * @throws IllegalArgumentException if {@code arrayType} is not an array type * or {@code arrayType} is not assignable to this method handle's trailing parameter type, * or {@code arrayLength} is not a legal array size, * or the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> * @throws WrongMethodTypeException if the implied {@code asType} call fails * @see #asSpreader * @see #asVarargsCollector */ public MethodHandle asCollector(Class<?> arrayType, int arrayLength) { return asCollector(type().parameterCount() - 1, arrayType, arrayLength); } /** * Makes an <em>array-collecting</em> method handle, which accepts a given number of positional arguments starting * at a given position, and collects them into an array argument. The new method handle adapts, as its * <i>target</i>, the current method handle. The type of the adapter will be the same as the type of the target, * except that the parameter at the position indicated by {@code collectArgPos} (usually of type {@code arrayType}) * is replaced by {@code arrayLength} parameters whose type is element type of {@code arrayType}. * <p> * This method behaves very much like {@link #asCollector(Class, int)}, but differs in that its {@code * collectArgPos} argument indicates at which position in the parameter list arguments should be collected. This * index is zero-based. * * @apiNote Examples: * <blockquote><pre>{@code StringWriter swr = new StringWriter(); MethodHandle swWrite = LOOKUP.findVirtual(StringWriter.class, "write", methodType(void.class, char[].class, int.class, int.class)).bindTo(swr); MethodHandle swWrite4 = swWrite.asCollector(0, char[].class, 4); swWrite4.invoke('A', 'B', 'C', 'D', 1, 2); assertEquals("BC", swr.toString()); swWrite4.invoke('P', 'Q', 'R', 'S', 0, 4); assertEquals("BCPQRS", swr.toString()); swWrite4.invoke('W', 'X', 'Y', 'Z', 3, 1); assertEquals("BCPQRSZ", swr.toString()); * }</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 collectArgPos the zero-based position in the parameter list at which to start collecting. * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments * @param arrayLength the number of arguments to collect into a new array argument * @return a new method handle which collects some arguments * into an array, before calling the original method handle * @throws NullPointerException if {@code arrayType} is a null reference * @throws IllegalArgumentException if {@code arrayType} is not an array type * or {@code arrayType} is not assignable to this method handle's array parameter type, * or {@code arrayLength} is not a legal array size, * or {@code collectArgPos} has an illegal value (negative, or greater than the number of arguments), * or the resulting method handle's type would have * <a href="MethodHandle.html#maxarity">too many parameters</a> * @throws WrongMethodTypeException if the implied {@code asType} call fails * * @see #asCollector(Class, int) * @since 9 */ public MethodHandle asCollector(int collectArgPos, Class<?> arrayType, int arrayLength) { asCollectorChecks(arrayType, collectArgPos, arrayLength); BoundMethodHandle mh = rebind(); MethodType resultType = type().asCollectorType(arrayType, collectArgPos, arrayLength); MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength); LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray); if (lform != null) { return mh.copyWith(resultType, lform); } lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType()); return mh.copyWithExtendL(resultType, lform, newArray); } /** * See if {@code asCollector} can be validly called with the given arguments. * Return false if the last parameter is not an exact match to arrayType. */ /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int pos, int arrayLength) { spreadArrayChecks(arrayType, arrayLength); int nargs = type().parameterCount(); if (pos < 0 || pos >= nargs) { throw newIllegalArgumentException("bad collect position"); } if (nargs != 0) { Class<?> param = type().parameterType(pos); if (param == arrayType) return true; if (param.isAssignableFrom(arrayType)) return false; } throw newIllegalArgumentException("array type not assignable to argument", this, arrayType); } /** * Makes a <em>variable arity</em> adapter which is able to accept * any number of trailing positional arguments and collect them * into an array argument. * <p> * The type and behavior of the adapter will be the same as * the type and behavior of the target, except that certain * {@code invoke} and {@code asType} requests can lead to * trailing positional arguments being collected into target's * trailing parameter. * Also, the * {@linkplain MethodType#lastParameterType last parameter type} * of the adapter will be * {@code arrayType}, even if the target has a different * last parameter type. * <p> * This transformation may return {@code this} if the method handle is * already of variable arity and its trailing parameter type * is identical to {@code arrayType}. * <p> * When called with {@link #invokeExact invokeExact}, the adapter invokes * the target with no argument changes. * (<em>Note:</em> This behavior is different from a * {@linkplain #asCollector fixed arity collector}, * since it accepts a whole array of indeterminate length, * rather than a fixed number of arguments.) * <p> * When called with plain, inexact {@link #invoke invoke}, if the caller * type is the same as the adapter, the adapter invokes the target as with * {@code invokeExact}. * (This is the normal behavior for {@code invoke} when types match.) * <p> * Otherwise, if the caller and adapter arity are the same, and the * trailing parameter type of the caller is a reference type identical to * or assignable to the trailing parameter type of the adapter, * the arguments and return values are converted pairwise, * as if by {@link #asType asType} on a fixed arity * method handle. * <p> * Otherwise, the arities differ, or the adapter's trailing parameter * type is not assignable from the corresponding caller type. * In this case, the adapter replaces all trailing arguments from * the original trailing argument position onward, by * a new array of type {@code arrayType}, whose elements * comprise (in order) the replaced arguments. * <p> * The caller type must provides as least enough arguments, * and of the correct type, to satisfy the target's requirement for * positional arguments before the trailing array argument. * Thus, the caller must supply, at a minimum, {@code N-1} arguments, * where {@code N} is the arity of the target. * Also, there must exist conversions from the incoming arguments * to the target's arguments. * As with other uses of plain {@code invoke}, if these basic * requirements are not fulfilled, a {@code WrongMethodTypeException} * may be thrown. * <p> * In all cases, what the target eventually returns is returned unchanged by the adapter. * <p> * In the final case, it is exactly as if the target method handle were * temporarily adapted with a {@linkplain #asCollector fixed arity collector} * to the arity required by the caller type. * (As with {@code asCollector}, if the array length is zero, * a shared constant may be used instead of a new array. * If the implied call to {@code asCollector} would throw * an {@code IllegalArgumentException} or {@code WrongMethodTypeException}, * the call to the variable arity adapter must throw * {@code WrongMethodTypeException}.) * <p> * The behavior of {@link #asType asType} is also specialized for * variable arity adapters, to maintain the invariant that * plain, inexact {@code invoke} is always equivalent to an {@code asType} * call to adjust the target type, followed by {@code invokeExact}. * Therefore, a variable arity adapter responds * to an {@code asType} request by building a fixed arity collector, * if and only if the adapter and requested type differ either * in arity or trailing argument type. * The resulting fixed arity collector has its type further adjusted * (if necessary) to the requested type by pairwise conversion, * as if by another application of {@code asType}. * <p> * When a method handle is obtained by executing an {@code ldc} instruction * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked * as a variable arity method (with the modifier bit {@code 0x0080}), * the method handle will accept multiple arities, as if the method handle * constant were created by means of a call to {@code asVarargsCollector}. * <p> * In order to create a collecting adapter which collects a predetermined * number of arguments, and whose type reflects this predetermined number, * use {@link #asCollector asCollector} instead. * <p> * No method handle transformations produce new method handles with * variable arity, unless they are documented as doing so. * Therefore, besides {@code asVarargsCollector} and {@code withVarargs}, * all methods in {@code MethodHandle} and {@code MethodHandles} * will return a method handle with fixed arity, * except in the cases where they are specified to return their original * operand (e.g., {@code asType} of the method handle's own type). * <p> * Calling {@code asVarargsCollector} on a method handle which is already * of variable arity will produce a method handle with the same type and behavior. * It may (or may not) return the original variable arity method handle. * <p> * Here is an example, of a list-making variable arity method handle: * <blockquote><pre>{@code MethodHandle deepToString = publicLookup() .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class)); MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class); assertEquals("[won]", (String) ts1.invokeExact( new Object[]{"won"})); assertEquals("[won]", (String) ts1.invoke( new Object[]{"won"})); assertEquals("[won]", (String) ts1.invoke( "won" )); assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"})); // findStatic of Arrays.asList(...) produces a variable arity method handle: MethodHandle asList = publicLookup() .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)); assertEquals(methodType(List.class, Object[].class), asList.type()); assert(asList.isVarargsCollector()); assertEquals("[]", asList.invoke().toString()); assertEquals("[1]", asList.invoke(1).toString()); assertEquals("[two, too]", asList.invoke("two", "too").toString()); String[] argv = { "three", "thee", "tee" }; assertEquals("[three, thee, tee]", asList.invoke(argv).toString()); assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString()); List ls = (List) asList.invoke((Object)argv); assertEquals(1, ls.size()); assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0))); * }</pre></blockquote> * <p style="font-size:smaller;"> * <em>Discussion:</em> * These rules are designed as a dynamically-typed variation * of the Java rules for variable arity methods. * In both cases, callers to a variable arity method or method handle * can either pass zero or more positional arguments, or else pass * pre-collected arrays of any length. Users should be aware of the * special role of the final argument, and of the effect of a * type match on that final argument, which determines whether * or not a single trailing argument is interpreted as a whole * array or a single element of an array to be collected. * Note that the dynamic type of the trailing argument has no * effect on this decision, only a comparison between the symbolic * type descriptor of the call site and the type descriptor of the method handle.) * * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments * @return a new method handle which can collect any number of trailing arguments * into an array, before calling the original method handle * @throws NullPointerException if {@code arrayType} is a null reference * @throws IllegalArgumentException if {@code arrayType} is not an array type * or {@code arrayType} is not assignable to this method handle's trailing parameter type * @see #asCollector * @see #isVarargsCollector * @see #withVarargs * @see #asFixedArity */ public MethodHandle asVarargsCollector(Class<?> arrayType) { Objects.requireNonNull(arrayType); boolean lastMatch = asCollectorChecks(arrayType, type().parameterCount() - 1, 0); if (isVarargsCollector() && lastMatch) return this; return MethodHandleImpl.makeVarargsCollector(this, arrayType); } /** * Determines if this method handle * supports {@linkplain #asVarargsCollector variable arity} calls. * Such method handles arise from the following sources: * <ul> * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector} * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method} * which resolves to a variable arity Java method or constructor * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle} * which resolves to a variable arity Java method or constructor * </ul> * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls * @see #asVarargsCollector * @see #asFixedArity */ public boolean isVarargsCollector() { return false; } /** * Makes a <em>fixed arity</em> method handle which is otherwise * equivalent to the current method handle. * <p> * If the current method handle is not of * {@linkplain #asVarargsCollector variable arity}, * the current method handle is returned. * This is true even if the current method handle * could not be a valid input to {@code asVarargsCollector}. * <p> * Otherwise, the resulting fixed-arity method handle has the same * type and behavior of the current method handle, * except that {@link #isVarargsCollector isVarargsCollector} * will be false. * The fixed-arity method handle may (or may not) be the * a previous argument to {@code asVarargsCollector}. * <p> * Here is an example, of a list-making variable arity method handle: * <blockquote><pre>{@code MethodHandle asListVar = publicLookup() .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class)) .asVarargsCollector(Object[].class); MethodHandle asListFix = asListVar.asFixedArity(); assertEquals("[1]", asListVar.invoke(1).toString()); Exception caught = null; try { asListFix.invoke((Object)1); } catch (Exception ex) { caught = ex; } assert(caught instanceof ClassCastException); assertEquals("[two, too]", asListVar.invoke("two", "too").toString()); try { asListFix.invoke("two", "too"); } catch (Exception ex) { caught = ex; } assert(caught instanceof WrongMethodTypeException); Object[] argv = { "three", "thee", "tee" }; assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString()); assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString()); assertEquals(1, ((List) asListVar.invoke((Object)argv)).size()); assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString()); * }</pre></blockquote> * * @return a new method handle which accepts only a fixed number of arguments * @see #asVarargsCollector * @see #isVarargsCollector * @see #withVarargs */ public MethodHandle asFixedArity() { assert (!isVarargsCollector()); return this; } /** * Binds a value {@code x} to the first argument of a method handle, without invoking it. * The new method handle adapts, as its <i>target</i>, * the current method handle by binding it to the given argument. * The type of the bound handle will be * the same as the type of the target, except that a single leading * reference parameter will be omitted. * <p> * When called, the bound handle inserts the given value {@code x} * as a new leading argument to the target. The other arguments are * also passed unchanged. * What the target eventually returns is returned unchanged by the bound handle. * <p> * The reference {@code x} must be convertible to the first parameter * type of the target. * <p> * <em>Note:</em> Because method handles are immutable, the target method handle * retains its original type and behavior. * <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 x the value to bind to the first argument of the target * @return a new method handle which prepends the given value to the incoming * argument list, before calling the original method handle * @throws IllegalArgumentException if the target does not have a * leading parameter type that is a reference type * @throws ClassCastException if {@code x} cannot be converted * to the leading parameter type of the target * @see MethodHandles#insertArguments */ public MethodHandle bindTo(Object x) { x = type.leadingReferenceParameter().cast(x); // throw CCE if needed return bindArgumentL(0, x); } /** * Return a nominal descriptor for this instance, if one can be * constructed, or an empty {@link Optional} if one cannot be. * * @return An {@link Optional} containing the resulting nominal descriptor, * or an empty {@link Optional} if one cannot be constructed. * @since 12 */ @Override public Optional<MethodHandleDesc> describeConstable() { MethodHandleInfo info; ClassDesc owner; String name; MethodTypeDesc type; boolean isInterface; try { info = IMPL_LOOKUP.revealDirect(this); isInterface = info.getDeclaringClass().isInterface(); owner = info.getDeclaringClass().describeConstable().orElseThrow(); type = info.getMethodType().describeConstable().orElseThrow(); name = info.getName(); } catch (Exception e) { return Optional.empty(); } switch (info.getReferenceKind()) { case REF_getField: return Optional.of( MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.GETTER, owner, name, type.returnType())); case REF_putField: return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.SETTER, owner, name, type.parameterType(0))); case REF_getStatic: return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.STATIC_GETTER, owner, name, type.returnType())); case REF_putStatic: return Optional.of(MethodHandleDesc.ofField(DirectMethodHandleDesc.Kind.STATIC_SETTER, owner, name, type.parameterType(0))); case REF_invokeVirtual: return Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.VIRTUAL, owner, name, type)); case REF_invokeStatic: return isInterface ? Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_STATIC, owner, name, type)) : Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.STATIC, owner, name, type)); case REF_invokeSpecial: return isInterface ? Optional.of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_SPECIAL, owner, name, type)) : Optional .of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.SPECIAL, owner, name, type)); case REF_invokeInterface: return Optional.of( MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.INTERFACE_VIRTUAL, owner, name, type)); case REF_newInvokeSpecial: return Optional .of(MethodHandleDesc.ofMethod(DirectMethodHandleDesc.Kind.CONSTRUCTOR, owner, name, type)); default: return Optional.empty(); } } /** * Returns a string representation of the method handle, * starting with the string {@code "MethodHandle"} and * ending with the string representation of the method handle's type. * In other words, this method returns a string equal to the value of: * <blockquote><pre>{@code * "MethodHandle" + type().toString() * }</pre></blockquote> * <p> * (<em>Note:</em> Future releases of this API may add further information * to the string representation. * Therefore, the present syntax should not be parsed by applications.) * * @return a string representation of the method handle */ @Override public String toString() { if (DEBUG_METHOD_HANDLE_NAMES) return "MethodHandle" + debugString(); return standardString(); } String standardString() { return "MethodHandle" + type; } /** Return a string with a several lines describing the method handle structure. * This string would be suitable for display in an IDE debugger. */ String debugString() { return type + " : " + internalForm() + internalProperties(); } //// Implementation methods. //// Sub-classes can override these default implementations. //// All these methods assume arguments are already validated. // Other transforms to do: convert, explicitCast, permute, drop, filter, fold, GWT, catch BoundMethodHandle bindArgumentL(int pos, Object value) { return rebind().bindArgumentL(pos, value); } /*non-public*/ MethodHandle setVarargs(MemberName member) throws IllegalAccessException { if (!member.isVarargs()) return this; try { return this.withVarargs(true); } catch (IllegalArgumentException ex) { throw member.makeAccessException("cannot make variable arity", null); } } /*non-public*/ MethodHandle viewAsType(MethodType newType, boolean strict) { // No actual conversions, just a new view of the same method. // Note that this operation must not produce a DirectMethodHandle, // because retyped DMHs, like any transformed MHs, // cannot be cracked into MethodHandleInfo. assert viewAsTypeChecks(newType, strict); BoundMethodHandle mh = rebind(); return mh.copyWith(newType, mh.form); } /*non-public*/ boolean viewAsTypeChecks(MethodType newType, boolean strict) { if (strict) { assert (type().isViewableAs(newType, true)) : Arrays.asList(this, newType); } else { assert (type().basicType().isViewableAs(newType.basicType(), true)) : Arrays.asList(this, newType); } return true; } // Decoding /*non-public*/ LambdaForm internalForm() { return form; } /*non-public*/ MemberName internalMemberName() { return null; // DMH returns DMH.member } /*non-public*/ Class<?> internalCallerClass() { return null; // caller-bound MH for @CallerSensitive method returns caller } /*non-public*/ MethodHandleImpl.Intrinsic intrinsicName() { // no special intrinsic meaning to most MHs return MethodHandleImpl.Intrinsic.NONE; } /*non-public*/ MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) { if (member != null) { return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial); } else if (internalMemberName() == null) { // The required internaMemberName is null, and this MH (like most) doesn't have one. return this; } else { // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH. MethodHandle result = rebind(); assert (result.internalMemberName() == null); return result; } } /*non-public*/ boolean isInvokeSpecial() { return false; // DMH.Special returns true } /*non-public*/ Object internalValues() { return null; } /*non-public*/ Object internalProperties() { // Override to something to follow this.form, like "\n& FOO=bar" return ""; } //// Method handle implementation methods. //// Sub-classes can override these default implementations. //// All these methods assume arguments are already validated. /*non-public*/ abstract MethodHandle copyWith(MethodType mt, LambdaForm lf); /** Require this method handle to be a BMH, or else replace it with a "wrapper" BMH. * Many transforms are implemented only for BMHs. * @return a behaviorally equivalent BMH */ abstract BoundMethodHandle rebind(); /** * Replace the old lambda form of this method handle with a new one. * The new one must be functionally equivalent to the old one. * Threads may continue running the old form indefinitely, * but it is likely that the new one will be preferred for new executions. * Use with discretion. */ /*non-public*/ void updateForm(LambdaForm newForm) { assert (newForm.customized == null || newForm.customized == this); if (form == newForm) return; newForm.prepare(); // as in MethodHandle.<init> UNSAFE.putReference(this, FORM_OFFSET, newForm); UNSAFE.fullFence(); } /** Craft a LambdaForm customized for this particular MethodHandle */ /*non-public*/ void customize() { final LambdaForm form = this.form; if (form.customized == null) { LambdaForm newForm = form.customize(this); updateForm(newForm); } else { assert (form.customized == this); } } private static final long FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class, "form"); }