Java tutorial
/* * Copyright (c) 2012, 2017, 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 java.io.Serializable; import java.util.Arrays; /** * <p>Methods to facilitate the creation of simple "function objects" that * implement one or more interfaces by delegation to a provided {@link MethodHandle}, * possibly after type adaptation and partial evaluation of arguments. These * methods are typically used as <em>bootstrap methods</em> for {@code invokedynamic} * call sites, to support the <em>lambda expression</em> and <em>method * reference expression</em> features of the Java Programming Language. * * <p>Indirect access to the behavior specified by the provided {@code MethodHandle} * proceeds in order through three phases: * <ul> * <li><em>Linkage</em> occurs when the methods in this class are invoked. * They take as arguments an interface to be implemented (typically a * <em>functional interface</em>, one with a single abstract method), a * name and signature of a method from that interface to be implemented, a * method handle describing the desired implementation behavior * for that method, and possibly other additional metadata, and produce a * {@link CallSite} whose target can be used to create suitable function * objects. Linkage may involve dynamically loading a new class that * implements the target interface. The {@code CallSite} can be considered a * "factory" for function objects and so these linkage methods are referred * to as "metafactories".</li> * * <li><em>Capture</em> occurs when the {@code CallSite}'s target is * invoked, typically through an {@code invokedynamic} call site, * producing a function object. This may occur many times for * a single factory {@code CallSite}. Capture may involve allocation of a * new function object, or may return an existing function object. The * behavior {@code MethodHandle} may have additional parameters beyond those * of the specified interface method; these are referred to as <em>captured * parameters</em>, which must be provided as arguments to the * {@code CallSite} target, and which may be early-bound to the behavior * {@code MethodHandle}. The number of captured parameters and their types * are determined during linkage. * The identity of a function object produced by invoking the * {@code CallSite}'s target is unpredictable, and therefore * identity-sensitive operations (such as reference equality, object * locking, and {@code System.identityHashCode()} may produce different * results in different implementations, or even upon different invocations * in the same implementation.</li> * * <li><em>Invocation</em> occurs when an implemented interface method * is invoked on a function object. This may occur many times for a single * function object. The method referenced by the behavior {@code MethodHandle} * is invoked with the captured arguments and any additional arguments * provided on invocation, as if by {@link MethodHandle#invoke(Object...)}.</li> * </ul> * * <p>It is sometimes useful to restrict the set of inputs or results permitted * at invocation. For example, when the generic interface {@code Predicate<T>} * is parameterized as {@code Predicate<String>}, the input must be a * {@code String}, even though the method to implement allows any {@code Object}. * At linkage time, an additional {@link MethodType} parameter describes the * "instantiated" method type; on invocation, the arguments and eventual result * are checked against this {@code MethodType}. * * <p>This class provides two forms of linkage methods: a standard version * ({@link #metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)}) * using an optimized protocol, and an alternate version * {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)}). * The alternate version is a generalization of the standard version, providing * additional control over the behavior of the generated function objects via * flags and additional arguments. The alternate version adds the ability to * manage the following attributes of function objects: * * <ul> * <li><em>Bridging.</em> It is sometimes useful to implement multiple * variations of the method signature, involving argument or return type * adaptation. This occurs when multiple distinct VM signatures for a method * are logically considered to be the same method by the language. The * flag {@code FLAG_BRIDGES} indicates that a list of additional * {@code MethodType}s will be provided, each of which will be implemented * by the resulting function object. These methods will share the same * name and instantiated type.</li> * * <li><em>Multiple interfaces.</em> If needed, more than one interface * can be implemented by the function object. (These additional interfaces * are typically marker interfaces with no methods.) The flag {@code FLAG_MARKERS} * indicates that a list of additional interfaces will be provided, each of * which should be implemented by the resulting function object.</li> * * <li><em>Serializability.</em> The generated function objects do not * generally support serialization. If desired, {@code FLAG_SERIALIZABLE} * can be used to indicate that the function objects should be serializable. * Serializable function objects will use, as their serialized form, * instances of the class {@code SerializedLambda}, which requires additional * assistance from the capturing class (the class described by the * {@link MethodHandles.Lookup} parameter {@code caller}); see * {@link SerializedLambda} for details.</li> * </ul> * * <p>Assume the linkage arguments are as follows: * <ul> * <li>{@code invokedType} (describing the {@code CallSite} signature) has * K parameters of types (D1..Dk) and return type Rd;</li> * <li>{@code samMethodType} (describing the implemented method type) has N * parameters, of types (U1..Un) and return type Ru;</li> * <li>{@code implMethod} (the {@code MethodHandle} providing the * implementation has M parameters, of types (A1..Am) and return type Ra * (if the method describes an instance method, the method type of this * method handle already includes an extra first argument corresponding to * the receiver);</li> * <li>{@code instantiatedMethodType} (allowing restrictions on invocation) * has N parameters, of types (T1..Tn) and return type Rt.</li> * </ul> * * <p>Then the following linkage invariants must hold: * <ul> * <li>Rd is an interface</li> * <li>{@code implMethod} is a <em>direct method handle</em></li> * <li>{@code samMethodType} and {@code instantiatedMethodType} have the same * arity N, and for i=1..N, Ti and Ui are the same type, or Ti and Ui are * both reference types and Ti is a subtype of Ui</li> * <li>Either Rt and Ru are the same type, or both are reference types and * Rt is a subtype of Ru</li> * <li>K + N = M</li> * <li>For i=1..K, Di = Ai</li> * <li>For i=1..N, Ti is adaptable to Aj, where j=i+k</li> * <li>The return type Rt is void, or the return type Ra is not void and is * adaptable to Rt</li> * </ul> * * <p>Further, at capture time, if {@code implMethod} corresponds to an instance * method, and there are any capture arguments ({@code K > 0}), then the first * capture argument (corresponding to the receiver) must be non-null. * * <p>A type Q is considered adaptable to S as follows: * <table class="striped"> * <caption style="display:none">adaptable types</caption> * <thead> * <tr><th scope="col">Q</th><th scope="col">S</th><th scope="col">Link-time checks</th><th scope="col">Invocation-time checks</th></tr> * </thead> * <tbody> * <tr> * <th scope="row">Primitive</th><th scope="row">Primitive</th> * <td>Q can be converted to S via a primitive widening conversion</td> * <td>None</td> * </tr> * <tr> * <th scope="row">Primitive</th><th scope="row">Reference</th> * <td>S is a supertype of the Wrapper(Q)</td> * <td>Cast from Wrapper(Q) to S</td> * </tr> * <tr> * <th scope="row">Reference</th><th scope="row">Primitive</th> * <td>for parameter types: Q is a primitive wrapper and Primitive(Q) * can be widened to S * <br>for return types: If Q is a primitive wrapper, check that * Primitive(Q) can be widened to S</td> * <td>If Q is not a primitive wrapper, cast Q to the base Wrapper(S); * for example Number for numeric types</td> * </tr> * <tr> * <th scope="row">Reference</th><th scope="row">Reference</th> * <td>for parameter types: S is a supertype of Q * <br>for return types: none</td> * <td>Cast from Q to S</td> * </tr> * </tbody> * </table> * * @apiNote These linkage methods are designed to support the evaluation * of <em>lambda expressions</em> and <em>method references</em> in the Java * Language. For every lambda expressions or method reference in the source code, * there is a target type which is a functional interface. Evaluating a lambda * expression produces an object of its target type. The recommended mechanism * for evaluating lambda expressions is to desugar the lambda body to a method, * invoke an invokedynamic call site whose static argument list describes the * sole method of the functional interface and the desugared implementation * method, and returns an object (the lambda object) that implements the target * type. (For method references, the implementation method is simply the * referenced method; no desugaring is needed.) * * <p>The argument list of the implementation method and the argument list of * the interface method(s) may differ in several ways. The implementation * methods may have additional arguments to accommodate arguments captured by * the lambda expression; there may also be differences resulting from permitted * adaptations of arguments, such as casting, boxing, unboxing, and primitive * widening. (Varargs adaptations are not handled by the metafactories; these are * expected to be handled by the caller.) * * <p>Invokedynamic call sites have two argument lists: a static argument list * and a dynamic argument list. The static argument list is stored in the * constant pool; the dynamic argument is pushed on the operand stack at capture * time. The bootstrap method has access to the entire static argument list * (which in this case, includes information describing the implementation method, * the target interface, and the target interface method(s)), as well as a * method signature describing the number and static types (but not the values) * of the dynamic arguments and the static return type of the invokedynamic site. * * @implNote The implementation method is described with a method handle. In * theory, any method handle could be used. Currently supported are direct method * handles representing invocation of virtual, interface, constructor and static * methods. * @since 1.8 */ public final class LambdaMetafactory { private LambdaMetafactory() { } /** Flag for alternate metafactories indicating the lambda object * must be serializable */ public static final int FLAG_SERIALIZABLE = 1 << 0; /** * Flag for alternate metafactories indicating the lambda object implements * other marker interfaces * besides Serializable */ public static final int FLAG_MARKERS = 1 << 1; /** * Flag for alternate metafactories indicating the lambda object requires * additional bridge methods */ public static final int FLAG_BRIDGES = 1 << 2; private static final Class<?>[] EMPTY_CLASS_ARRAY = new Class<?>[0]; private static final MethodType[] EMPTY_MT_ARRAY = new MethodType[0]; // LambdaMetafactory bootstrap methods are startup sensitive, and may be // special cased in java.lang.invokeBootstrapMethodInvoker to ensure // methods are invoked with exact type information to avoid generating // code for runtime checks. Take care any changes or additions here are // reflected there as appropriate. /** * Facilitates the creation of simple "function objects" that implement one * or more interfaces by delegation to a provided {@link MethodHandle}, * after appropriate type adaptation and partial evaluation of arguments. * Typically used as a <em>bootstrap method</em> for {@code invokedynamic} * call sites, to support the <em>lambda expression</em> and <em>method * reference expression</em> features of the Java Programming Language. * * <p>This is the standard, streamlined metafactory; additional flexibility * is provided by {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)}. * A general description of the behavior of this method is provided * {@link LambdaMetafactory above}. * * <p>When the target of the {@code CallSite} returned from this method is * invoked, the resulting function objects are instances of a class which * implements the interface named by the return type of {@code invokedType}, * declares a method with the name given by {@code invokedName} and the * signature given by {@code samMethodType}. It may also override additional * methods from {@code Object}. * * @param caller Represents a lookup context with the accessibility * privileges of the caller. Specifically, the lookup context * must have * <a href="MethodHandles.Lookup.html#privacc">private access</a> * privileges. * When used with {@code invokedynamic}, this is stacked * automatically by the VM. * @param invokedName The name of the method to implement. When used with * {@code invokedynamic}, this is provided by the * {@code NameAndType} of the {@code InvokeDynamic} * structure and is stacked automatically by the VM. * @param invokedType The expected signature of the {@code CallSite}. The * parameter types represent the types of capture variables; * the return type is the interface to implement. When * used with {@code invokedynamic}, this is provided by * the {@code NameAndType} of the {@code InvokeDynamic} * structure and is stacked automatically by the VM. * In the event that the implementation method is an * instance method and this signature has any parameters, * the first parameter in the invocation signature must * correspond to the receiver. * @param samMethodType Signature and return type of method to be implemented * by the function object. * @param implMethod A direct method handle describing the implementation * method which should be called (with suitable adaptation * of argument types, return types, and with captured * arguments prepended to the invocation arguments) at * invocation time. * @param instantiatedMethodType The signature and return type that should * be enforced dynamically at invocation time. * This may be the same as {@code samMethodType}, * or may be a specialization of it. * @return a CallSite whose target can be used to perform capture, generating * instances of the interface named by {@code invokedType} * @throws LambdaConversionException If any of the linkage invariants * described {@link LambdaMetafactory above} * are violated, or the lookup context * does not have private access privileges. */ public static CallSite metafactory(MethodHandles.Lookup caller, String invokedName, MethodType invokedType, MethodType samMethodType, MethodHandle implMethod, MethodType instantiatedMethodType) throws LambdaConversionException { AbstractValidatingLambdaMetafactory mf; mf = new InnerClassLambdaMetafactory(caller, invokedType, invokedName, samMethodType, implMethod, instantiatedMethodType, false, EMPTY_CLASS_ARRAY, EMPTY_MT_ARRAY); mf.validateMetafactoryArgs(); return mf.buildCallSite(); } /** * Facilitates the creation of simple "function objects" that implement one * or more interfaces by delegation to a provided {@link MethodHandle}, * after appropriate type adaptation and partial evaluation of arguments. * Typically used as a <em>bootstrap method</em> for {@code invokedynamic} * call sites, to support the <em>lambda expression</em> and <em>method * reference expression</em> features of the Java Programming Language. * * <p>This is the general, more flexible metafactory; a streamlined version * is provided by {@link #metafactory(java.lang.invoke.MethodHandles.Lookup, * String, MethodType, MethodType, MethodHandle, MethodType)}. * A general description of the behavior of this method is provided * {@link LambdaMetafactory above}. * * <p>The argument list for this method includes three fixed parameters, * corresponding to the parameters automatically stacked by the VM for the * bootstrap method in an {@code invokedynamic} invocation, and an {@code Object[]} * parameter that contains additional parameters. The declared argument * list for this method is: * * <pre>{@code * CallSite altMetafactory(MethodHandles.Lookup caller, * String invokedName, * MethodType invokedType, * Object... args) * }</pre> * * <p>but it behaves as if the argument list is as follows: * * <pre>{@code * CallSite altMetafactory(MethodHandles.Lookup caller, * String invokedName, * MethodType invokedType, * MethodType samMethodType, * MethodHandle implMethod, * MethodType instantiatedMethodType, * int flags, * int markerInterfaceCount, // IF flags has MARKERS set * Class... markerInterfaces, // IF flags has MARKERS set * int bridgeCount, // IF flags has BRIDGES set * MethodType... bridges // IF flags has BRIDGES set * ) * }</pre> * * <p>Arguments that appear in the argument list for * {@link #metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)} * have the same specification as in that method. The additional arguments * are interpreted as follows: * <ul> * <li>{@code flags} indicates additional options; this is a bitwise * OR of desired flags. Defined flags are {@link #FLAG_BRIDGES}, * {@link #FLAG_MARKERS}, and {@link #FLAG_SERIALIZABLE}.</li> * <li>{@code markerInterfaceCount} is the number of additional interfaces * the function object should implement, and is present if and only if the * {@code FLAG_MARKERS} flag is set.</li> * <li>{@code markerInterfaces} is a variable-length list of additional * interfaces to implement, whose length equals {@code markerInterfaceCount}, * and is present if and only if the {@code FLAG_MARKERS} flag is set.</li> * <li>{@code bridgeCount} is the number of additional method signatures * the function object should implement, and is present if and only if * the {@code FLAG_BRIDGES} flag is set.</li> * <li>{@code bridges} is a variable-length list of additional * methods signatures to implement, whose length equals {@code bridgeCount}, * and is present if and only if the {@code FLAG_BRIDGES} flag is set.</li> * </ul> * * <p>Each class named by {@code markerInterfaces} is subject to the same * restrictions as {@code Rd}, the return type of {@code invokedType}, * as described {@link LambdaMetafactory above}. Each {@code MethodType} * named by {@code bridges} is subject to the same restrictions as * {@code samMethodType}, as described {@link LambdaMetafactory above}. * * <p>When FLAG_SERIALIZABLE is set in {@code flags}, the function objects * will implement {@code Serializable}, and will have a {@code writeReplace} * method that returns an appropriate {@link SerializedLambda}. The * {@code caller} class must have an appropriate {@code $deserializeLambda$} * method, as described in {@link SerializedLambda}. * * <p>When the target of the {@code CallSite} returned from this method is * invoked, the resulting function objects are instances of a class with * the following properties: * <ul> * <li>The class implements the interface named by the return type * of {@code invokedType} and any interfaces named by {@code markerInterfaces}</li> * <li>The class declares methods with the name given by {@code invokedName}, * and the signature given by {@code samMethodType} and additional signatures * given by {@code bridges}</li> * <li>The class may override methods from {@code Object}, and may * implement methods related to serialization.</li> * </ul> * * @param caller Represents a lookup context with the accessibility * privileges of the caller. Specifically, the lookup context * must have * <a href="MethodHandles.Lookup.html#privacc">private access</a> * privileges. * When used with {@code invokedynamic}, this is stacked * automatically by the VM. * @param invokedName The name of the method to implement. When used with * {@code invokedynamic}, this is provided by the * {@code NameAndType} of the {@code InvokeDynamic} * structure and is stacked automatically by the VM. * @param invokedType The expected signature of the {@code CallSite}. The * parameter types represent the types of capture variables; * the return type is the interface to implement. When * used with {@code invokedynamic}, this is provided by * the {@code NameAndType} of the {@code InvokeDynamic} * structure and is stacked automatically by the VM. * In the event that the implementation method is an * instance method and this signature has any parameters, * the first parameter in the invocation signature must * correspond to the receiver. * @param args An {@code Object[]} array containing the required * arguments {@code samMethodType}, {@code implMethod}, * {@code instantiatedMethodType}, {@code flags}, and any * optional arguments, as described * {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)} above} * @return a CallSite whose target can be used to perform capture, generating * instances of the interface named by {@code invokedType} * @throws LambdaConversionException If any of the linkage invariants * described {@link LambdaMetafactory above} * are violated, or the lookup context * does not have private access privileges. */ public static CallSite altMetafactory(MethodHandles.Lookup caller, String invokedName, MethodType invokedType, Object... args) throws LambdaConversionException { MethodType samMethodType = (MethodType) args[0]; MethodHandle implMethod = (MethodHandle) args[1]; MethodType instantiatedMethodType = (MethodType) args[2]; int flags = (Integer) args[3]; Class<?>[] markerInterfaces; MethodType[] bridges; int argIndex = 4; if ((flags & FLAG_MARKERS) != 0) { int markerCount = (Integer) args[argIndex++]; markerInterfaces = new Class<?>[markerCount]; System.arraycopy(args, argIndex, markerInterfaces, 0, markerCount); argIndex += markerCount; } else markerInterfaces = EMPTY_CLASS_ARRAY; if ((flags & FLAG_BRIDGES) != 0) { int bridgeCount = (Integer) args[argIndex++]; bridges = new MethodType[bridgeCount]; System.arraycopy(args, argIndex, bridges, 0, bridgeCount); argIndex += bridgeCount; } else bridges = EMPTY_MT_ARRAY; boolean isSerializable = ((flags & FLAG_SERIALIZABLE) != 0); if (isSerializable) { boolean foundSerializableSupertype = Serializable.class.isAssignableFrom(invokedType.returnType()); for (Class<?> c : markerInterfaces) foundSerializableSupertype |= Serializable.class.isAssignableFrom(c); if (!foundSerializableSupertype) { markerInterfaces = Arrays.copyOf(markerInterfaces, markerInterfaces.length + 1); markerInterfaces[markerInterfaces.length - 1] = Serializable.class; } } AbstractValidatingLambdaMetafactory mf = new InnerClassLambdaMetafactory(caller, invokedType, invokedName, samMethodType, implMethod, instantiatedMethodType, isSerializable, markerInterfaces, bridges); mf.validateMetafactoryArgs(); return mf.buildCallSite(); } }