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java/lang/invoke/LambdaMetafactory.java

/*
 * Copyright (c) 2012, 2017, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 *
 */

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();
    }
}

java/lang/invoke/LambdaMetafactory.java

 

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