openjdk/jdk/src/share/classes/java/lang/invoke/MethodHandleProxies.java

/*
 * Copyright (c) 2008, 2011, 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.lang.reflect.*;
import sun.invoke.WrapperInstance;

/**
 * This class consists exclusively of static methods that help adapt
 * method handles to other JVM types, such as interfaces.
 */
public class MethodHandleProxies {

    private MethodHandleProxies() { }  // do not instantiate

    /**
     * Produces an instance of the given single-method interface which redirects
     * its calls to the given method handle.
     * <p>
     * A single-method interface is an interface which declares a uniquely named method.
     * When determining the uniquely named method of a single-method interface,
     * the public {@code Object} methods ({@code toString}, {@code equals}, {@code hashCode})
     * are disregarded.  For example, {@link java.util.Comparator} is a single-method interface,
     * even though it re-declares the {@code Object.equals} method.
     * <p>
     * The interface must be public.  No additional access checks are performed.
     * <p>
     * The resulting instance of the required type will respond to
     * invocation of the type's uniquely named method by calling
     * the given target on the incoming arguments,
     * and returning or throwing whatever the target
     * returns or throws.  The invocation will be as if by
     * {@code target.invoke}.
     * The target's type will be checked before the
     * instance is created, as if by a call to {@code asType},
     * which may result in a {@code WrongMethodTypeException}.
     * <p>
     * The uniquely named method is allowed to be multiply declared,
     * with distinct type descriptors.  (E.g., it can be overloaded,
     * or can possess bridge methods.)  All such declarations are
     * connected directly to the target method handle.
     * Argument and return types are adjusted by {@code asType}
     * for each individual declaration.
     * <p>
     * The wrapper instance will implement the requested interface
     * and its super-types, but no other single-method interfaces.
     * This means that the instance will not unexpectedly
     * pass an {@code instanceof} test for any unrequested type.
     * <p style="font-size:smaller;">
     * <em>Implementation Note:</em>
     * Therefore, each instance must implement a unique single-method interface.
     * Implementations may not bundle together
     * multiple single-method interfaces onto single implementation classes
     * in the style of {@link java.awt.AWTEventMulticaster}.
     * <p>
     * The method handle may throw an <em>undeclared exception</em>,
     * which means any checked exception (or other checked throwable)
     * not declared by the requested type's single abstract method.
     * If this happens, the throwable will be wrapped in an instance of
     * {@link java.lang.reflect.UndeclaredThrowableException UndeclaredThrowableException}
     * and thrown in that wrapped form.
     * <p>
     * Like {@link java.lang.Integer#valueOf Integer.valueOf},
     * {@code asInterfaceInstance} is a factory method whose results are defined
     * by their behavior.
     * It is not guaranteed to return a new instance for every call.
     * <p>
     * Because of the possibility of {@linkplain java.lang.reflect.Method#isBridge bridge methods}
     * and other corner cases, the interface may also have several abstract methods
     * with the same name but having distinct descriptors (types of returns and parameters).
     * In this case, all the methods are bound in common to the one given target.
     * The type check and effective {@code asType} conversion is applied to each
     * method type descriptor, and all abstract methods are bound to the target in common.
     * Beyond this type check, no further checks are made to determine that the
     * abstract methods are related in any way.
     * <p>
     * Future versions of this API may accept additional types,
     * such as abstract classes with single abstract methods.
     * Future versions of this API may also equip wrapper instances
     * with one or more additional public "marker" interfaces.
     *
     * @param target the method handle to invoke from the wrapper
     * @param intfc the desired type of the wrapper, a single-method interface
     * @return a correctly-typed wrapper for the given target
     * @throws NullPointerException if either argument is null
     * @throws IllegalArgumentException if the {@code intfc} is not a
     *         valid argument to this method
     * @throws WrongMethodTypeException if the target cannot
     *         be converted to the type required by the requested interface
     */
    // Other notes to implementors:
    // <p>
    // No stable mapping is promised between the single-method interface and
    // the implementation class C.  Over time, several implementation
    // classes might be used for the same type.
    // <p>
    // If the implementation is able
    // to prove that a wrapper of the required type
    // has already been created for a given
    // method handle, or for another method handle with the
    // same behavior, the implementation may return that wrapper in place of
    // a new wrapper.
    // <p>
    // This method is designed to apply to common use cases
    // where a single method handle must interoperate with
    // an interface that implements a function-like
    // API.  Additional variations, such as single-abstract-method classes with
    // private constructors, or interfaces with multiple but related
    // entry points, must be covered by hand-written or automatically
    // generated adapter classes.
    //
    public static
    <T> T asInterfaceInstance(final Class<T> intfc, final MethodHandle target) {
        // POC implementation only; violates the above contract several ways
        final Method sm = getSingleMethod(intfc);
        if (sm == null)
            throw new IllegalArgumentException("not a single-method interface: "+intfc.getName());
        MethodType smMT = MethodType.methodType(sm.getReturnType(), sm.getParameterTypes());
        MethodHandle checkTarget = target.asType(smMT);  // make throw WMT
        checkTarget = checkTarget.asType(checkTarget.type().changeReturnType(Object.class));
        final MethodHandle vaTarget = checkTarget.asSpreader(Object[].class, smMT.parameterCount());
        return intfc.cast(Proxy.newProxyInstance(
                intfc.getClassLoader(),
                new Class[]{ intfc, WrapperInstance.class },
                new InvocationHandler() {
                    private Object getArg(String name) {
                        if ((Object)name == "getWrapperInstanceTarget")  return target;
                        if ((Object)name == "getWrapperInstanceType")    return intfc;
                        throw new AssertionError();
                    }
                    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                        if (method.getDeclaringClass() == WrapperInstance.class)
                            return getArg(method.getName());
                        if (method.equals(sm))
                            return vaTarget.invokeExact(args);
                        if (isObjectMethod(method))
                            return callObjectMethod(this, method, args);
                        throw new InternalError();
                    }
                }));
    }

    /**
     * Determines if the given object was produced by a call to {@link #asInterfaceInstance asInterfaceInstance}.
     * @param x any reference
     * @return true if the reference is not null and points to an object produced by {@code asInterfaceInstance}
     */
    public static
    boolean isWrapperInstance(Object x) {
        return x instanceof WrapperInstance;
    }

    private static WrapperInstance asWrapperInstance(Object x) {
        try {
            if (x != null)
                return (WrapperInstance) x;
        } catch (ClassCastException ex) {
        }
        throw new IllegalArgumentException("not a wrapper instance");
    }

    /**
     * Produces or recovers a target method handle which is behaviorally
     * equivalent to the unique method of this wrapper instance.
     * The object {@code x} must have been produced by a call to {@link #asInterfaceInstance asInterfaceInstance}.
     * This requirement may be tested via {@link #isWrapperInstance isWrapperInstance}.
     * @param x any reference
     * @return a method handle implementing the unique method
     * @throws IllegalArgumentException if the reference x is not to a wrapper instance
     */
    public static
    MethodHandle wrapperInstanceTarget(Object x) {
        return asWrapperInstance(x).getWrapperInstanceTarget();
    }

    /**
     * Recovers the unique single-method interface type for which this wrapper instance was created.
     * The object {@code x} must have been produced by a call to {@link #asInterfaceInstance asInterfaceInstance}.
     * This requirement may be tested via {@link #isWrapperInstance isWrapperInstance}.
     * @param x any reference
     * @return the single-method interface type for which the wrapper was created
     * @throws IllegalArgumentException if the reference x is not to a wrapper instance
     */
    public static
    Class<?> wrapperInstanceType(Object x) {
        return asWrapperInstance(x).getWrapperInstanceType();
    }

    private static
    boolean isObjectMethod(Method m) {
        switch (m.getName()) {
        case "toString":
            return (m.getReturnType() == String.class
                    && m.getParameterTypes().length == 0);
        case "hashCode":
            return (m.getReturnType() == int.class
                    && m.getParameterTypes().length == 0);
        case "equals":
            return (m.getReturnType() == boolean.class
                    && m.getParameterTypes().length == 1
                    && m.getParameterTypes()[0] == Object.class);
        }
        return false;
    }

    private static
    Object callObjectMethod(Object self, Method m, Object[] args) {
        assert(isObjectMethod(m)) : m;
        switch (m.getName()) {
        case "toString":
            return self.getClass().getName() + "@" + Integer.toHexString(self.hashCode());
        case "hashCode":
            return System.identityHashCode(self);
        case "equals":
            return (self == args[0]);
        }
        return null;
    }

    private static
    Method getSingleMethod(Class<?> intfc) {
        if (!intfc.isInterface())  return null;
        Method sm = null;
        for (Method m : intfc.getMethods()) {
            int mod = m.getModifiers();
            if (Modifier.isAbstract(mod)) {
                if (sm != null && !isObjectMethod(sm))
                    return null;  // too many abstract methods
                sm = m;
            }
        }
        return sm;
    }
}