Java example source code file (MethodHandleProxies.java)

This example Java source code file (MethodHandleProxies.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

arraylist, bcp, class, classloader, illegalargumentexception, invocationhandler, method, methodhandle, methodhandleproxies, methodtype, object, reflection, security, string, throwable, util, wrapperinstance

The MethodHandleProxies.java Java example source code

/*
 * Copyright (c) 2008, 2013, 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 java.security.AccessController;
import java.security.PrivilegedAction;
import sun.invoke.WrapperInstance;
import java.util.ArrayList;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.reflect.misc.ReflectUtil;

/**
 * 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:
     * 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,
     * 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.
     * <p>
     * If a security manager is installed, this method is caller sensitive.
     * During any invocation of the target method handle via the returned wrapper,
     * the original creator of the wrapper (the caller) will be visible
     * to context checks requested by the security manager.
     *
     * @param <T> the desired type of the wrapper, a single-method interface
     * @param intfc a class object representing {@code T}
     * @param target the method handle to invoke from the wrapper
     * @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.
    //
    @CallerSensitive
    public static
    <T> T asInterfaceInstance(final Class intfc, final MethodHandle target) {
        if (!intfc.isInterface() || !Modifier.isPublic(intfc.getModifiers()))
            throw new IllegalArgumentException("not a public interface: "+intfc.getName());
        final MethodHandle mh;
        if (System.getSecurityManager() != null) {
            final Class<?> caller = Reflection.getCallerClass();
            final ClassLoader ccl = caller != null ? caller.getClassLoader() : null;
            ReflectUtil.checkProxyPackageAccess(ccl, intfc);
            mh = ccl != null ? bindCaller(target, caller) : target;
        } else {
            mh = target;
        }
        ClassLoader proxyLoader = intfc.getClassLoader();
        if (proxyLoader == null) {
            ClassLoader cl = Thread.currentThread().getContextClassLoader(); // avoid use of BCP
            proxyLoader = cl != null ? cl : ClassLoader.getSystemClassLoader();
        }
        final Method[] methods = getSingleNameMethods(intfc);
        if (methods == null)
            throw new IllegalArgumentException("not a single-method interface: "+intfc.getName());
        final MethodHandle[] vaTargets = new MethodHandle[methods.length];
        for (int i = 0; i < methods.length; i++) {
            Method sm = methods[i];
            MethodType smMT = MethodType.methodType(sm.getReturnType(), sm.getParameterTypes());
            MethodHandle checkTarget = mh.asType(smMT);  // make throw WMT
            checkTarget = checkTarget.asType(checkTarget.type().changeReturnType(Object.class));
            vaTargets[i] = checkTarget.asSpreader(Object[].class, smMT.parameterCount());
        }
        final InvocationHandler ih = 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 {
                    for (int i = 0; i < methods.length; i++) {
                        if (method.equals(methods[i]))
                            return vaTargets[i].invokeExact(args);
                    }
                    if (method.getDeclaringClass() == WrapperInstance.class)
                        return getArg(method.getName());
                    if (isObjectMethod(method))
                        return callObjectMethod(proxy, method, args);
                    throw new InternalError("bad proxy method: "+method);
                }
            };

        final Object proxy;
        if (System.getSecurityManager() != null) {
            // sun.invoke.WrapperInstance is a restricted interface not accessible
            // by any non-null class loader.
            final ClassLoader loader = proxyLoader;
            proxy = AccessController.doPrivileged(new PrivilegedAction<Object>() {
                public Object run() {
                    return Proxy.newProxyInstance(
                            loader,
                            new Class<?>[]{ intfc, WrapperInstance.class },
                            ih);
                }
            });
        } else {
            proxy = Proxy.newProxyInstance(proxyLoader,
                                           new Class<?>[]{ intfc, WrapperInstance.class },
                                           ih);
        }
        return intfc.cast(proxy);
    }

    private static MethodHandle bindCaller(MethodHandle target, Class<?> hostClass) {
        MethodHandle cbmh = MethodHandleImpl.bindCaller(target, hostClass);
        if (target.isVarargsCollector()) {
            MethodType type = cbmh.type();
            int arity = type.parameterCount();
            return cbmh.asVarargsCollector(type.parameterType(arity-1));
        }
        return cbmh;
    }

    /**
     * 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[] getSingleNameMethods(Class<?> intfc) {
        ArrayList<Method> methods = new ArrayList();
        String uniqueName = null;
        for (Method m : intfc.getMethods()) {
            if (isObjectMethod(m))  continue;
            if (!Modifier.isAbstract(m.getModifiers()))  continue;
            String mname = m.getName();
            if (uniqueName == null)
                uniqueName = mname;
            else if (!uniqueName.equals(mname))
                return null;  // too many abstract methods
            methods.add(m);
        }
        if (uniqueName == null)  return null;
        return methods.toArray(new Method[methods.size()]);
    }
}