com.google.common.reflect.AbstractInvocationHandler.java Source code

Introduction

Here is the source code for com.google.common.reflect.AbstractInvocationHandler.java

Source

/*
 * Copyright (C) 2012 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.reflect;

import com.google.common.annotations.Beta;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.Arrays;

import javax.annotation.Nullable;

/**
 * Abstract implementation of {@link InvocationHandler} that handles {@link Object#equals},
 * {@link Object#hashCode} and {@link Object#toString}. For example: <pre>
 * class Unsupported extends AbstractInvocationHandler {
 *   protected Object handleInvocation(
 *       Object proxy, Method method, Object[] args) {
 *     throw new UnsupportedOperationException();
 *   }
 * }
 *
 * CharSequence unsupported = Reflection.newProxy(CharSequence.class, new Unsupported());
 * </pre>
 *
 * @author Ben Yu
 * @since 12.0
 */
@Beta
public abstract class AbstractInvocationHandler implements InvocationHandler {

    private static final Object[] NO_ARGS = {};

    /**
     * {@inheritDoc}
     *
     * <p><ul>
     * <li>{@code proxy.hashCode()} delegates to {@link AbstractInvocationHandler#hashCode}
     * <li>{@code proxy.toString()} delegates to {@link AbstractInvocationHandler#toString}
     * <li>{@code proxy.equals(argument)} returns true if: <ul>
     *   <li>{@code proxy} and {@code argument} are of the same type
     *   <li>and {@link AbstractInvocationHandler#equals} returns true for the {@link
     *       InvocationHandler} of {@code argument}
     *   </ul>
     * <li>other method calls are dispatched to {@link #handleInvocation}.
     * </ul>
     */
    @Override
    public final Object invoke(Object proxy, Method method, @Nullable Object[] args) throws Throwable {
        if (args == null) {
            args = NO_ARGS;
        }
        if (args.length == 0 && method.getName().equals("hashCode")) {
            return hashCode();
        }
        if (args.length == 1 && method.getName().equals("equals")
                && method.getParameterTypes()[0] == Object.class) {
            Object arg = args[0];
            if (arg == null) {
                return false;
            }
            if (proxy == arg) {
                return true;
            }
            return isProxyOfSameInterfaces(arg, proxy.getClass()) && equals(Proxy.getInvocationHandler(arg));
        }
        if (args.length == 0 && method.getName().equals("toString")) {
            return toString();
        }
        return handleInvocation(proxy, method, args);
    }

    /**
     * {@link #invoke} delegates to this method upon any method invocation on the proxy instance,
     * except {@link Object#equals}, {@link Object#hashCode} and {@link Object#toString}. The result
     * will be returned as the proxied method's return value.
     * 
     * <p>Unlike {@link #invoke}, {@code args} will never be null. When the method has no parameter,
     * an empty array is passed in.
     */
    protected abstract Object handleInvocation(Object proxy, Method method, Object[] args) throws Throwable;

    /**
     * By default delegates to {@link Object#equals} so instances are only equal if they are
     * identical. {@code proxy.equals(argument)} returns true if: <ul>
     * <li>{@code proxy} and {@code argument} are of the same type
     * <li>and this method returns true for the {@link InvocationHandler} of {@code argument}
     * </ul>
     * <p>Subclasses can override this method to provide custom equality.
     */
    @Override
    public boolean equals(Object obj) {
        return super.equals(obj);
    }

    /**
     * By default delegates to {@link Object#hashCode}. The dynamic proxies' {@code hashCode()} will
     * delegate to this method. Subclasses can override this method to provide custom equality.
     */
    @Override
    public int hashCode() {
        return super.hashCode();
    }

    /**
     * By default delegates to {@link Object#toString}. The dynamic proxies' {@code toString()} will
     * delegate to this method. Subclasses can override this method to provide custom string
     * representation for the proxies.
     */
    @Override
    public String toString() {
        return super.toString();
    }

    private static boolean isProxyOfSameInterfaces(Object arg, Class<?> proxyClass) {
        return proxyClass.isInstance(arg)
                // Equal proxy instances should mostly be instance of proxyClass
                // Under some edge cases (such as the proxy of JDK types serialized and then deserialized)
                // the proxy type may not be the same.
                // We first check isProxyClass() so that the common case of comparing with non-proxy objects
                // is efficient.
                || (Proxy.isProxyClass(arg.getClass())
                        && Arrays.equals(arg.getClass().getInterfaces(), proxyClass.getInterfaces()));
    }
}