    001    /*
    002     * Copyright (C) 2012 The Guava Authors
    003     *
    004     * Licensed under the Apache License, Version 2.0 (the "License");
    005     * you may not use this file except in compliance with the License.
    006     * You may obtain a copy of the License at
    007     *
    008     * http://www.apache.org/licenses/LICENSE-2.0
    009     *
    010     * Unless required by applicable law or agreed to in writing, software
    011     * distributed under the License is distributed on an "AS IS" BASIS,
    012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    013     * See the License for the specific language governing permissions and
    014     * limitations under the License.
    015     */
    016
    017    package com.google.common.reflect;
    018
    019    import com.google.common.annotations.Beta;
    020
    021    import java.lang.reflect.InvocationHandler;
    022    import java.lang.reflect.Method;
    023
    024    import javax.annotation.Nullable;
    025
    026    /**
    027     * Abstract implementation of {@link InvocationHandler} that handles {@link Object#equals},
    028     * {@link Object#hashCode} and {@link Object#toString}.
    029     *
    030     * @author Ben Yu
    031     * @since 12.0
    032     */
    033    @Beta
    034    public abstract class AbstractInvocationHandler implements InvocationHandler {
    035
    036      private static final Object[] NO_ARGS = {};
    037
    038      /**
    039       * {@inheritDoc}
    040       *
    041       * <p>{@link Object#equals}, {@link Object#hashCode} are implemented according to referential
    042       * equality (the default behavior of {@link Object}). {@link Object#toString} delegates to
    043       * {@link #toString} that can be overridden by subclasses.
    044       */
    045      @Override public final Object invoke(Object proxy, Method method, @Nullable Object[] args)
    046          throws Throwable {
    047        if (args == null) {
    048          args = NO_ARGS;
    049        }
    050        if (args.length == 0 && method.getName().equals("hashCode")) {
    051          return System.identityHashCode(proxy);
    052        }
    053        if (args.length == 1
    054            && method.getName().equals("equals")
    055            && method.getParameterTypes()[0] == Object.class) {
    056          return proxy == args[0];
    057        }
    058        if (args.length == 0 && method.getName().equals("toString")) {
    059          return toString();
    060        }
    061        return handleInvocation(proxy, method, args);
    062      }
    063
    064      /**
    065       * {@link #invoke} delegates to this method upon any method invocation on the proxy instance,
    066       * except {@link Object#equals}, {@link Object#hashCode} and {@link Object#toString}. The result
    067       * will be returned as the proxied method's return value.
    068       *
    069       * <p>Unlike {@link #invoke}, {@code args} will never be null. When the method has no parameter,
    070       * an empty array is passed in.
    071       */
    072      protected abstract Object handleInvocation(Object proxy, Method method, Object[] args)
    073          throws Throwable;
    074
    075      /**
    076       * The dynamic proxies' {@link Object#toString} will delegate to this method. Subclasses can
    077       * override this to provide custom string representation of the proxies.
    078       */
    079      @Override public String toString() {
    080        return super.toString();
    081      }
    082    }