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    }