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
017package com.google.common.reflect;
018
019import com.google.common.annotations.Beta;
020
021import java.lang.reflect.InvocationHandler;
022import java.lang.reflect.Method;
023import java.lang.reflect.Proxy;
024import java.util.Arrays;
025
026import javax.annotation.Nullable;
027
028/**
029 * Abstract implementation of {@link InvocationHandler} that handles {@link Object#equals},
030 * {@link Object#hashCode} and {@link Object#toString}. For example: <pre>
031 * class Unsupported extends AbstractInvocationHandler {
032 *   protected Object handleInvocation(
033 *       Object proxy, Method method, Object[] args) {
034 *     throw new UnsupportedOperationException();
035 *   }
036 * }
037 *
038 * CharSequence unsupported = Reflection.newProxy(CharSequence.class, new Unsupported());
039 * </pre>
040 *
041 * @author Ben Yu
042 * @since 12.0
043 */
044@Beta
045public abstract class AbstractInvocationHandler implements InvocationHandler {
046
047  private static final Object[] NO_ARGS = {};
048
049  /**
050   * {@inheritDoc}
051   *
052   * <p><ul>
053   * <li>{@code proxy.hashCode()} delegates to {@link AbstractInvocationHandler#hashCode}
054   * <li>{@code proxy.toString()} delegates to {@link AbstractInvocationHandler#toString}
055   * <li>{@code proxy.equals(argument)} returns true if: <ul>
056   *   <li>{@code proxy} and {@code argument} are of the same type
057   *   <li>and {@link AbstractInvocationHandler#equals} returns true for the {@link
058   *       InvocationHandler} of {@code argument}
059   *   </ul>
060   * <li>other method calls are dispatched to {@link #handleInvocation}.
061   * </ul>
062   */
063  @Override public final Object invoke(Object proxy, Method method, @Nullable Object[] args)
064      throws Throwable {
065    if (args == null) {
066      args = NO_ARGS;
067    }
068    if (args.length == 0 && method.getName().equals("hashCode")) {
069      return hashCode();
070    }
071    if (args.length == 1
072        && method.getName().equals("equals")
073        && method.getParameterTypes()[0] == Object.class) {
074      Object arg = args[0];
075      if (arg == null) {
076        return false;
077      }
078      if (proxy == arg) {
079        return true;
080      }
081      return isProxyOfSameInterfaces(arg, proxy.getClass())
082          && equals(Proxy.getInvocationHandler(arg));
083    }
084    if (args.length == 0 && method.getName().equals("toString")) {
085      return toString();
086    }
087    return handleInvocation(proxy, method, args);
088  }
089
090  /**
091   * {@link #invoke} delegates to this method upon any method invocation on the proxy instance,
092   * except {@link Object#equals}, {@link Object#hashCode} and {@link Object#toString}. The result
093   * will be returned as the proxied method's return value.
094   * 
095   * <p>Unlike {@link #invoke}, {@code args} will never be null. When the method has no parameter,
096   * an empty array is passed in.
097   */
098  protected abstract Object handleInvocation(Object proxy, Method method, Object[] args)
099      throws Throwable;
100
101  /**
102   * By default delegates to {@link Object#equals} so instances are only equal if they are
103   * identical. {@code proxy.equals(argument)} returns true if: <ul>
104   * <li>{@code proxy} and {@code argument} are of the same type
105   * <li>and this method returns true for the {@link InvocationHandler} of {@code argument}
106   * </ul>
107   * <p>Subclasses can override this method to provide custom equality.
108   */
109  @Override public boolean equals(Object obj) {
110    return super.equals(obj);
111  }
112
113  /**
114   * By default delegates to {@link Object#hashCode}. The dynamic proxies' {@code hashCode()} will
115   * delegate to this method. Subclasses can override this method to provide custom equality.
116   */
117  @Override public int hashCode() {
118    return super.hashCode();
119  }
120
121  /**
122   * By default delegates to {@link Object#toString}. The dynamic proxies' {@code toString()} will
123   * delegate to this method. Subclasses can override this method to provide custom string
124   * representation for the proxies.
125   */
126  @Override public String toString() {
127    return super.toString();
128  }
129
130  private static boolean isProxyOfSameInterfaces(Object arg, Class<?> proxyClass) {
131    return proxyClass.isInstance(arg)
132        // Equal proxy instances should mostly be instance of proxyClass
133        // Under some edge cases (such as the proxy of JDK types serialized and then deserialized)
134        // the proxy type may not be the same.
135        // We first check isProxyClass() so that the common case of comparing with non-proxy objects
136        // is efficient.
137        || (Proxy.isProxyClass(arg.getClass())
138            && Arrays.equals(arg.getClass().getInterfaces(), proxyClass.getInterfaces()));
139  }
140}