1/*
2 * Copyright (C) 2012 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.google.common.reflect;
18
19import com.google.common.annotations.Beta;
20
21import java.lang.reflect.InvocationHandler;
22import java.lang.reflect.Method;
23import java.lang.reflect.Proxy;
24import java.util.Arrays;
25
26import javax.annotation.Nullable;
27
28/**
29 * Abstract implementation of {@link InvocationHandler} that handles {@link Object#equals},
30 * {@link Object#hashCode} and {@link Object#toString}. For example: <pre>
31 * class Unsupported extends AbstractInvocationHandler {
32 *   protected Object handleInvocation(
33 *       Object proxy, Method method, Object[] args) {
34 *     throw new UnsupportedOperationException();
35 *   }
36 * }
37 *
38 * CharSequence unsupported = Reflection.newProxy(CharSequence.class, new Unsupported());
39 * </pre>
40 *
41 * @author Ben Yu
42 * @since 12.0
43 */
44@Beta
45public abstract class AbstractInvocationHandler implements InvocationHandler {
46
47  private static final Object[] NO_ARGS = {};
48
49  /**
50   * {@inheritDoc}
51   *
52   * <p><ul>
53   * <li>{@code proxy.hashCode()} delegates to {@link AbstractInvocationHandler#hashCode}
54   * <li>{@code proxy.toString()} delegates to {@link AbstractInvocationHandler#toString}
55   * <li>{@code proxy.equals(argument)} returns true if: <ul>
56   *   <li>{@code proxy} and {@code argument} are of the same type
57   *   <li>and {@link AbstractInvocationHandler#equals} returns true for the {@link
58   *       InvocationHandler} of {@code argument}
59   *   </ul>
60   * <li>other method calls are dispatched to {@link #handleInvocation}.
61   * </ul>
62   */
63  @Override public final Object invoke(Object proxy, Method method, @Nullable Object[] args)
64      throws Throwable {
65    if (args == null) {
66      args = NO_ARGS;
67    }
68    if (args.length == 0 && method.getName().equals("hashCode")) {
69      return hashCode();
70    }
71    if (args.length == 1
72        && method.getName().equals("equals")
73        && method.getParameterTypes()[0] == Object.class) {
74      Object arg = args[0];
75      if (arg == null) {
76        return false;
77      }
78      if (proxy == arg) {
79        return true;
80      }
81      return isProxyOfSameInterfaces(arg, proxy.getClass())
82          && equals(Proxy.getInvocationHandler(arg));
83    }
84    if (args.length == 0 && method.getName().equals("toString")) {
85      return toString();
86    }
87    return handleInvocation(proxy, method, args);
88  }
89
90  /**
91   * {@link #invoke} delegates to this method upon any method invocation on the proxy instance,
92   * except {@link Object#equals}, {@link Object#hashCode} and {@link Object#toString}. The result
93   * will be returned as the proxied method's return value.
94   *
95   * <p>Unlike {@link #invoke}, {@code args} will never be null. When the method has no parameter,
96   * an empty array is passed in.
97   */
98  protected abstract Object handleInvocation(Object proxy, Method method, Object[] args)
99      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}
141