001/*
002 * Copyright (C) 2012 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.reflect;
016
017import com.google.common.annotations.Beta;
018import java.lang.reflect.InvocationHandler;
019import java.lang.reflect.Method;
020import java.lang.reflect.Proxy;
021import java.util.Arrays;
022import org.checkerframework.checker.nullness.compatqual.NullableDecl;
023
024/**
025 * Abstract implementation of {@link InvocationHandler} that handles {@link Object#equals}, {@link
026 * Object#hashCode} and {@link Object#toString}. For example:
027 *
028 * <pre>
029 * class Unsupported extends AbstractInvocationHandler {
030 *   protected Object handleInvocation(Object proxy, Method method, Object[] args) {
031 *     throw new UnsupportedOperationException();
032 *   }
033 * }
034 *
035 * CharSequence unsupported = Reflection.newProxy(CharSequence.class, new Unsupported());
036 * </pre>
037 *
038 * @author Ben Yu
039 * @since 12.0
040 */
041@Beta
042public abstract class AbstractInvocationHandler implements InvocationHandler {
043
044  private static final Object[] NO_ARGS = {};
045
046  /**
047   * {@inheritDoc}
048   *
049   * <ul>
050   *   <li>{@code proxy.hashCode()} delegates to {@link AbstractInvocationHandler#hashCode}
051   *   <li>{@code proxy.toString()} delegates to {@link AbstractInvocationHandler#toString}
052   *   <li>{@code proxy.equals(argument)} returns true if:
053   *       <ul>
054   *         <li>{@code proxy} and {@code argument} are of the same type
055   *         <li>and {@link AbstractInvocationHandler#equals} returns true for the {@link
056   *             InvocationHandler} of {@code argument}
057   *       </ul>
058   *   <li>other method calls are dispatched to {@link #handleInvocation}.
059   * </ul>
060   */
061  @Override
062  public final Object invoke(Object proxy, Method method, @NullableDecl Object[] args)
063      throws Throwable {
064    if (args == null) {
065      args = NO_ARGS;
066    }
067    if (args.length == 0 && method.getName().equals("hashCode")) {
068      return hashCode();
069    }
070    if (args.length == 1
071        && method.getName().equals("equals")
072        && method.getParameterTypes()[0] == Object.class) {
073      Object arg = args[0];
074      if (arg == null) {
075        return false;
076      }
077      if (proxy == arg) {
078        return true;
079      }
080      return isProxyOfSameInterfaces(arg, proxy.getClass())
081          && equals(Proxy.getInvocationHandler(arg));
082    }
083    if (args.length == 0 && method.getName().equals("toString")) {
084      return toString();
085    }
086    return handleInvocation(proxy, method, args);
087  }
088
089  /**
090   * {@link #invoke} delegates to this method upon any method invocation on the proxy instance,
091   * except {@link Object#equals}, {@link Object#hashCode} and {@link Object#toString}. The result
092   * will be returned as the proxied method's return value.
093   *
094   * <p>Unlike {@link #invoke}, {@code args} will never be null. When the method has no parameter,
095   * an empty array is passed in.
096   */
097  protected abstract Object handleInvocation(Object proxy, Method method, Object[] args)
098      throws Throwable;
099
100  /**
101   * By default delegates to {@link Object#equals} so instances are only equal if they are
102   * identical. {@code proxy.equals(argument)} returns true if:
103   *
104   * <ul>
105   *   <li>{@code proxy} and {@code argument} are of the same type
106   *   <li>and this method returns true for the {@link InvocationHandler} of {@code argument}
107   * </ul>
108   *
109   * <p>Subclasses can override this method to provide custom equality.
110   */
111  @Override
112  public boolean equals(Object obj) {
113    return super.equals(obj);
114  }
115
116  /**
117   * By default delegates to {@link Object#hashCode}. The dynamic proxies' {@code hashCode()} will
118   * delegate to this method. Subclasses can override this method to provide custom equality.
119   */
120  @Override
121  public int hashCode() {
122    return super.hashCode();
123  }
124
125  /**
126   * By default delegates to {@link Object#toString}. The dynamic proxies' {@code toString()} will
127   * delegate to this method. Subclasses can override this method to provide custom string
128   * representation for the proxies.
129   */
130  @Override
131  public String toString() {
132    return super.toString();
133  }
134
135  private static boolean isProxyOfSameInterfaces(Object arg, Class<?> proxyClass) {
136    return proxyClass.isInstance(arg)
137        // Equal proxy instances should mostly be instance of proxyClass
138        // Under some edge cases (such as the proxy of JDK types serialized and then deserialized)
139        // the proxy type may not be the same.
140        // We first check isProxyClass() so that the common case of comparing with non-proxy objects
141        // is efficient.
142        || (Proxy.isProxyClass(arg.getClass())
143            && Arrays.equals(arg.getClass().getInterfaces(), proxyClass.getInterfaces()));
144  }
145}