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 java.lang.reflect.InvocationHandler;
018import java.lang.reflect.Method;
019import java.lang.reflect.Proxy;
020import java.util.Arrays;
021import javax.annotation.CheckForNull;
022import org.checkerframework.checker.nullness.qual.Nullable;
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 */
041public abstract class AbstractInvocationHandler implements InvocationHandler {
042  /** Constructor for use by subclasses. */
043  public AbstractInvocationHandler() {}
044
045  private static final Object[] NO_ARGS = {};
046
047  /**
048   * {@inheritDoc}
049   *
050   * <ul>
051   *   <li>{@code proxy.hashCode()} delegates to {@link AbstractInvocationHandler#hashCode}
052   *   <li>{@code proxy.toString()} delegates to {@link AbstractInvocationHandler#toString}
053   *   <li>{@code proxy.equals(argument)} returns true if:
054   *       <ul>
055   *         <li>{@code proxy} and {@code argument} are of the same type
056   *         <li>and {@link AbstractInvocationHandler#equals} returns true for the {@link
057   *             InvocationHandler} of {@code argument}
058   *       </ul>
059   *   <li>other method calls are dispatched to {@link #handleInvocation}.
060   * </ul>
061   */
062  @Override
063  @CheckForNull
064  public final Object invoke(Object proxy, Method method, @CheckForNull @Nullable Object[] args)
065      throws Throwable {
066    if (args == null) {
067      args = NO_ARGS;
068    }
069    if (args.length == 0 && method.getName().equals("hashCode")) {
070      return hashCode();
071    }
072    if (args.length == 1
073        && method.getName().equals("equals")
074        && method.getParameterTypes()[0] == Object.class) {
075      Object arg = args[0];
076      if (arg == null) {
077        return false;
078      }
079      if (proxy == arg) {
080        return true;
081      }
082      return isProxyOfSameInterfaces(arg, proxy.getClass())
083          && equals(Proxy.getInvocationHandler(arg));
084    }
085    if (args.length == 0 && method.getName().equals("toString")) {
086      return toString();
087    }
088    return handleInvocation(proxy, method, args);
089  }
090
091  /**
092   * {@link #invoke} delegates to this method upon any method invocation on the proxy instance,
093   * except {@link Object#equals}, {@link Object#hashCode} and {@link Object#toString}. The result
094   * will be returned as the proxied method's return value.
095   *
096   * <p>Unlike {@link #invoke}, {@code args} will never be null. When the method has no parameter,
097   * an empty array is passed in.
098   */
099  @CheckForNull
100  protected abstract Object handleInvocation(Object proxy, Method method, @Nullable Object[] args)
101      throws Throwable;
102
103  /**
104   * By default delegates to {@link Object#equals} so instances are only equal if they are
105   * identical. {@code proxy.equals(argument)} returns true if:
106   *
107   * <ul>
108   *   <li>{@code proxy} and {@code argument} are of the same type
109   *   <li>and this method returns true for the {@link InvocationHandler} of {@code argument}
110   * </ul>
111   *
112   * <p>Subclasses can override this method to provide custom equality.
113   */
114  @Override
115  public boolean equals(@CheckForNull Object obj) {
116    return super.equals(obj);
117  }
118
119  /**
120   * By default delegates to {@link Object#hashCode}. The dynamic proxies' {@code hashCode()} will
121   * delegate to this method. Subclasses can override this method to provide custom equality.
122   */
123  @Override
124  public int hashCode() {
125    return super.hashCode();
126  }
127
128  /**
129   * By default delegates to {@link Object#toString}. The dynamic proxies' {@code toString()} will
130   * delegate to this method. Subclasses can override this method to provide custom string
131   * representation for the proxies.
132   */
133  @Override
134  public String toString() {
135    return super.toString();
136  }
137
138  private static boolean isProxyOfSameInterfaces(Object arg, Class<?> proxyClass) {
139    return proxyClass.isInstance(arg)
140        // Equal proxy instances should mostly be instance of proxyClass
141        // Under some edge cases (such as the proxy of JDK types serialized and then deserialized)
142        // the proxy type may not be the same.
143        // We first check isProxyClass() so that the common case of comparing with non-proxy objects
144        // is efficient.
145        || (Proxy.isProxyClass(arg.getClass())
146            && Arrays.equals(arg.getClass().getInterfaces(), proxyClass.getInterfaces()));
147  }
148}