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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  
18  package org.apache.commons.proxy.invoker;
19  
20  import org.apache.commons.proxy.Invoker;
21  import org.apache.commons.proxy.ObjectProvider;
22  
23  import java.lang.reflect.Method;
24  
25  /**
26   * An invoker which supports <a href="http://en.wikipedia.org/wiki/Duck_typing">&quot;duck typing&quot;</a>, meaning
27   * that it finds a matching method on the object returned from the target provider and invokes it.  This class is
28   * useful for adapting an existing class to an interface it does not implement.
29   * <p>
30   * <b>Example:</b>
31   * </p>
32   * <p>
33   * <pre>
34   * public class LegacyDuck // Does not implement interface!
35   * {
36   *   public void quack()
37   *   {
38   *     // Quacking logic...
39   *   }
40   * }
41   * <p/>
42   * public interface Duck
43   * {
44   *   public void quack();
45   * }
46   * <p/>
47   * ObjectProvider targetProvider = new ConstantProvider(new LegacyDuck()); // Always returns a "legacy" duck
48   * DuckTypingInvoker invoker = new DuckTypingInvoker(targetProvider);
49   * Duck duck = ( Duck )proxyFactory.createInvokerProxy( invoker, new Class[] { Duck.class } );
50   * </pre>
51   * </p>
52   */
53  public class DuckTypingInvoker implements Invoker
54  {
55  //----------------------------------------------------------------------------------------------------------------------
56  // Fields
57  //----------------------------------------------------------------------------------------------------------------------
58  
59      private final ObjectProvider targetProvider;
60  
61  //----------------------------------------------------------------------------------------------------------------------
62  // Constructors
63  //----------------------------------------------------------------------------------------------------------------------
64  
65      public DuckTypingInvoker( final ObjectProvider targetProvider )
66      {
67          this.targetProvider = targetProvider;
68      }
69  
70  //----------------------------------------------------------------------------------------------------------------------
71  // Interface Invoker
72  //----------------------------------------------------------------------------------------------------------------------
73  
74      public Object invoke( final Object proxy, final Method method, final Object[] arguments ) throws Throwable
75      {
76          final Object target = targetProvider.getObject();
77          final Class targetClass = target.getClass();
78          try
79          {
80              final Method targetMethod = targetClass.getMethod( method.getName(), method.getParameterTypes() );
81              if ( method.getReturnType().isAssignableFrom( targetMethod.getReturnType() ) )
82              {
83                  return targetMethod.invoke( target, arguments );
84              }
85              throw new UnsupportedOperationException(
86                      "Target type " + targetClass.getName() + " method has incompatible return type." );
87          }
88          catch ( NoSuchMethodException e )
89          {
90              throw new UnsupportedOperationException(
91                      "Target type " + targetClass.getName() + " does not have a method matching " + method + "." );
92          }
93      }
94  }