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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.provider;
19  
20  import org.apache.commons.proxy.ObjectProvider;
21  
22  /**
23   * Wraps another object provider, making sure to only call it once, returning the value returned from the wrapped
24   * provider on all subsequent invocations.
25   *
26   * @author James Carman
27   * @since 1.0
28   */
29  public class SingletonProvider extends ProviderDecorator
30  {
31  //----------------------------------------------------------------------------------------------------------------------
32  // Fields
33  //----------------------------------------------------------------------------------------------------------------------
34  
35      private Object instance;
36  
37  //----------------------------------------------------------------------------------------------------------------------
38  // Constructors
39  //----------------------------------------------------------------------------------------------------------------------
40  
41      public SingletonProvider( ObjectProvider inner )
42      {
43          super( inner );
44      }
45  
46  //----------------------------------------------------------------------------------------------------------------------
47  // ObjectProvider Implementation
48  //----------------------------------------------------------------------------------------------------------------------
49  
50      public Object getObject()
51      {
52          synchronized( this )
53          {
54              if( instance == null )
55              {
56                  instance = super.getObject();
57                  inner = null; // Garbage collection
58              }
59          }
60          return instance;
61      }
62  }
63