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.proxy2.provider;
19
20 import org.apache.commons.proxy2.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 * @since 1.0
27 */
28 public class SingletonProvider<T> extends ProviderDecorator<T>
29 {
30 //******************************************************************************************************************
31 // Fields
32 //******************************************************************************************************************
33
34 private static final long serialVersionUID = 1L;
35
36 private T instance;
37
38 //******************************************************************************************************************
39 // Constructors
40 //******************************************************************************************************************
41
42 /**
43 * Create a new SingletonProvider instance.
44 *
45 * @param inner
46 */
47 public SingletonProvider(ObjectProvider<? extends T> inner)
48 {
49 super(inner);
50 }
51
52 //******************************************************************************************************************
53 // ObjectProvider Implementation
54 //******************************************************************************************************************
55
56 /**
57 * {@inheritDoc}
58 */
59 @Override
60 public T getObject()
61 {
62 synchronized (this)
63 {
64 if (instance == null)
65 {
66 instance = super.getObject();
67 // Garbage collection
68 setInner(null);
69 }
70 return instance;
71 }
72 }
73 }