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1   /*
2    * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4    *
5    * This code is free software; you can redistribute it and/or modify it
6    * under the terms of the GNU General Public License version 2 only, as
7    * published by the Free Software Foundation.  Oracle designates this
8    * particular file as subject to the "Classpath" exception as provided
9    * by Oracle in the LICENSE file that accompanied this code.
10   *
11   * This code is distributed in the hope that it will be useful, but WITHOUT
12   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14   * version 2 for more details (a copy is included in the LICENSE file that
15   * accompanied this code).
16   *
17   * You should have received a copy of the GNU General Public License version
18   * 2 along with this work; if not, write to the Free Software Foundation,
19   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20   *
21   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22   * or visit www.oracle.com if you need additional information or have any
23   * questions.
24   */
25  
26  package com.sun.istack.internal;
27  
28  import java.util.concurrent.ConcurrentLinkedQueue;
29  import java.lang.ref.WeakReference;
30  
31  /**
32   * Pool of reusable objects that are indistinguishable from each other,
33   * such as JAXB marshallers.
34   *
35   * @author Kohsuke Kawaguchi
36   */
37  public interface Pool<T> {
38  
39      /**
40       * Gets a new object from the pool.
41       *
42       * <p>
43       * If no object is available in the pool, this method creates a new one.
44       */
45      @NotNull T take();
46  
47      /**
48       * Returns an object back to the pool.
49       */
50      void recycle(@NotNull T t);
51  
52      /**
53       * Default implementation that uses {@link ConcurrentLinkedQueue}
54       * as the data store.
55       *
56       * <h2>Note for Implementors</h2>
57       * <p>
58       * Don't rely on the fact that this class extends from {@link ConcurrentLinkedQueue}.
59       */
60      public abstract class Impl<T> implements Pool<T> {
61  
62          private volatile WeakReference<ConcurrentLinkedQueue<T>> queue;
63  
64          /**
65           * Gets a new object from the pool.
66           *
67           * <p>
68           * If no object is available in the pool, this method creates a new one.
69           *
70           * @return
71           *      always non-null.
72           */
73          public final @NotNull T take() {
74              T t = getQueue().poll();
75              if(t==null) {
76                  return create();
77              }
78              return t;
79          }
80  
81          /**
82           * Returns an object back to the pool.
83           */
84          public final void recycle(T t) {
85              getQueue().offer(t);
86          }
87  
88          private ConcurrentLinkedQueue<T> getQueue() {
89              WeakReference<ConcurrentLinkedQueue<T>> q = queue;
90              if (q != null) {
91                  ConcurrentLinkedQueue<T> d = q.get();
92                  if (d != null) {
93                      return d;
94                  }
95              }
96              // overwrite the queue
97              ConcurrentLinkedQueue<T> d = new ConcurrentLinkedQueue<T>();
98              queue = new WeakReference<ConcurrentLinkedQueue<T>>(d);
99  
100             return d;
101         }
102 
103         /**
104          * Creates a new instance of object.
105          *
106          * <p>
107          * This method is used when someone wants to
108          * {@link #take() take} an object from an empty pool.
109          *
110          * <p>
111          * Also note that multiple threads may call this method
112          * concurrently.
113          */
114         protected abstract @NotNull T create();
115     }
116 }