AbstractPool


1   /*****************************************
2    *                                       *
3    *  JBoss Portal: The OpenSource Portal  *
4    *                                       *
5    *       Forums JBoss Portlet            *
6    *                                       *
7    *   Distributable under GPL license.    *
8    *   See terms of license at gnu.org.    *
9    *                                       *
10   *****************************************/
11  package org.jboss.portlet.forums.helper;
12  
13  
14  /**
15   * A simple object pool designed for extensibility.
16   * There is no resizer thread so far.
17   *
18   * @author <a HREF="mailto:julien@jboss.org">Julien Viet</a>
19   * @version $Revision: 1.1.1.1 $
20   */
21  public abstract class AbstractPool
22  {
23     // Attributes ----------------------------------------------------
24  
25     /** The lock. */
26     private final Object   lock = new Object  ();
27  
28     /** Pool stack. */
29     private Object  [] stack;
30  
31     /** Pool index. */
32     private int index;
33  
34     // Constructors --------------------------------------------------
35     protected AbstractPool()
36     {
37     }
38  
39     // Public --------------------------------------------------------
40  
41     /**
42      * Returns an object instance. If the Pool is empty
43      * a new object will be created.
44      *
45      * @return A pooled instance.
46      */
47     public Object   acquire()
48     {
49        // The instance we return
50        Object   instance = null;
51  
52        // Synchronize the stack to keep a consistent state
53        synchronized (lock)
54        {
55           if (index > -1)
56           {
57              instance          = stack[index];
58              stack[index--]    = null;
59           }
60        }
61  
62        // If no instance where available create one
63        if (instance == null)
64        {
65           instance = _create();
66        }
67  
68        // Set the object acquired
69        _acquire(instance);
70  
71        // Finally return the instance
72        return instance;
73     }
74  
75     /**
76      * Release an object and pools it if the pool is
77      * not full, otherwise discard it.
78      *
79      * @param instance the released instance.
80      */
81     public void release(Object   instance)
82     {
83        if (instance == null)
84        {
85           // This is a bug from the user of that pool
86           // That should not release a null object
87           return;
88        }
89  
90        // First release the instance
91        _release(instance);
92  
93        // Keep object state consistent
94        synchronized (lock)
95        {
96           if (index < (stack.length - 1))
97           {
98              stack[++index]    = instance;
99              instance          = null;
100          }
101       }
102 
103       // If the instance is not null it is not in the
104       // pool anymore, so destroy it
105       if (instance != null)
106       {
107          _destroy(instance);
108       }
109    }
110 
111    // Protected -----------------------------------------------------
112 
113    /**
114     * Initialize the pool.
115     *
116     * @param maxSize the max size of object in the pool
117     * @param initialSize the initial size. A negative value is
118     * considered as zero, a value greater than maxSize is
119     * considered as maxSize.
120     */
121    protected void initialize(int maxSize,
122                              int initialSize)
123    {
124       // Arguments check
125       if (maxSize < 1)
126       {
127          throw new IllegalArgumentException  ("maxSize must be greater than 0");
128       }
129 
130       initialSize    = (initialSize < 0) ? 0 : initialSize;
131       initialSize    = (initialSize > maxSize) ? maxSize : initialSize;
132 
133       // Initialization
134       stack         = new Object  [maxSize];
135       this.index    = -1;
136       while (initialSize-- > 0)
137       {
138          stack[++index] = _create();
139       }
140    }
141 
142    /**
143     * The method does nothing bu default,
144     * called when the client release an instance.
145     */
146    protected void _destroy(Object   instance)
147    {
148    }
149 
150    /**
151     * The method does nothing bu default,
152     * called when the client release an instance.
153     */
154    protected void _acquire(Object   instance)
155    {
156    }
157 
158    /**
159     * The method does nothing bu default,
160     * called when the client release an instance.
161     */
162    protected void _release(Object   instance)
163    {
164    }
165 
166    // Abstract ------------------------------------------------------
167 
168    /**
169     * Called when the pool need a new instance.
170     */
171    protected abstract Object   _create();
172 }
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