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1   /*
2    * Copyright (c) 1997, 2003, 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
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24   */
25  
26  package java.lang.ref;
27  
28  
29  /**
30   * Soft reference objects, which are cleared at the discretion of the garbage
31   * collector in response to memory demand.  Soft references are most often used
32   * to implement memory-sensitive caches.
33   *
34   * <p> Suppose that the garbage collector determines at a certain point in time
35   * that an object is <a href="package-summary.html#reachability">softly
36   * reachable</a>.  At that time it may choose to clear atomically all soft
37   * references to that object and all soft references to any other
38   * softly-reachable objects from which that object is reachable through a chain
39   * of strong references.  At the same time or at some later time it will
40   * enqueue those newly-cleared soft references that are registered with
41   * reference queues.
42   *
43   * <p> All soft references to softly-reachable objects are guaranteed to have
44   * been cleared before the virtual machine throws an
45   * <code>OutOfMemoryError</code>.  Otherwise no constraints are placed upon the
46   * time at which a soft reference will be cleared or the order in which a set
47   * of such references to different objects will be cleared.  Virtual machine
48   * implementations are, however, encouraged to bias against clearing
49   * recently-created or recently-used soft references.
50   *
51   * <p> Direct instances of this class may be used to implement simple caches;
52   * this class or derived subclasses may also be used in larger data structures
53   * to implement more sophisticated caches.  As long as the referent of a soft
54   * reference is strongly reachable, that is, is actually in use, the soft
55   * reference will not be cleared.  Thus a sophisticated cache can, for example,
56   * prevent its most recently used entries from being discarded by keeping
57   * strong referents to those entries, leaving the remaining entries to be
58   * discarded at the discretion of the garbage collector.
59   *
60   * @author   Mark Reinhold
61   * @since    1.2
62   */
63  
64  public class SoftReference<T> extends Reference<T> {
65  
66      /**
67       * Timestamp clock, updated by the garbage collector
68       */
69      static private long clock;
70  
71      /**
72       * Timestamp updated by each invocation of the get method.  The VM may use
73       * this field when selecting soft references to be cleared, but it is not
74       * required to do so.
75       */
76      private long timestamp;
77  
78      /**
79       * Creates a new soft reference that refers to the given object.  The new
80       * reference is not registered with any queue.
81       *
82       * @param referent object the new soft reference will refer to
83       */
84      public SoftReference(T referent) {
85          super(referent);
86          this.timestamp = clock;
87      }
88  
89      /**
90       * Creates a new soft reference that refers to the given object and is
91       * registered with the given queue.
92       *
93       * @param referent object the new soft reference will refer to
94       * @param q the queue with which the reference is to be registered,
95       *          or <tt>null</tt> if registration is not required
96       *
97       */
98      public SoftReference(T referent, ReferenceQueue<? super T> q) {
99          super(referent, q);
100         this.timestamp = clock;
101     }
102 
103     /**
104      * Returns this reference object's referent.  If this reference object has
105      * been cleared, either by the program or by the garbage collector, then
106      * this method returns <code>null</code>.
107      *
108      * @return   The object to which this reference refers, or
109      *           <code>null</code> if this reference object has been cleared
110      */
111     public T get() {
112         T o = super.get();
113         if (o != null && this.timestamp != clock)
114             this.timestamp = clock;
115         return o;
116     }
117 
118 }