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Java > Open Source Codes > java > lang > ref > SoftReference


1 /*
2  * @(#)SoftReference.java 1.34 03/12/19
3  *
4  * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5  * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6  */
7
8 package java.lang.ref;
9
10
11 /**
12  * Soft reference objects, which are cleared at the discretion of the garbage
13  * collector in response to memory demand. Soft references are most often used
14  * to implement memory-sensitive caches.
15  *
16  * <p> Suppose that the garbage collector determines at a certain point in time
17  * that an object is <a HREF="package-summary.html#reachability">softly
18  * reachable</a>. At that time it may choose to clear atomically all soft
19  * references to that object and all soft references to any other
20  * softly-reachable objects from which that object is reachable through a chain
21  * of strong references. At the same time or at some later time it will
22  * enqueue those newly-cleared soft references that are registered with
23  * reference queues.
24  *
25  * <p> All soft references to softly-reachable objects are guaranteed to have
26  * been cleared before the virtual machine throws an
27  * <code>OutOfMemoryError</code>. Otherwise no constraints are placed upon the
28  * time at which a soft reference will be cleared or the order in which a set
29  * of such references to different objects will be cleared. Virtual machine
30  * implementations are, however, encouraged to bias against clearing
31  * recently-created or recently-used soft references.
32  *
33  * <p> Direct instances of this class may be used to implement simple caches;
34  * this class or derived subclasses may also be used in larger data structures
35  * to implement more sophisticated caches. As long as the referent of a soft
36  * reference is strongly reachable, that is, is actually in use, the soft
37  * reference will not be cleared. Thus a sophisticated cache can, for example,
38  * prevent its most recently used entries from being discarded by keeping
39  * strong referents to those entries, leaving the remaining entries to be
40  * discarded at the discretion of the garbage collector.
41  *
42  * @version 1.34, 12/19/03
43  * @author Mark Reinhold
44  * @since 1.2
45  */
46
47 public class SoftReference<T> extends Reference JavaDoc<T> {
48
49     /* Timestamp clock, updated by the garbage collector
50      */
51     static private long clock;
52
53     /* Timestamp updated by each invocation of the get method. The VM may use
54      * this field when selecting soft references to be cleared, but it is not
55      * required to do so.
56      */
57     private long timestamp;
58
59     /**
60      * Creates a new soft reference that refers to the given object. The new
61      * reference is not registered with any queue.
62      *
63      * @param referent object the new soft reference will refer to
64      */
65     public SoftReference(T referent) {
66     super(referent);
67     this.timestamp = clock;
68     }
69
70     /**
71      * Creates a new soft reference that refers to the given object and is
72      * registered with the given queue.
73      *
74      * @param referent object the new soft reference will refer to
75      * @param q the queue with which the reference is to be registered,
76      * or <tt>null</tt> if registration is not required
77      *
78      */
79     public SoftReference(T referent, ReferenceQueue JavaDoc<? super T> q) {
80     super(referent, q);
81     this.timestamp = clock;
82     }
83
84     /**
85      * Returns this reference object's referent. If this reference object has
86      * been cleared, either by the program or by the garbage collector, then
87      * this method returns <code>null</code>.
88      *
89      * @return The object to which this reference refers, or
90      * <code>null</code> if this reference object has been cleared
91      */
92     public T get() {
93     T o = super.get();
94     if (o != null) this.timestamp = clock;
95     return o;
96     }
97
98 }
99
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