SoftReference.java revision 12745:f068a4ffddd2 
1/*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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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).
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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25
26package 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}.  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
64public class SoftReference<T> extends Reference<T> {
65
66    /**
67     * Timestamp clock, updated by the garbage collector
68     */
69    private static 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 {@code null} 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}.
107     *
108     * @return   The object to which this reference refers, or
109     *           {@code null} 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}
119