1 /*
2 * Copyright 1997-2003 Sun Microsystems, Inc. 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. Sun designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
22 * CA 95054 USA or visit www.sun.com if you need additional information or
23 * have any questions.
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 /* Timestamp clock, updated by the garbage collector
67 */
68 static private long clock;
69
70 /* Timestamp updated by each invocation of the get method. The VM may use
71 * this field when selecting soft references to be cleared, but it is not
72 * required to do so.
73 */
74 private long timestamp;
75
76 /**
77 * Creates a new soft reference that refers to the given object. The new
78 * reference is not registered with any queue.
79 *
80 * @param referent object the new soft reference will refer to
81 */
82 public SoftReference(T referent) {
83 super(referent);
84 this.timestamp = clock;
85 }
86
87 /**
88 * Creates a new soft reference that refers to the given object and is
89 * registered with the given queue.
90 *
91 * @param referent object the new soft reference will refer to
92 * @param q the queue with which the reference is to be registered,
93 * or <tt>null</tt> if registration is not required
94 *
95 */
96 public SoftReference(T referent, ReferenceQueue<? super T> q) {
97 super(referent, q);
98 this.timestamp = clock;
99 }
100
101 /**
102 * Returns this reference object's referent. If this reference object has
103 * been cleared, either by the program or by the garbage collector, then
104 * this method returns <code>null</code>.
105 *
106 * @return The object to which this reference refers, or
107 * <code>null</code> if this reference object has been cleared
108 */
109 public T get() {
110 T o = super.get();
111 if (o != null) this.timestamp = clock;
112 return o;
113 }
114
115 }
Save This Page