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
22 * or visit www.oracle.com if you need additional information or have any
23 * 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 * <p>
34 * The referent must not be an instance of an inline class; such a value
35 * can never have another reference to it and cannot be held in a reference type.
36 *
37 * <p> Suppose that the garbage collector determines at a certain point in time
38 * that an object is <a href="package-summary.html#reachability">softly
39 * reachable</a>. At that time it may choose to clear atomically all soft
40 * references to that object and all soft references to any other
41 * softly-reachable objects from which that object is reachable through a chain
42 * of strong references. At the same time or at some later time it will
43 * enqueue those newly-cleared soft references that are registered with
44 * reference queues.
45 *
46 * <p> All soft references to softly-reachable objects are guaranteed to have
47 * been cleared before the virtual machine throws an
48 * {@code OutOfMemoryError}. Otherwise no constraints are placed upon the
49 * time at which a soft reference will be cleared or the order in which a set
50 * of such references to different objects will be cleared. Virtual machine
51 * implementations are, however, encouraged to bias against clearing
52 * recently-created or recently-used soft references.
53 *
54 * <p> Direct instances of this class may be used to implement simple caches;
55 * this class or derived subclasses may also be used in larger data structures
56 * to implement more sophisticated caches. As long as the referent of a soft
57 * reference is strongly reachable, that is, is actually in use, the soft
58 * reference will not be cleared. Thus a sophisticated cache can, for example,
59 * prevent its most recently used entries from being discarded by keeping
60 * strong referents to those entries, leaving the remaining entries to be
61 * discarded at the discretion of the garbage collector.
62 *
63 * @author Mark Reinhold
64 * @since 1.2
65 */
66
67 public class SoftReference<T> extends Reference<T> {
68
69 /**
70 * Timestamp clock, updated by the garbage collector
71 */
72 private static long clock;
73
74 /**
75 * Timestamp updated by each invocation of the get method. The VM may use
76 * this field when selecting soft references to be cleared, but it is not
77 * required to do so.
78 */
79 private long timestamp;
80
81 /**
82 * Creates a new soft reference that refers to the given object. The new
83 * reference is not registered with any queue.
84 *
85 * @param referent object the new soft reference will refer to
86 * @throws IllegalArgumentException if the referent is an instance of an
87 * {@link Class#isInlineClass() inlineClass}
88 */
89 public SoftReference(T referent) {
90 super(referent);
91 this.timestamp = clock;
92 }
93
94 /**
95 * Creates a new soft reference that refers to the given object and is
96 * registered with the given queue.
97 *
98 * @param referent object the new soft reference will refer to
99 * @param q the queue with which the reference is to be registered,
100 * or {@code null} if registration is not required
101 * @throws IllegalArgumentException if the referent is an instance of an
102 * {@link Class#isInlineClass() inlineClass}
103 */
104 public SoftReference(T referent, ReferenceQueue<? super T> q) {
105 super(referent, q);
106 this.timestamp = clock;
107 }
108
109 /**
110 * Returns this reference object's referent. If this reference object has
111 * been cleared, either by the program or by the garbage collector, then
112 * this method returns {@code null}.
113 *
114 * @return The object to which this reference refers, or
115 * {@code null} if this reference object has been cleared
116 */
117 public T get() {
118 T o = super.get();
119 if (o != null && this.timestamp != clock)
120 this.timestamp = clock;
121 return o;
122 }
123
124 }