1 /*
2 * Copyright (c) 1997, 2022, 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 *
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 * @param <T> the type of the referent
60 *
61 * @author Mark Reinhold
62 * @since 1.2
63 */
64
65 public non-sealed class SoftReference<T> extends Reference<T> {
66
67 /**
68 * Timestamp clock, updated by the garbage collector
69 */
70 private static long clock;
71
72 /**
73 * Timestamp updated by each invocation of the get method. The VM may use
74 * this field when selecting soft references to be cleared, but it is not
75 * required to do so.
76 */
77 private long timestamp;
78
79 /**
80 * Creates a new soft reference that refers to the given object. The new
81 * reference is not registered with any queue.
82 *
83 * @param referent object the new soft reference will refer to
84 */
85 public SoftReference(T referent) {
86 super(referent);
87 this.timestamp = clock;
88 }
89
90 /**
91 * Creates a new soft reference that refers to the given object and is
92 * registered with the given queue.
93 *
94 * @param referent object the new soft reference will refer to
95 * @param q the queue with which the reference is to be registered,
96 * or {@code null} if registration is not required
97 *
98 */
99 public SoftReference(T referent, ReferenceQueue<? super T> q) {
100 super(referent, q);
101 this.timestamp = clock;
102 }
103
104 /**
105 * Returns this reference object's referent. If this reference object has
106 * been cleared, either by the program or by the garbage collector, then
107 * this method returns {@code null}.
108 *
109 * @return The object to which this reference refers, or
110 * {@code null} if this reference object has been cleared
111 */
112 public T get() {
113 T o = super.get();
114 if (o != null && this.timestamp != clock)
115 this.timestamp = clock;
116 return o;
117 }
118
119 }