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 import java.util.Objects;
30
31 /**
32 * Soft reference objects, which are cleared at the discretion of the garbage
33 * collector in response to memory demand. Soft references are most often used
34 * to implement memory-sensitive caches.
35 *
36 * <div class="preview-block">
37 * <div class="preview-comment">
38 * The referent must have {@linkplain Objects#hasIdentity(Object) object identity}.
39 * When preview features are enabled, attempts to create a reference
40 * to a {@linkplain Class#isValue value object} result in an {@link IdentityException}.
41 * </div>
42 * </div>
43 *
44 * <p> Suppose that the garbage collector determines at a certain point in time
45 * that an object is <a href="package-summary.html#reachability">softly
46 * reachable</a>. At that time it may choose to clear atomically all soft
47 * references to that object and all soft references to any other
48 * softly-reachable objects from which that object is reachable through a chain
49 * of strong references. At the same time or at some later time it will
50 * enqueue those newly-cleared soft references that are registered with
51 * reference queues.
52 *
53 * <p> All soft references to softly-reachable objects are guaranteed to have
54 * been cleared before the virtual machine throws an
55 * {@code OutOfMemoryError}. Otherwise no constraints are placed upon the
56 * time at which a soft reference will be cleared or the order in which a set
57 * of such references to different objects will be cleared. Virtual machine
58 * implementations are, however, encouraged to bias against clearing
59 * recently-created or recently-used soft references.
60 *
61 * <p> Direct instances of this class may be used to implement simple caches;
62 * this class or derived subclasses may also be used in larger data structures
63 * to implement more sophisticated caches. As long as the referent of a soft
64 * reference is strongly reachable, that is, is actually in use, the soft
65 * reference will not be cleared. Thus a sophisticated cache can, for example,
66 * prevent its most recently used entries from being discarded by keeping
67 * strong referents to those entries, leaving the remaining entries to be
68 * discarded at the discretion of the garbage collector.
69 * @param <T> the type of the referent
70 *
71 * @author Mark Reinhold
72 * @since 1.2
73 */
74
75 public non-sealed class SoftReference<T> extends Reference<T> {
76
77 /**
78 * Timestamp clock, updated by the garbage collector
79 */
80 private static long clock;
81
82 /**
83 * Timestamp updated by each invocation of the get method. The VM may use
84 * this field when selecting soft references to be cleared, but it is not
85 * required to do so.
86 */
87 private long timestamp;
88
89 /**
90 * Creates a new soft reference that refers to the given object. The new
91 * reference is not registered with any queue.
92 *
93 * @param referent object the new soft reference will refer to
94 * @throws IdentityException if the referent is not an
95 * {@link java.util.Objects#hasIdentity(Object) identity object}
96 */
97 public SoftReference(T referent) {
98 super(referent);
99 this.timestamp = clock;
100 }
101
102 /**
103 * Creates a new soft reference that refers to the given object and is
104 * registered with the given queue.
105 *
106 * @param referent object the new soft reference will refer to
107 * @param q the queue with which the reference is to be registered,
108 * or {@code null} if registration is not required
109 * @throws IdentityException if the referent is not an
110 * {@link java.util.Objects#hasIdentity(Object) identity object}
111 */
112 public SoftReference(T referent, ReferenceQueue<? super T> q) {
113 super(referent, q);
114 this.timestamp = clock;
115 }
116
117 /**
118 * Returns this reference object's referent. If this reference object has
119 * been cleared, either by the program or by the garbage collector, then
120 * this method returns {@code null}.
121 *
122 * @return The object to which this reference refers, or
123 * {@code null} if this reference object has been cleared
124 */
125 public T get() {
126 T o = super.get();
127 if (o != null && this.timestamp != clock)
128 this.timestamp = clock;
129 return o;
130 }
131
132 }