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
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.
32 *
33 * <p> Suppose that the garbage collector determines at a certain point in time
34 * that an object is <a href="package-summary.html#reachability">softly
35 * reachable</a>.  At that time it may choose to clear atomically all soft
36 * references to that object and all soft references to any other
37 * softly-reachable objects from which that object is reachable through a chain
38 * of strong references.  At the same time or at some later time it will
39 * enqueue those newly-cleared soft references that are registered with
40 * reference queues.
41 *
42 * <p> All soft references to softly-reachable objects are guaranteed to have
43 * been cleared before the virtual machine throws an
44 * <code>OutOfMemoryError</code>.  Otherwise no constraints are placed upon the
45 * time at which a soft reference will be cleared or the order in which a set
46 * of such references to different objects will be cleared.  Virtual machine
47 * implementations are, however, encouraged to bias against clearing
48 * recently-created or recently-used soft references.
49 *
50 * <h3>Avoid Soft References for Caching</h3>
51 * In practice, soft references are inefficient for caching. The runtime doesn't
52 * have enough information on which references to clear and which to keep. Most
53 * fatally, it doesn't know what to do when given the choice between clearing a
54 * soft reference and growing the heap.
55 *
56 * <p>The lack of information on the value to your application of each reference
57 * limits the usefulness of soft references. References that are cleared too
58 * early cause unnecessary work; those that are cleared too late waste memory.
59 *
60 * <p>Most applications should use an {@code android.util.LruCache} instead of
61 * soft references. LruCache has an effective eviction policy and lets the user
62 * tune how much memory is allotted.
63 *
64 * @author   Mark Reinhold
65 * @since    1.2
66 */
67
68public class SoftReference<T> extends Reference<T> {
69
70    /**
71     * Timestamp clock, updated by the garbage collector
72     */
73    static private long clock;
74
75    /**
76     * Timestamp updated by each invocation of the get method.  The VM may use
77     * this field when selecting soft references to be cleared, but it is not
78     * required to do so.
79     */
80    private long timestamp;
81
82    /**
83     * Creates a new soft reference that refers to the given object.  The new
84     * reference is not registered with any queue.
85     *
86     * @param referent object the new soft reference will refer to
87     */
88    public SoftReference(T referent) {
89        super(referent);
90        this.timestamp = clock;
91    }
92
93    /**
94     * Creates a new soft reference that refers to the given object and is
95     * registered with the given queue.
96     *
97     * @param referent object the new soft reference will refer to
98     * @param q the queue with which the reference is to be registered,
99     *          or <tt>null</tt> if registration is not required
100     *
101     */
102    public SoftReference(T referent, ReferenceQueue<? super T> q) {
103        super(referent, q);
104        this.timestamp = clock;
105    }
106
107    /**
108     * Returns this reference object's referent.  If this reference object has
109     * been cleared, either by the program or by the garbage collector, then
110     * this method returns <code>null</code>.
111     *
112     * @return   The object to which this reference refers, or
113     *           <code>null</code> if this reference object has been cleared
114     */
115    public T get() {
116        T o = super.get();
117        if (o != null && this.timestamp != clock)
118            this.timestamp = clock;
119        return o;
120    }
121
122}
123