001 /*
002 * Copyright 1997-2003 Sun Microsystems, Inc. All Rights Reserved.
003 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004 *
005 * This code is free software; you can redistribute it and/or modify it
006 * under the terms of the GNU General Public License version 2 only, as
007 * published by the Free Software Foundation. Sun designates this
008 * particular file as subject to the "Classpath" exception as provided
009 * by Sun in the LICENSE file that accompanied this code.
010 *
011 * This code is distributed in the hope that it will be useful, but WITHOUT
012 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014 * version 2 for more details (a copy is included in the LICENSE file that
015 * accompanied this code).
016 *
017 * You should have received a copy of the GNU General Public License version
018 * 2 along with this work; if not, write to the Free Software Foundation,
019 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020 *
021 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022 * CA 95054 USA or visit www.sun.com if you need additional information or
023 * have any questions.
024 */
025
026 package java.lang.ref;
027
028 /**
029 * Soft reference objects, which are cleared at the discretion of the garbage
030 * collector in response to memory demand. Soft references are most often used
031 * to implement memory-sensitive caches.
032 *
033 * <p> Suppose that the garbage collector determines at a certain point in time
034 * that an object is <a href="package-summary.html#reachability">softly
035 * reachable</a>. At that time it may choose to clear atomically all soft
036 * references to that object and all soft references to any other
037 * softly-reachable objects from which that object is reachable through a chain
038 * of strong references. At the same time or at some later time it will
039 * enqueue those newly-cleared soft references that are registered with
040 * reference queues.
041 *
042 * <p> All soft references to softly-reachable objects are guaranteed to have
043 * been cleared before the virtual machine throws an
044 * <code>OutOfMemoryError</code>. Otherwise no constraints are placed upon the
045 * time at which a soft reference will be cleared or the order in which a set
046 * of such references to different objects will be cleared. Virtual machine
047 * implementations are, however, encouraged to bias against clearing
048 * recently-created or recently-used soft references.
049 *
050 * <p> Direct instances of this class may be used to implement simple caches;
051 * this class or derived subclasses may also be used in larger data structures
052 * to implement more sophisticated caches. As long as the referent of a soft
053 * reference is strongly reachable, that is, is actually in use, the soft
054 * reference will not be cleared. Thus a sophisticated cache can, for example,
055 * prevent its most recently used entries from being discarded by keeping
056 * strong referents to those entries, leaving the remaining entries to be
057 * discarded at the discretion of the garbage collector.
058 *
059 * @version 1.41, 05/05/07
060 * @author Mark Reinhold
061 * @since 1.2
062 */
063
064 public class SoftReference<T> extends Reference<T> {
065
066 /* Timestamp clock, updated by the garbage collector
067 */
068 static private long clock;
069
070 /* Timestamp updated by each invocation of the get method. The VM may use
071 * this field when selecting soft references to be cleared, but it is not
072 * required to do so.
073 */
074 private long timestamp;
075
076 /**
077 * Creates a new soft reference that refers to the given object. The new
078 * reference is not registered with any queue.
079 *
080 * @param referent object the new soft reference will refer to
081 */
082 public SoftReference(T referent) {
083 super (referent);
084 this .timestamp = clock;
085 }
086
087 /**
088 * Creates a new soft reference that refers to the given object and is
089 * registered with the given queue.
090 *
091 * @param referent object the new soft reference will refer to
092 * @param q the queue with which the reference is to be registered,
093 * or <tt>null</tt> if registration is not required
094 *
095 */
096 public SoftReference(T referent, ReferenceQueue<? super T> q) {
097 super (referent, q);
098 this .timestamp = clock;
099 }
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)
112 this.timestamp = clock;
113 return o;
114 }
115
116 }
|