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