1 /* 2 * Copyright (c) 1998, 2016, 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 /** 27 * Provides reference-object classes, which support a limited degree 28 * of interaction with the garbage collector. A program may use a 29 * reference object to maintain a reference to some other object in 30 * such a way that the latter object may still be reclaimed by the 31 * collector. A program may also arrange to be notified some time 32 * after the collector has determined that the reachability of a given 33 * object has changed. 34 * 35 *<h2>Package Specification</h2> 36 * 37 * A <em>reference object</em> encapsulates a reference to some other 38 * object so that the reference itself may be examined and manipulated 39 * like any other object. Three types of reference objects are 40 * provided, each weaker than the last: <em>soft</em>, <em>weak</em>, 41 * and <em>phantom</em>. Each type corresponds to a different level 42 * of reachability, as defined below. Soft references are for 43 * implementing memory-sensitive caches, weak references are for 44 * implementing canonicalizing mappings that do not prevent their keys 45 * (or values) from being reclaimed, and phantom references are for 46 * scheduling post-mortem cleanup actions. 47 * Post-mortem cleanup actions can be registered and managed by a 48 * {@link java.lang.ref.Cleaner}. 49 * 50 * <p> Each reference-object type is implemented by a subclass of the 51 * abstract base {@link java.lang.ref.Reference} class. 52 * An instance of one of these subclasses encapsulates a single 53 * reference to a particular object, called the <em>referent</em>. 54 * Every reference object provides methods for getting and clearing 55 * the reference. Aside from the clearing operation reference objects 56 * are otherwise immutable, so no {@code set} operation is 57 * provided. A program may further subclass these subclasses, adding 58 * whatever fields and methods are required for its purposes, or it 59 * may use these subclasses without change. 60 * 61 * <h3>Notification</h3> 62 * 63 * A program may request to be notified of changes in an object's 64 * reachability by <em>registering</em> an appropriate reference 65 * object with a <em>reference queue</em> at the time the reference 66 * object is created. Some time after the garbage collector 67 * determines that the reachability of the referent has changed to the 68 * value corresponding to the type of the reference, it will clear the 69 * reference and add it to the associated queue. At this point, the 70 * reference is considered to be <em>enqueued</em>. The program may remove 71 * references from a queue either by polling or by blocking until a 72 * reference becomes available. Reference queues are implemented by 73 * the {@link java.lang.ref.ReferenceQueue} class. 74 * 75 * <p> The relationship between a registered reference object and its 76 * queue is one-sided. That is, a queue does not keep track of the 77 * references that are registered with it. If a registered reference 78 * becomes unreachable itself, then it will never be enqueued. It is 79 * the responsibility of the program using reference objects to ensure 80 * that the objects remain reachable for as long as the program is 81 * interested in their referents. 82 * 83 * <p> While some programs will choose to dedicate a thread to 84 * removing reference objects from one or more queues and processing 85 * them, this is by no means necessary. A tactic that often works 86 * well is to examine a reference queue in the course of performing 87 * some other fairly-frequent action. For example, a hashtable that 88 * uses weak references to implement weak keys could poll its 89 * reference queue each time the table is accessed. This is how the 90 * {@link java.util.WeakHashMap} class works. Because 91 * the {@link java.lang.ref.ReferenceQueue#poll 92 * ReferenceQueue.poll} method simply checks an internal data 93 * structure, this check will add little overhead to the hashtable 94 * access methods. 95 * 96 * <a id="reachability"></a> 97 * <h3>Reachability</h3> 98 * 99 * Going from strongest to weakest, the different levels of 100 * reachability reflect the life cycle of an object. They are 101 * operationally defined as follows: 102 * 103 * <ul> 104 * 105 * <li> An object is <em>strongly reachable</em> if it can be reached 106 * by some thread without traversing any reference objects. A 107 * newly-created object is strongly reachable by the thread that 108 * created it. 109 * 110 * <li> An object is <em>softly reachable</em> if it is not strongly 111 * reachable but can be reached by traversing a soft reference. 112 * 113 * <li> An object is <em>weakly reachable</em> if it is neither 114 * strongly nor softly reachable but can be reached by traversing a 115 * weak reference. When the weak references to a weakly-reachable 116 * object are cleared, the object becomes eligible for finalization. 117 * 118 * <li> An object is <em>phantom reachable</em> if it is neither 119 * strongly, softly, nor weakly reachable, it has been finalized, and 120 * some phantom reference refers to it. 121 * 122 * <li> Finally, an object is <em>unreachable</em>, and therefore 123 * eligible for reclamation, when it is not reachable in any of the 124 * above ways. 125 * 126 * </ul> 127 * 128 * @author Mark Reinhold 129 * @since 1.2 130 */ 131 package java.lang.ref; 132