1 /*
   2  * Copyright (c) 1997, 2019, 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  * <p>
  34  * The referent must not be an instance of an inline class; such a value
  35  * can never have another reference to it and cannot be held in a reference type.
  36  *
  37  * <p> Suppose that the garbage collector determines at a certain point in time
  38  * that an object is <a href="package-summary.html#reachability">softly
  39  * reachable</a>.  At that time it may choose to clear atomically all soft
  40  * references to that object and all soft references to any other
  41  * softly-reachable objects from which that object is reachable through a chain
  42  * of strong references.  At the same time or at some later time it will
  43  * enqueue those newly-cleared soft references that are registered with
  44  * reference queues.
  45  *
  46  * <p> All soft references to softly-reachable objects are guaranteed to have
  47  * been cleared before the virtual machine throws an
  48  * {@code OutOfMemoryError}.  Otherwise no constraints are placed upon the
  49  * time at which a soft reference will be cleared or the order in which a set
  50  * of such references to different objects will be cleared.  Virtual machine
  51  * implementations are, however, encouraged to bias against clearing
  52  * recently-created or recently-used soft references.
  53  *
  54  * <p> Direct instances of this class may be used to implement simple caches;
  55  * this class or derived subclasses may also be used in larger data structures
  56  * to implement more sophisticated caches.  As long as the referent of a soft
  57  * reference is strongly reachable, that is, is actually in use, the soft
  58  * reference will not be cleared.  Thus a sophisticated cache can, for example,
  59  * prevent its most recently used entries from being discarded by keeping
  60  * strong referents to those entries, leaving the remaining entries to be
  61  * discarded at the discretion of the garbage collector.
  62  *
  63  * @author   Mark Reinhold
  64  * @since    1.2
  65  */
  66 
  67 public class SoftReference<T> extends Reference<T> {
  68 
  69     /**
  70      * Timestamp clock, updated by the garbage collector
  71      */
  72     private static long clock;
  73 
  74     /**
  75      * Timestamp updated by each invocation of the get method.  The VM may use
  76      * this field when selecting soft references to be cleared, but it is not
  77      * required to do so.
  78      */
  79     private long timestamp;
  80 
  81     /**
  82      * Creates a new soft reference that refers to the given object.  The new
  83      * reference is not registered with any queue.
  84      *
  85      * @param referent object the new soft reference will refer to
  86      * @throws IllegalArgumentException if the referent is an instance of an
  87      *         {@link Class#isInlineClass() inlineClass}
  88      */
  89     public SoftReference(T referent) {
  90         super(referent);
  91         this.timestamp = clock;
  92     }
  93 
  94     /**
  95      * Creates a new soft reference that refers to the given object and is
  96      * registered with the given queue.
  97      *
  98      * @param referent object the new soft reference will refer to
  99      * @param q the queue with which the reference is to be registered,
 100      *          or {@code null} if registration is not required
 101      * @throws IllegalArgumentException if the referent is an instance of an
 102      *         {@link Class#isInlineClass() inlineClass}
 103      */
 104     public SoftReference(T referent, ReferenceQueue<? super T> q) {
 105         super(referent, q);
 106         this.timestamp = clock;
 107     }
 108 
 109     /**
 110      * Returns this reference object's referent.  If this reference object has
 111      * been cleared, either by the program or by the garbage collector, then
 112      * this method returns {@code null}.
 113      *
 114      * @return   The object to which this reference refers, or
 115      *           {@code null} if this reference object has been cleared
 116      */
 117     public T get() {
 118         T o = super.get();
 119         if (o != null && this.timestamp != clock)
 120             this.timestamp = clock;
 121         return o;
 122     }
 123 
 124 }