1 /*
   2  * Copyright 1997-2003 Sun Microsystems, Inc.  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.  Sun designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
  22  * CA 95054 USA or visit www.sun.com if you need additional information or
  23  * have any 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     /* Timestamp clock, updated by the garbage collector
  67      */
  68     static private long clock;
  69 
  70     /* Timestamp updated by each invocation of the get method.  The VM may use
  71      * this field when selecting soft references to be cleared, but it is not
  72      * required to do so.
  73      */
  74     private long timestamp;
  75 
  76     /**
  77      * Creates a new soft reference that refers to the given object.  The new
  78      * reference is not registered with any queue.
  79      *
  80      * @param referent object the new soft reference will refer to
  81      */
  82     public SoftReference(T referent) {
  83         super(referent);
  84         this.timestamp = clock;
  85     }
  86 
  87     /**
  88      * Creates a new soft reference that refers to the given object and is
  89      * registered with the given queue.
  90      *
  91      * @param referent object the new soft reference will refer to
  92      * @param q the queue with which the reference is to be registered,
  93      *          or <tt>null</tt> if registration is not required
  94      *
  95      */
  96     public SoftReference(T referent, ReferenceQueue<? super T> q) {
  97         super(referent, q);
  98         this.timestamp = clock;
  99     }
 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 && this.timestamp != clock) 
 112             this.timestamp = clock;
 113         return o;
 114     }
 115 
 116 }