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        */
   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        */
   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        */
  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   }