/*

  * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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  */

 package java.lang.ref;

 /**

  * Soft reference objects, which are cleared at the discretion of the garbage

  * collector in response to memory demand.  Soft references are most often used

  * to implement memory-sensitive caches.

  *

  * <p> Suppose that the garbage collector determines at a certain point in time

  * that an object is <a href="package-summary.html#reachability">softly

  * reachable</a>.  At that time it may choose to clear atomically all soft

  * references to that object and all soft references to any other

  * softly-reachable objects from which that object is reachable through a chain

  * of strong references.  At the same time or at some later time it will

  * enqueue those newly-cleared soft references that are registered with

  * reference queues.

  *

  * <p> All soft references to softly-reachable objects are guaranteed to have

  * been cleared before the virtual machine throws an

  * <code>OutOfMemoryError</code>.  Otherwise no constraints are placed upon the

  * time at which a soft reference will be cleared or the order in which a set

  * of such references to different objects will be cleared.  Virtual machine

  * implementations are, however, encouraged to bias against clearing

  * recently-created or recently-used soft references.

  *

  * <p> Direct instances of this class may be used to implement simple caches;

  * this class or derived subclasses may also be used in larger data structures

  * to implement more sophisticated caches.  As long as the referent of a soft

  * reference is strongly reachable, that is, is actually in use, the soft

  * reference will not be cleared.  Thus a sophisticated cache can, for example,

  * prevent its most recently used entries from being discarded by keeping

  * strong referents to those entries, leaving the remaining entries to be

  * discarded at the discretion of the garbage collector.

  *

  * @author   Mark Reinhold

  * @since    1.2

  */

 public class SoftReference<T> extends Reference<T> {

     /**

      * Timestamp clock, updated by the garbage collector

      */

     static private long clock;

     /**

      * Timestamp updated by each invocation of the get method.  The VM may use

      * this field when selecting soft references to be cleared, but it is not

      * required to do so.

      */

     private long timestamp;

     /**

      * Creates a new soft reference that refers to the given object.  The new

      * reference is not registered with any queue.

      *

      * @param referent object the new soft reference will refer to

      */

     public SoftReference(T referent) {

         super(referent);

         this.timestamp = clock;

     }

     /**

      * Creates a new soft reference that refers to the given object and is

      * registered with the given queue.

      *

      * @param referent object the new soft reference will refer to

      * @param q the queue with which the reference is to be registered,

      *          or <tt>null</tt> if registration is not required

      *

      */

     public SoftReference(T referent, ReferenceQueue<? super T> q) {

         super(referent, q);

         this.timestamp = clock;

     }

     /**

      * Returns this reference object's referent.  If this reference object has

      * been cleared, either by the program or by the garbage collector, then

      * this method returns <code>null</code>.

      *

      * @return   The object to which this reference refers, or

      *           <code>null</code> if this reference object has been cleared

      */

     public T get() {

         T o = super.get();

         if (o != null && this.timestamp != clock)

             this.timestamp = clock;

         return o;

     }

 }