/*
 
* 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
 
* or visit www.oracle.com if you need additional information or have any
 
* questions.
 
*/

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;
    
}

}