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

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) {
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;