/*

 * Copyright 1997-2003 Sun Microsystems, Inc.  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.  Sun designates this

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

 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.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;

        return o;

    }

}