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 * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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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}. 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
 */
 private static 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 {@code null} 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}.
 *
 * @return The object to which this reference refers, or
 * {@code null} 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;
 }

}