KickJava   Java API By Example, From Geeks To Geeks.

Java > Open Source Codes > java > lang > ref > PhantomReference


1 /*
2  * @(#)PhantomReference.java 1.19 03/12/19
3  *
4  * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5  * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6  */
7
8 package java.lang.ref;
9
10
11 /**
12  * Phantom reference objects, which are enqueued after the collector
13  * determines that their referents may otherwise be reclaimed. Phantom
14  * references are most often used for scheduling pre-mortem cleanup actions in
15  * a more flexible way than is possible with the Java finalization mechanism.
16  *
17  * <p> If the garbage collector determines at a certain point in time that the
18  * referent of a phantom reference is <a
19  * HREF="package-summary.html#reachability">phantom reachable</a>, then at that
20  * time or at some later time it will enqueue the reference.
21  *
22  * <p> In order to ensure that a reclaimable object remains so, the referent of
23  * a phantom reference may not be retrieved: The <code>get</code> method of a
24  * phantom reference always returns <code>null</code>.
25  *
26  * <p> Unlike soft and weak references, phantom references are not
27  * automatically cleared by the garbage collector as they are enqueued. An
28  * object that is reachable via phantom references will remain so until all
29  * such references are cleared or themselves become unreachable.
30  *
31  * @version 1.19, 12/19/03
32  * @author Mark Reinhold
33  * @since 1.2
34  */
35
36 public class PhantomReference<T> extends Reference JavaDoc<T> {
37
38     /**
39      * Returns this reference object's referent. Because the referent of a
40      * phantom reference is always inaccessible, this method always returns
41      * <code>null</code>.
42      *
43      * @return <code>null</code>
44      */
45     public T get() {
46     return null;
47     }
48
49     /**
50      * Creates a new phantom reference that refers to the given object and
51      * is registered with the given queue.
52      *
53      * <p> It is possible to create a phantom reference with a <tt>null</tt>
54      * queue, but such a reference is completely useless: Its <tt>get</tt>
55      * method will always return null and, since it does not have a queue, it
56      * will never be enqueued.
57      *
58      * @param referent the object the new phantom reference will refer to
59      * @param q the queue with which the reference is to be registered,
60      * or <tt>null</tt> if registration is not required
61      */
62     public PhantomReference(T referent, ReferenceQueue JavaDoc<? super T> q) {
63     super(referent, q);
64     }
65
66 }
67
Popular Tags