1/*
2 * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.  Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26package java.lang.ref;
27
28
29/**
30 * Phantom reference objects, which are enqueued after the collector
31 * determines that their referents may otherwise be reclaimed.  Phantom
32 * references are most often used for scheduling pre-mortem cleanup actions in
33 * a more flexible way than is possible with the Java finalization mechanism.
34 *
35 * <p> If the garbage collector determines at a certain point in time that the
36 * referent of a phantom reference is <a
37 * href="package-summary.html#reachability">phantom reachable</a>, then at that
38 * time or at some later time it will enqueue the reference.
39 *
40 * <p> In order to ensure that a reclaimable object remains so, the referent of
41 * a phantom reference may not be retrieved: The <code>get</code> method of a
42 * phantom reference always returns <code>null</code>.
43 *
44 * <p> Unlike soft and weak references, phantom references are not
45 * automatically cleared by the garbage collector as they are enqueued.  An
46 * object that is reachable via phantom references will remain so until all
47 * such references are cleared or themselves become unreachable.
48 *
49 * @author   Mark Reinhold
50 * @since    1.2
51 */
52
53public class PhantomReference<T> extends Reference<T> {
54
55    /**
56     * Returns this reference object's referent.  Because the referent of a
57     * phantom reference is always inaccessible, this method always returns
58     * <code>null</code>.
59     *
60     * @return  <code>null</code>
61     */
62    public T get() {
63        return null;
64    }
65
66    /**
67     * Creates a new phantom reference that refers to the given object and
68     * is registered with the given queue.
69     *
70     * <p> It is possible to create a phantom reference with a <tt>null</tt>
71     * queue, but such a reference is completely useless: Its <tt>get</tt>
72     * method will always return null and, since it does not have a queue, it
73     * will never be enqueued.
74     *
75     * @param referent the object the new phantom reference will refer to
76     * @param q the queue with which the reference is to be registered,
77     *          or <tt>null</tt> if registration is not required
78     */
79    public PhantomReference(T referent, ReferenceQueue<? super T> q) {
80        super(referent, q);
81    }
82
83}
84