1 /*
2 * Copyright (c) 1997, 2022, 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
26 package java.lang.ref;
27
28 import java.util.Objects;
29 import jdk.internal.vm.annotation.IntrinsicCandidate;
30
31 /**
32 * Phantom reference objects, which are enqueued after the collector
33 * determines that their referents may otherwise be reclaimed. Phantom
34 * references are most often used to schedule post-mortem cleanup actions.
35 * <p>
36 * The referent must be an {@linkplain Objects#isIdentityObject(Object) identity object}.
37 *
38 * <p> Suppose the garbage collector determines at a certain point in time
39 * that an object is <a href="package-summary.html#reachability">
40 * phantom reachable</a>. At that time it will atomically clear
41 * all phantom references to that object and all phantom references to
42 * any other phantom-reachable objects from which that object is reachable.
43 * At the same time or at some later time it will enqueue those newly-cleared
44 * phantom references that are registered with reference queues.
45 *
46 * <p> In order to ensure that a reclaimable object remains so, the referent of
47 * a phantom reference may not be retrieved: The {@code get} method of a
48 * phantom reference always returns {@code null}.
49 * The {@link #refersTo(Object) refersTo} method can be used to test
50 * whether some object is the referent of a phantom reference.
51 * @param <T> the type of the referent
52 *
53 * @author Mark Reinhold
54 * @since 1.2
55 */
56
57 public non-sealed class PhantomReference<T> extends Reference<T> {
58
59 /**
60 * Returns this reference object's referent. Because the referent of a
61 * phantom reference is always inaccessible, this method always returns
62 * {@code null}.
63 *
64 * @return {@code null}
65 */
66 public T get() {
67 return null;
68 }
69
70 /* Override the implementation of Reference.refersTo.
71 * Phantom references are weaker than finalization, so the referent
72 * access needs to be handled differently for garbage collectors that
73 * do reference processing concurrently.
74 */
75 @Override
76 boolean refersToImpl(T obj) {
77 return refersTo0(obj);
78 }
79
80 @IntrinsicCandidate
81 private native boolean refersTo0(Object o);
82
83 /**
84 * Creates a new phantom reference that refers to the given object and
85 * is registered with the given queue.
86 *
87 * <p> It is possible to create a phantom reference with a {@code null}
88 * queue. Such a reference will never be enqueued.
89 *
90 * @param referent the object the new phantom reference will refer to
91 * @param q the queue with which the reference is to be registered,
92 * or {@code null} if registration is not required
93 * @throws IdentityException if the referent is not an
94 * {@link java.util.Objects#isIdentityObject(Object) identity object}
95 */
96 public PhantomReference(T referent, ReferenceQueue<? super T> q) {
97 super(referent, q);
98 }
99
100 }