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 *
36 * <div class="preview-block">
37 * <div class="preview-comment">
38 * The referent must have {@linkplain Objects#hasIdentity(Object) object identity}.
39 * When preview features are enabled, attempts to create a reference
40 * to a {@linkplain Class#isValue value object} result in an {@link IdentityException}.
41 * </div>
42 * </div>
43 *
44 * <p> Suppose the garbage collector determines at a certain point in time
45 * that an object is <a href="package-summary.html#reachability">
46 * phantom reachable</a>. At that time it will atomically clear
47 * all phantom references to that object and all phantom references to
48 * any other phantom-reachable objects from which that object is reachable.
49 * At the same time or at some later time it will enqueue those newly-cleared
50 * phantom references that are registered with reference queues.
51 *
52 * <p> In order to ensure that a reclaimable object remains so, the referent of
53 * a phantom reference may not be retrieved: The {@code get} method of a
54 * phantom reference always returns {@code null}.
55 * The {@link #refersTo(Object) refersTo} method can be used to test
56 * whether some object is the referent of a phantom reference.
57 * @param <T> the type of the referent
58 *
59 * @author Mark Reinhold
60 * @since 1.2
61 */
62
63 public non-sealed class PhantomReference<T> extends Reference<T> {
64
65 /**
66 * Returns this reference object's referent. Because the referent of a
67 * phantom reference is always inaccessible, this method always returns
68 * {@code null}.
69 *
70 * @return {@code null}
71 */
72 public T get() {
73 return null;
74 }
75
76 /* Override the implementation of Reference.refersTo.
77 * Phantom references are weaker than finalization, so the referent
78 * access needs to be handled differently for garbage collectors that
79 * do reference processing concurrently.
80 */
81 @Override
82 boolean refersToImpl(T obj) {
83 return refersTo0(obj);
84 }
85
86 @IntrinsicCandidate
87 private native boolean refersTo0(Object o);
88
89 /* Override the implementation of Reference.clear.
90 * Phantom references are weaker than finalization, so the referent
91 * access needs to be handled differently for garbage collectors that
92 * do reference processing concurrently.
93 */
94 @Override
95 void clearImpl() {
96 clear0();
97 }
98
99 @IntrinsicCandidate
100 private native void clear0();
101
102 /**
103 * Creates a new phantom reference that refers to the given object and
104 * is registered with the given queue.
105 *
106 * <p> It is possible to create a phantom reference with a {@code null}
107 * queue. Such a reference will never be enqueued.
108 *
109 * @param referent the object the new phantom reference will refer to
110 * @param q the queue with which the reference is to be registered,
111 * or {@code null} if registration is not required
112 * @throws IdentityException if the referent is not an
113 * {@link java.util.Objects#hasIdentity(Object) identity object}
114 */
115 public PhantomReference(T referent, ReferenceQueue<? super T> q) {
116 super(referent, q);
117 }
118
119 }