1 /*
2 * Copyright (c) 1997, 2025, 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
30 /**
31 * Weak reference objects, which do not prevent their referents from being
32 * made finalizable, finalized, and then reclaimed. Weak references are most
33 * often used to implement canonicalizing mappings.
34 *
35 * <div class="preview-block">
36 * <div class="preview-comment">
37 * The referent must have {@linkplain Objects#hasIdentity(Object) object identity}.
38 * When preview features are enabled, attempts to create a reference
39 * to a {@linkplain Class#isValue value object} result in an {@link IdentityException}.
40 * </div>
41 * </div>
42 *
43 * <p> Suppose that the garbage collector determines at a certain point in time
44 * that an object is <a href="package-summary.html#reachability">weakly
45 * reachable</a>. At that time it will atomically clear all weak references to
46 * that object and all weak references to any other weakly-reachable objects
47 * from which that object is reachable through a chain of strong and soft
48 * references. At the same time it will declare all of the formerly
49 * weakly-reachable objects to be finalizable. At the same time or at some
50 * later time it will enqueue those newly-cleared weak references that are
51 * registered with reference queues.
52 * @param <T> the type of the referent
53 *
54 * @author Mark Reinhold
55 * @since 1.2
56 */
57
58 public non-sealed class WeakReference<@jdk.internal.RequiresIdentity T> extends Reference<T> {
59
60 /**
61 * Creates a new weak reference that refers to the given object. The new
62 * reference is not registered with any queue.
63 *
64 * @param referent object the new weak reference will refer to
65 * @throws IdentityException if the referent is not an
66 * {@link java.util.Objects#hasIdentity(Object) identity object}
67 */
68 public WeakReference(@jdk.internal.RequiresIdentity T referent) {
69 super(referent);
70 }
71
72 /**
73 * Creates a new weak reference that refers to the given object and is
74 * registered with the given queue.
75 *
76 * @param referent object the new weak reference will refer to
77 * @param q the queue with which the reference is to be registered,
78 * or {@code null} if registration is not required
79 * @throws IdentityException if the referent is not an
80 * {@link java.util.Objects#hasIdentity(Object) identity object}
81 */
82 public WeakReference(@jdk.internal.RequiresIdentity T referent, ReferenceQueue<? super T> q) {
83 super(referent, q);
84 }
85
86 }