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 
29 import java.util.Objects;
30 
31 /**
32  * Weak reference objects, which do not prevent their referents from being
33  * made finalizable, finalized, and then reclaimed.  Weak references are most
34  * often used to implement canonicalizing mappings.
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 that the garbage collector determines at a certain point in time
45  * that an object is <a href="package-summary.html#reachability">weakly
46  * reachable</a>.  At that time it will atomically clear all weak references to
47  * that object and all weak references to any other weakly-reachable objects
48  * from which that object is reachable through a chain of strong and soft
49  * references.  At the same time it will declare all of the formerly
50  * weakly-reachable objects to be finalizable.  At the same time or at some
51  * later time it will enqueue those newly-cleared weak references that are
52  * registered with reference queues.
53  * @param <T> the type of the referent
54  *
55  * @author   Mark Reinhold
56  * @since    1.2
57  */
58 
59 public non-sealed class WeakReference<T> extends Reference<T> {
60 
61     /**
62      * Creates a new weak reference that refers to the given object.  The new
63      * reference is not registered with any queue.
64      *
65      * @param referent object the new weak reference will refer to
66      * @throws IdentityException if the referent is not an
67      *         {@link java.util.Objects#hasIdentity(Object) identity object}
68      */
69     public WeakReference(T referent) {
70         super(referent);
71     }
72 
73     /**
74      * Creates a new weak reference that refers to the given object and is
75      * registered with the given queue.
76      *
77      * @param referent object the new weak reference will refer to
78      * @param q the queue with which the reference is to be registered,
79      *          or {@code null} if registration is not required
80      * @throws IdentityException if the referent is not an
81      *         {@link java.util.Objects#hasIdentity(Object) identity object}
82      */
83     public WeakReference(T referent, ReferenceQueue<? super T> q) {
84         super(referent, q);
85     }
86 
87 }