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| 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 | |||
| 26 | package 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 | |||
| 53 | public 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 | } | ||