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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 | +} | |