1 /* 2 * Copyright (c) 1997, 2015, 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 to schedule post-mortem cleanup actions. 33 * 34 * <p> Suppose the garbage collector determines at a certain point in time 35 * that an object is <a href="package-summary.html#reachability"> 36 * phantom reachable</a>. At that time it will atomically clear 37 * all phantom references to that object and all phantom references to 38 * any other phantom-reachable objects from which that object is reachable. 39 * At the same time or at some later time it will enqueue those newly-cleared 40 * phantom references that are registered with reference queues. 41 * 42 * <p> In order to ensure that a reclaimable object remains so, the referent of 43 * a phantom reference may not be retrieved: The {@code get} method of a 44 * phantom reference always returns {@code null}. 45 * 46 * @author Mark Reinhold 47 * @since 1.2 48 */ 49 50 public class PhantomReference<T> extends Reference<T> { 51 52 /** 53 * Returns this reference object's referent. Because the referent of a 54 * phantom reference is always inaccessible, this method always returns 55 * {@code null}. 56 * 57 * @return {@code null} 58 */ get()59 public T get() { 60 return null; 61 } 62 63 /** 64 * Creates a new phantom reference that refers to the given object and 65 * is registered with the given queue. 66 * 67 * <p> It is possible to create a phantom reference with a {@code null} 68 * queue, but such a reference is completely useless: Its {@code get} 69 * method will always return {@code null} and, since it does not have a queue, 70 * it will never be enqueued. 71 * 72 * @param referent the object the new phantom reference will refer to 73 * @param q the queue with which the reference is to be registered, 74 * or {@code null} if registration is not required 75 */ PhantomReference(T referent, ReferenceQueue<? super T> q)76 public PhantomReference(T referent, ReferenceQueue<? super T> q) { 77 super(referent, q); 78 } 79 80 } 81