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 */ get()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 */ PhantomReference(T referent, ReferenceQueue<? super T> q)79 public PhantomReference(T referent, ReferenceQueue<? super T> q) { 80 super(referent, q); 81 } 82 83 } 84