| /* |
| * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| */ |
| |
| package java.lang.ref; |
| |
| |
| /** |
| * Phantom reference objects, which are enqueued after the collector |
| * determines that their referents may otherwise be reclaimed. Phantom |
| * references are most often used to schedule post-mortem cleanup actions. |
| * |
| * <p> Suppose the garbage collector determines at a certain point in time |
| * that an object is <a href="package-summary.html#reachability"> |
| * phantom reachable</a>. At that time it will atomically clear |
| * all phantom references to that object and all phantom references to |
| * any other phantom-reachable objects from which that object is reachable. |
| * At the same time or at some later time it will enqueue those newly-cleared |
| * phantom references that are registered with reference queues. |
| * |
| * <p> In order to ensure that a reclaimable object remains so, the referent of |
| * a phantom reference may not be retrieved: The {@code get} method of a |
| * phantom reference always returns {@code null}. |
| * |
| * @author Mark Reinhold |
| * @since 1.2 |
| */ |
| |
| public class PhantomReference<T> extends Reference<T> { |
| |
| /** |
| * Returns this reference object's referent. Because the referent of a |
| * phantom reference is always inaccessible, this method always returns |
| * {@code null}. |
| * |
| * @return {@code null} |
| */ |
| public T get() { |
| return null; |
| } |
| |
| /** |
| * Creates a new phantom reference that refers to the given object and |
| * is registered with the given queue. |
| * |
| * <p> It is possible to create a phantom reference with a {@code null} |
| * queue, but such a reference is completely useless: Its {@code get} |
| * method will always return {@code null} and, since it does not have a queue, |
| * it will never be enqueued. |
| * |
| * @param referent the object the new phantom reference will refer to |
| * @param q the queue with which the reference is to be registered, |
| * or {@code null} if registration is not required |
| */ |
| public PhantomReference(T referent, ReferenceQueue<? super T> q) { |
| super(referent, q); |
| } |
| |
| } |