jaroslav@601: /*
jaroslav@601:  * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.
jaroslav@601:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jaroslav@601:  *
jaroslav@601:  * This code is free software; you can redistribute it and/or modify it
jaroslav@601:  * under the terms of the GNU General Public License version 2 only, as
jaroslav@601:  * published by the Free Software Foundation.  Oracle designates this
jaroslav@601:  * particular file as subject to the "Classpath" exception as provided
jaroslav@601:  * by Oracle in the LICENSE file that accompanied this code.
jaroslav@601:  *
jaroslav@601:  * This code is distributed in the hope that it will be useful, but WITHOUT
jaroslav@601:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jaroslav@601:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
jaroslav@601:  * version 2 for more details (a copy is included in the LICENSE file that
jaroslav@601:  * accompanied this code).
jaroslav@601:  *
jaroslav@601:  * You should have received a copy of the GNU General Public License version
jaroslav@601:  * 2 along with this work; if not, write to the Free Software Foundation,
jaroslav@601:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jaroslav@601:  *
jaroslav@601:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jaroslav@601:  * or visit www.oracle.com if you need additional information or have any
jaroslav@601:  * questions.
jaroslav@601:  */
jaroslav@601: 
jaroslav@601: package java.lang.ref;
jaroslav@601: 
jaroslav@601: 
jaroslav@601: /**
jaroslav@601:  * Phantom reference objects, which are enqueued after the collector
jaroslav@601:  * determines that their referents may otherwise be reclaimed.  Phantom
jaroslav@601:  * references are most often used for scheduling pre-mortem cleanup actions in
jaroslav@601:  * a more flexible way than is possible with the Java finalization mechanism.
jaroslav@601:  *
jaroslav@601:  * <p> If the garbage collector determines at a certain point in time that the
jaroslav@601:  * referent of a phantom reference is <a
jaroslav@601:  * href="package-summary.html#reachability">phantom reachable</a>, then at that
jaroslav@601:  * time or at some later time it will enqueue the reference.
jaroslav@601:  *
jaroslav@601:  * <p> In order to ensure that a reclaimable object remains so, the referent of
jaroslav@601:  * a phantom reference may not be retrieved: The <code>get</code> method of a
jaroslav@601:  * phantom reference always returns <code>null</code>.
jaroslav@601:  *
jaroslav@601:  * <p> Unlike soft and weak references, phantom references are not
jaroslav@601:  * automatically cleared by the garbage collector as they are enqueued.  An
jaroslav@601:  * object that is reachable via phantom references will remain so until all
jaroslav@601:  * such references are cleared or themselves become unreachable.
jaroslav@601:  *
jaroslav@601:  * @author   Mark Reinhold
jaroslav@601:  * @since    1.2
jaroslav@601:  */
jaroslav@601: 
jaroslav@601: public class PhantomReference<T> extends Reference<T> {
jaroslav@601: 
jaroslav@601:     /**
jaroslav@601:      * Returns this reference object's referent.  Because the referent of a
jaroslav@601:      * phantom reference is always inaccessible, this method always returns
jaroslav@601:      * <code>null</code>.
jaroslav@601:      *
jaroslav@601:      * @return  <code>null</code>
jaroslav@601:      */
jaroslav@601:     public T get() {
jaroslav@601:         return null;
jaroslav@601:     }
jaroslav@601: 
jaroslav@601:     /**
jaroslav@601:      * Creates a new phantom reference that refers to the given object and
jaroslav@601:      * is registered with the given queue.
jaroslav@601:      *
jaroslav@601:      * <p> It is possible to create a phantom reference with a <tt>null</tt>
jaroslav@601:      * queue, but such a reference is completely useless: Its <tt>get</tt>
jaroslav@601:      * method will always return null and, since it does not have a queue, it
jaroslav@601:      * will never be enqueued.
jaroslav@601:      *
jaroslav@601:      * @param referent the object the new phantom reference will refer to
jaroslav@601:      * @param q the queue with which the reference is to be registered,
jaroslav@601:      *          or <tt>null</tt> if registration is not required
jaroslav@601:      */
jaroslav@601:     public PhantomReference(T referent, ReferenceQueue<? super T> q) {
jaroslav@601:         super(referent, q);
jaroslav@601:     }
jaroslav@601: 
jaroslav@601: }