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