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<!--
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Copyright (c) 1998, 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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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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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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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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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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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<html>
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<body bgcolor="white">
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Provides reference-object classes, which support a limited degree of
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interaction with the garbage collector. A program may use a reference object
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to maintain a reference to some other object in such a way that the latter
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object may still be reclaimed by the collector. A program may also arrange to
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be notified some time after the collector has determined that the reachability
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of a given object has changed.
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<h2>Package Specification</h2>
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A <em>reference object</em> encapsulates a reference to some other object so
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that the reference itself may be examined and manipulated like any other
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object. Three types of reference objects are provided, each weaker than the
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last: <em>soft</em>, <em>weak</em>, and <em>phantom</em>. Each type
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corresponds to a different level of reachability, as defined below. Soft
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references are for implementing memory-sensitive caches, weak references are
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for implementing canonicalizing mappings that do not prevent their keys (or
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values) from being reclaimed, and phantom references are for scheduling
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pre-mortem cleanup actions in a more flexible way than is possible with the
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Java finalization mechanism.
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<p> Each reference-object type is implemented by a subclass of the abstract
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base <code>{@link java.lang.ref.Reference}</code> class. An instance of one of
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these subclasses encapsulates a single reference to a particular object, called
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the <em>referent</em>. Every reference object provides methods for getting and
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clearing the reference. Aside from the clearing operation reference objects
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are otherwise immutable, so no <code>set</code> operation is provided. A
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program may further subclass these subclasses, adding whatever fields and
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methods are required for its purposes, or it may use these subclasses without
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change.
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<h3>Notification</h3>
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A program may request to be notified of changes in an object's reachability by
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<em>registering</em> an appropriate reference object with a <em>reference
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queue</em> at the time the reference object is created. Some time after the
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garbage collector determines that the reachability of the referent has changed
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to the value corresponding to the type of the reference, it will add the
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reference to the associated queue. At this point, the reference is considered
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to be <em>enqueued</em>. The program may remove references from a queue either
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by polling or by blocking until a reference becomes available. Reference
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queues are implemented by the <code>{@link java.lang.ref.ReferenceQueue}</code>
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class.
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<p> The relationship between a registered reference object and its queue is
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one-sided. That is, a queue does not keep track of the references that are
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registered with it. If a registered reference becomes unreachable itself, then
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it will never be enqueued. It is the responsibility of the program using
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reference objects to ensure that the objects remain reachable for as long as
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the program is interested in their referents.
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<p> While some programs will choose to dedicate a thread to removing reference
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objects from one or more queues and processing them, this is by no means
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necessary. A tactic that often works well is to examine a reference queue in
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the course of performing some other fairly-frequent action. For example, a
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hashtable that uses weak references to implement weak keys could poll its
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reference queue each time the table is accessed. This is how the <code>{@link
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java.util.WeakHashMap}</code> class works. Because the <code>{@link
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java.lang.ref.ReferenceQueue#poll ReferenceQueue.poll}</code> method simply
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checks an internal data structure, this check will add little overhead to the
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hashtable access methods.
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<h3>Automatically-cleared references</h3>
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Soft and weak references are automatically cleared by the collector before
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being added to the queues with which they are registered, if any. Therefore
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soft and weak references need not be registered with a queue in order to be
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useful, while phantom references do. An object that is reachable via phantom
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references will remain so until all such references are cleared or themselves
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become unreachable.
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<a name="reachability"></a>
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<h3>Reachability</h3>
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Going from strongest to weakest, the different levels of reachability reflect
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the life cycle of an object. They are operationally defined as follows:
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<ul>
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<li> An object is <em>strongly reachable</em> if it can be reached by some
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thread without traversing any reference objects. A newly-created object is
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strongly reachable by the thread that created it.
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<li> An object is <em>softly reachable</em> if it is not strongly reachable but
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can be reached by traversing a soft reference.
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<li> An object is <em>weakly reachable</em> if it is neither strongly nor
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softly reachable but can be reached by traversing a weak reference. When the
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weak references to a weakly-reachable object are cleared, the object becomes
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eligible for finalization.
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<li> An object is <em>phantom reachable</em> if it is neither strongly, softly,
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nor weakly reachable, it has been finalized, and some phantom reference refers
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to it.
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<li> Finally, an object is <em>unreachable</em>, and therefore eligible for
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reclamation, when it is not reachable in any of the above ways.
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</ul>
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@author Mark Reinhold
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@since 1.2
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<!--
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<h2>Related Documentation</h2>
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For overviews, tutorials, examples, guides, and tool documentation, please see:
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<ul>
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<li><a href="">##### REFER TO NON-SPEC DOCUMENTATION HERE #####</a>
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</ul>
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-->
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</body>
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</html>
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