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 Provides reference-object classes, which support a limited degree of
 interaction with the garbage collector.  A program may use a reference object
 to maintain a reference to some other object in such a way that the latter
 object may still be reclaimed by the collector.  A program may also arrange to
 be notified some time after the collector has determined that the reachability
 of a given object has changed.


 <h2>Package Specification</h2>

 A <em>reference object</em> encapsulates a reference to some other object so
 that the reference itself may be examined and manipulated like any other
 object.  Three types of reference objects are provided, each weaker than the
 last: <em>soft</em>, <em>weak</em>, and <em>phantom</em>.  Each type
 corresponds to a different level of reachability, as defined below.  Soft
 references are for implementing memory-sensitive caches, weak references are
 for implementing canonicalizing mappings that do not prevent their keys (or
 values) from being reclaimed, and phantom references are for scheduling
 pre-mortem cleanup actions in a more flexible way than is possible with the
 Java finalization mechanism.

 <p> Each reference-object type is implemented by a subclass of the abstract
 base <code>{@link java.lang.ref.Reference}</code> class.  An instance of one of
 these subclasses encapsulates a single reference to a particular object, called
 the <em>referent</em>.  Every reference object provides methods for getting and
 clearing the reference.  Aside from the clearing operation reference objects
 are otherwise immutable, so no <code>set</code> operation is provided.  A
 program may further subclass these subclasses, adding whatever fields and
 methods are required for its purposes, or it may use these subclasses without
 change.


 <h3>Notification</h3>

 A program may request to be notified of changes in an object's reachability by
 <em>registering</em> an appropriate reference object with a <em>reference
 queue</em> at the time the reference object is created.  Some time after the
 garbage collector determines that the reachability of the referent has changed
 to the value corresponding to the type of the reference, it will add the
 reference to the associated queue.  At this point, the reference is considered
 to be <em>enqueued</em>.  The program may remove references from a queue either
 by polling or by blocking until a reference becomes available.  Reference
 queues are implemented by the <code>{@link java.lang.ref.ReferenceQueue}</code>
 class.

 <p> The relationship between a registered reference object and its queue is
 one-sided.  That is, a queue does not keep track of the references that are
 registered with it.  If a registered reference becomes unreachable itself, then
 it will never be enqueued.  It is the responsibility of the program using
 reference objects to ensure that the objects remain reachable for as long as
 the program is interested in their referents.

 <p> While some programs will choose to dedicate a thread to removing reference
 objects from one or more queues and processing them, this is by no means
 necessary.  A tactic that often works well is to examine a reference queue in
 the course of performing some other fairly-frequent action.  For example, a
 hashtable that uses weak references to implement weak keys could poll its
 reference queue each time the table is accessed.  This is how the <code>{@link
 java.util.WeakHashMap}</code> class works.  Because the <code>{@link
 java.lang.ref.ReferenceQueue#poll ReferenceQueue.poll}</code> method simply
 checks an internal data structure, this check will add little overhead to the
 hashtable access methods.


 <h3>Automatically-cleared references</h3>

 Soft and weak references are automatically cleared by the collector before
 being added to the queues with which they are registered, if any.  Therefore
 soft and weak references need not be registered with a queue in order to be
 useful, while phantom references do.  An object that is reachable via phantom
 references will remain so until all such references are cleared or themselves
 become unreachable.


 <a name="reachability"></a>
 <h3>Reachability</h3>

 Going from strongest to weakest, the different levels of reachability reflect
 the life cycle of an object.  They are operationally defined as follows:

 <ul>

 <li> An object is <em>strongly reachable</em> if it can be reached by some
 thread without traversing any reference objects.  A newly-created object is
 strongly reachable by the thread that created it.

 <li> An object is <em>softly reachable</em> if it is not strongly reachable but
 can be reached by traversing a soft reference.

 <li> An object is <em>weakly reachable</em> if it is neither strongly nor
 softly reachable but can be reached by traversing a weak reference.  When the
 weak references to a weakly-reachable object are cleared, the object becomes
 eligible for finalization.

 <li> An object is <em>phantom reachable</em> if it is neither strongly, softly,
 nor weakly reachable, it has been finalized, and some phantom reference refers
 to it.

 <li> Finally, an object is <em>unreachable</em>, and therefore eligible for
 reclamation, when it is not reachable in any of the above ways.

 </ul>


 @author	  Mark Reinhold
 @since	  1.2

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	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
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	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.

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	or visit www.oracle.com if you need additional information or have any
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	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
	<html>
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	Provides reference-object classes, which support a limited degree of
	interaction with the garbage collector. A program may use a reference object
	to maintain a reference to some other object in such a way that the latter
	object may still be reclaimed by the collector. A program may also arrange to
	be notified some time after the collector has determined that the reachability
	of a given object has changed.


	<h2>Package Specification</h2>

	A <em>reference object</em> encapsulates a reference to some other object so
	that the reference itself may be examined and manipulated like any other
	object. Three types of reference objects are provided, each weaker than the
	last: <em>soft</em>, <em>weak</em>, and <em>phantom</em>. Each type
	corresponds to a different level of reachability, as defined below. Soft
	references are for implementing memory-sensitive caches, weak references are
	for implementing canonicalizing mappings that do not prevent their keys (or
	values) from being reclaimed, and phantom references are for scheduling
	pre-mortem cleanup actions in a more flexible way than is possible with the
	Java finalization mechanism.

	<p> Each reference-object type is implemented by a subclass of the abstract
	base <code>{@link java.lang.ref.Reference}</code> class. An instance of one of
	these subclasses encapsulates a single reference to a particular object, called
	the <em>referent</em>. Every reference object provides methods for getting and
	clearing the reference. Aside from the clearing operation reference objects
	are otherwise immutable, so no <code>set</code> operation is provided. A
	program may further subclass these subclasses, adding whatever fields and
	methods are required for its purposes, or it may use these subclasses without
	change.


	<h3>Notification</h3>

	A program may request to be notified of changes in an object's reachability by
	<em>registering</em> an appropriate reference object with a <em>reference
	queue</em> at the time the reference object is created. Some time after the
	garbage collector determines that the reachability of the referent has changed
	to the value corresponding to the type of the reference, it will add the
	reference to the associated queue. At this point, the reference is considered
	to be <em>enqueued</em>. The program may remove references from a queue either
	by polling or by blocking until a reference becomes available. Reference
	queues are implemented by the <code>{@link java.lang.ref.ReferenceQueue}</code>
	class.

	<p> The relationship between a registered reference object and its queue is
	one-sided. That is, a queue does not keep track of the references that are
	registered with it. If a registered reference becomes unreachable itself, then
	it will never be enqueued. It is the responsibility of the program using
	reference objects to ensure that the objects remain reachable for as long as
	the program is interested in their referents.

	<p> While some programs will choose to dedicate a thread to removing reference
	objects from one or more queues and processing them, this is by no means
	necessary. A tactic that often works well is to examine a reference queue in
	the course of performing some other fairly-frequent action. For example, a
	hashtable that uses weak references to implement weak keys could poll its
	reference queue each time the table is accessed. This is how the <code>{@link
	java.util.WeakHashMap}</code> class works. Because the <code>{@link
	java.lang.ref.ReferenceQueue#poll ReferenceQueue.poll}</code> method simply
	checks an internal data structure, this check will add little overhead to the
	hashtable access methods.


	<h3>Automatically-cleared references</h3>

	Soft and weak references are automatically cleared by the collector before
	being added to the queues with which they are registered, if any. Therefore
	soft and weak references need not be registered with a queue in order to be
	useful, while phantom references do. An object that is reachable via phantom
	references will remain so until all such references are cleared or themselves
	become unreachable.


	<a name="reachability"></a>
	<h3>Reachability</h3>

	Going from strongest to weakest, the different levels of reachability reflect
	the life cycle of an object. They are operationally defined as follows:

	<ul>

	<li> An object is <em>strongly reachable</em> if it can be reached by some
	thread without traversing any reference objects. A newly-created object is
	strongly reachable by the thread that created it.

	<li> An object is <em>softly reachable</em> if it is not strongly reachable but
	can be reached by traversing a soft reference.

	<li> An object is <em>weakly reachable</em> if it is neither strongly nor
	softly reachable but can be reached by traversing a weak reference. When the
	weak references to a weakly-reachable object are cleared, the object becomes
	eligible for finalization.

	<li> An object is <em>phantom reachable</em> if it is neither strongly, softly,
	nor weakly reachable, it has been finalized, and some phantom reference refers
	to it.

	<li> Finally, an object is <em>unreachable</em>, and therefore eligible for
	reclamation, when it is not reachable in any of the above ways.

	</ul>


	@author Mark Reinhold
	@since 1.2

	<!--
	<h2>Related Documentation</h2>

	For overviews, tutorials, examples, guides, and tool documentation, please see:
	<ul>
	<li><a href="">##### REFER TO NON-SPEC DOCUMENTATION HERE #####</a>
	</ul>
	-->
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