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  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

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  * 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

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 package java.lang.ref;

 import sun.misc.Cleaner;

 /**

  * Abstract base class for reference objects.  This class defines the

  * operations common to all reference objects.  Because reference objects are

  * implemented in close cooperation with the garbage collector, this class may

  * not be subclassed directly.

  *

  * @author   Mark Reinhold

  * @since    1.2

  */

 public abstract class Reference<T> {

     /* A Reference instance is in one of four possible internal states:

      *

      *     Active: Subject to special treatment by the garbage collector.  Some

      *     time after the collector detects that the reachability of the

      *     referent has changed to the appropriate state, it changes the

      *     instance's state to either Pending or Inactive, depending upon

      *     whether or not the instance was registered with a queue when it was

      *     created.  In the former case it also adds the instance to the

      *     pending-Reference list.  Newly-created instances are Active.

      *

      *     Pending: An element of the pending-Reference list, waiting to be

      *     enqueued by the Reference-handler thread.  Unregistered instances

      *     are never in this state.

      *

      *     Enqueued: An element of the queue with which the instance was

      *     registered when it was created.  When an instance is removed from

      *     its ReferenceQueue, it is made Inactive.  Unregistered instances are

      *     never in this state.

      *

      *     Inactive: Nothing more to do.  Once an instance becomes Inactive its

      *     state will never change again.

      *

      * The state is encoded in the queue and next fields as follows:

      *

      *     Active: queue = ReferenceQueue with which instance is registered, or

      *     ReferenceQueue.NULL if it was not registered with a queue; next =

      *     null.

      *

      *     Pending: queue = ReferenceQueue with which instance is registered;

      *     next = Following instance in queue, or this if at end of list.

      *

      *     Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance

      *     in queue, or this if at end of list.

      *

      *     Inactive: queue = ReferenceQueue.NULL; next = this.

      *

      * With this scheme the collector need only examine the next field in order

      * to determine whether a Reference instance requires special treatment: If

      * the next field is null then the instance is active; if it is non-null,

      * then the collector should treat the instance normally.

      *

      * To ensure that concurrent collector can discover active Reference

      * objects without interfering with application threads that may apply

      * the enqueue() method to those objects, collectors should link

      * discovered objects through the discovered field.

      */

     private T referent;         /* Treated specially by GC */

     ReferenceQueue<? super T> queue;

     Reference next;

     transient private Reference<T> discovered;  /* used by VM */

     /* Object used to synchronize with the garbage collector.  The collector

      * must acquire this lock at the beginning of each collection cycle.  It is

      * therefore critical that any code holding this lock complete as quickly

      * as possible, allocate no new objects, and avoid calling user code.

      */

     static private class Lock { };

     private static Lock lock = new Lock();

     /* List of References waiting to be enqueued.  The collector adds

      * References to this list, while the Reference-handler thread removes

      * them.  This list is protected by the above lock object.

      */

     private static Reference pending = null;

     /* High-priority thread to enqueue pending References

      */

     private static class ReferenceHandler extends Thread {

         ReferenceHandler(ThreadGroup g, String name) {

             super(g, name);

         }

         public void run() {

             for (;;) {

                 Reference r;

                 synchronized (lock) {

                     if (pending != null) {

                         r = pending;

                         Reference rn = r.next;

                         pending = (rn == r) ? null : rn;

                         r.next = r;

                     } else {

                         try {

                             lock.wait();

                         } catch (InterruptedException x) { }

                         continue;

                     }

                 }

                 // Fast path for cleaners

                 if (r instanceof Cleaner) {

                     ((Cleaner)r).clean();

                     continue;

                 }

                 ReferenceQueue q = r.queue;

                 if (q != ReferenceQueue.NULL) q.enqueue(r);

             }

         }

     }

     static {

         ThreadGroup tg = Thread.currentThread().getThreadGroup();

         for (ThreadGroup tgn = tg;

              tgn != null;

              tg = tgn, tgn = tg.getParent());

         Thread handler = new ReferenceHandler(tg, "Reference Handler");

         /* If there were a special system-only priority greater than

          * MAX_PRIORITY, it would be used here

          */

         handler.setPriority(Thread.MAX_PRIORITY);

         handler.setDaemon(true);

         handler.start();

     }

     /* -- Referent accessor and setters -- */

     /**

      * Returns this reference object's referent.  If this reference object has

      * been cleared, either by the program or by the garbage collector, then

      * this method returns <code>null</code>.

      *

      * @return   The object to which this reference refers, or

      *           <code>null</code> if this reference object has been cleared

      */

     public T get() {

         return this.referent;

     }

     /**

      * Clears this reference object.  Invoking this method will not cause this

      * object to be enqueued.

      *

      * <p> This method is invoked only by Java code; when the garbage collector

      * clears references it does so directly, without invoking this method.

      */

     public void clear() {

         this.referent = null;

     }

     /* -- Queue operations -- */

     /**

      * Tells whether or not this reference object has been enqueued, either by

      * the program or by the garbage collector.  If this reference object was

      * not registered with a queue when it was created, then this method will

      * always return <code>false</code>.

      *

      * @return   <code>true</code> if and only if this reference object has

      *           been enqueued

      */

     public boolean isEnqueued() {

         /* In terms of the internal states, this predicate actually tests

            whether the instance is either Pending or Enqueued */

         synchronized (this) {

             return (this.queue != ReferenceQueue.NULL) && (this.next != null);

         }

     }

     /**

      * Adds this reference object to the queue with which it is registered,

      * if any.

      *

      * <p> This method is invoked only by Java code; when the garbage collector

      * enqueues references it does so directly, without invoking this method.

      *

      * @return   <code>true</code> if this reference object was successfully

      *           enqueued; <code>false</code> if it was already enqueued or if

      *           it was not registered with a queue when it was created

      */

     public boolean enqueue() {

         return this.queue.enqueue(this);

     }

     /* -- Constructors -- */

     Reference(T referent) {

         this(referent, null);

     }

     Reference(T referent, ReferenceQueue<? super T> queue) {

         this.referent = referent;

         this.queue = (queue == null) ? ReferenceQueue.NULL : queue;

     }

 }