/*

  * Copyright (c) 1997, 2005, Oracle and/or its affiliates. All rights reserved.

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

  *

  * This code is free software; you can redistribute it and/or modify it

  * 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;

 /**

  * Reference queues, to which registered reference objects are appended by the

  * garbage collector after the appropriate reachability changes are detected.

  *

  * @author   Mark Reinhold

  * @since    1.2

  */

 public class ReferenceQueue<T> {

     /**

      * Constructs a new reference-object queue.

      */

     public ReferenceQueue() { }

     private static class Null extends ReferenceQueue {

         boolean enqueue(Reference r) {

             return false;

         }

     }

     static ReferenceQueue NULL = new Null();

     static ReferenceQueue ENQUEUED = new Null();

     static private class Lock { };

     private Lock lock = new Lock();

     private volatile Reference<? extends T> head = null;

     private long queueLength = 0;

     boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */

         synchronized (r) {

             if (r.queue == ENQUEUED) return false;

             synchronized (lock) {

                 r.queue = ENQUEUED;

                 r.next = (head == null) ? r : head;

                 head = r;

                 queueLength++;

                 lock.notifyAll();

                 return true;

             }

         }

     }

     private Reference<? extends T> reallyPoll() {       /* Must hold lock */

         if (head != null) {

             Reference<? extends T> r = head;

             head = (r.next == r) ? null : r.next;

             r.queue = NULL;

             r.next = r;

             queueLength--;

             return r;

         }

         return null;

     }

     /**

      * Polls this queue to see if a reference object is available.  If one is

      * available without further delay then it is removed from the queue and

      * returned.  Otherwise this method immediately returns <tt>null</tt>.

      *

      * @return  A reference object, if one was immediately available,

      *          otherwise <code>null</code>

      */

     public Reference<? extends T> poll() {

         if (head == null)

             return null;

         synchronized (lock) {

             return reallyPoll();

         }

     }

     /**

      * Removes the next reference object in this queue, blocking until either

      * one becomes available or the given timeout period expires.

      *

      * <p> This method does not offer real-time guarantees: It schedules the

      * timeout as if by invoking the {@link Object#wait(long)} method.

      *

      * @param  timeout  If positive, block for up to <code>timeout</code>

      *                  milliseconds while waiting for a reference to be

      *                  added to this queue.  If zero, block indefinitely.

      *

      * @return  A reference object, if one was available within the specified

      *          timeout period, otherwise <code>null</code>

      *

      * @throws  IllegalArgumentException

      *          If the value of the timeout argument is negative

      *

      * @throws  InterruptedException

      *          If the timeout wait is interrupted

      */

     public Reference<? extends T> remove(long timeout)

         throws IllegalArgumentException, InterruptedException

     {

         if (timeout < 0) {

             throw new IllegalArgumentException("Negative timeout value");

         }

         synchronized (lock) {

             Reference<? extends T> r = reallyPoll();

             if (r != null) return r;

             for (;;) {

                 lock.wait(timeout);

                 r = reallyPoll();

                 if (r != null) return r;

                 if (timeout != 0) return null;

             }

         }

     }

     /**

      * Removes the next reference object in this queue, blocking until one

      * becomes available.

      *

      * @return A reference object, blocking until one becomes available

      * @throws  InterruptedException  If the wait is interrupted

      */

     public Reference<? extends T> remove() throws InterruptedException {

         return remove(0);

     }

 }