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    1   /*
    2    * Copyright (c) 1997, 2005, Oracle and/or its affiliates. All rights reserved.
    3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
    4    *
    5    * This code is free software; you can redistribute it and/or modify it
    6    * under the terms of the GNU General Public License version 2 only, as
    7    * published by the Free Software Foundation.  Oracle designates this
    8    * particular file as subject to the "Classpath" exception as provided
    9    * by Oracle in the LICENSE file that accompanied this code.
   10    *
   11    * This code is distributed in the hope that it will be useful, but WITHOUT
   12    * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   14    * version 2 for more details (a copy is included in the LICENSE file that
   15    * accompanied this code).
   16    *
   17    * You should have received a copy of the GNU General Public License version
   18    * 2 along with this work; if not, write to the Free Software Foundation,
   19    * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   20    *
   21    * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   22    * or visit www.oracle.com if you need additional information or have any
   23    * questions.
   24    */
   25   
   26   package java.lang.ref;
   27   
   28   /**
   29    * Reference queues, to which registered reference objects are appended by the
   30    * garbage collector after the appropriate reachability changes are detected.
   31    *
   32    * @author   Mark Reinhold
   33    * @since    1.2
   34    */
   35   
   36   public class ReferenceQueue<T> {
   37   
   38       /**
   39        * Constructs a new reference-object queue.
   40        */
   41       public ReferenceQueue() { }
   42   
   43       private static class Null extends ReferenceQueue {
   44           boolean enqueue(Reference r) {
   45               return false;
   46           }
   47       }
   48   
   49       static ReferenceQueue NULL = new Null();
   50       static ReferenceQueue ENQUEUED = new Null();
   51   
   52       static private class Lock { };
   53       private Lock lock = new Lock();
   54       private volatile Reference<? extends T> head = null;
   55       private long queueLength = 0;
   56   
   57       boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
   58           synchronized (r) {
   59               if (r.queue == ENQUEUED) return false;
   60               synchronized (lock) {
   61                   r.queue = ENQUEUED;
   62                   r.next = (head == null) ? r : head;
   63                   head = r;
   64                   queueLength++;
   65                   if (r instanceof FinalReference) {
   66                       sun.misc.VM.addFinalRefCount(1);
   67                   }
   68                   lock.notifyAll();
   69                   return true;
   70               }
   71           }
   72       }
   73   
   74       private Reference<? extends T> reallyPoll() {       /* Must hold lock */
   75           if (head != null) {
   76               Reference<? extends T> r = head;
   77               head = (r.next == r) ? null : r.next;
   78               r.queue = NULL;
   79               r.next = r;
   80               queueLength--;
   81               if (r instanceof FinalReference) {
   82                   sun.misc.VM.addFinalRefCount(-1);
   83               }
   84               return r;
   85           }
   86           return null;
   87       }
   88   
   89       /**
   90        * Polls this queue to see if a reference object is available.  If one is
   91        * available without further delay then it is removed from the queue and
   92        * returned.  Otherwise this method immediately returns <tt>null</tt>.
   93        *
   94        * @return  A reference object, if one was immediately available,
   95        *          otherwise <code>null</code>
   96        */
   97       public Reference<? extends T> poll() {
   98           if (head == null)
   99               return null;
  100           synchronized (lock) {
  101               return reallyPoll();
  102           }
  103       }
  104   
  105       /**
  106        * Removes the next reference object in this queue, blocking until either
  107        * one becomes available or the given timeout period expires.
  108        *
  109        * <p> This method does not offer real-time guarantees: It schedules the
  110        * timeout as if by invoking the {@link Object#wait(long)} method.
  111        *
  112        * @param  timeout  If positive, block for up to <code>timeout</code>
  113        *                  milliseconds while waiting for a reference to be
  114        *                  added to this queue.  If zero, block indefinitely.
  115        *
  116        * @return  A reference object, if one was available within the specified
  117        *          timeout period, otherwise <code>null</code>
  118        *
  119        * @throws  IllegalArgumentException
  120        *          If the value of the timeout argument is negative
  121        *
  122        * @throws  InterruptedException
  123        *          If the timeout wait is interrupted
  124        */
  125       public Reference<? extends T> remove(long timeout)
  126           throws IllegalArgumentException, InterruptedException
  127       {
  128           if (timeout < 0) {
  129               throw new IllegalArgumentException("Negative timeout value");
  130           }
  131           synchronized (lock) {
  132               Reference<? extends T> r = reallyPoll();
  133               if (r != null) return r;
  134               for (;;) {
  135                   lock.wait(timeout);
  136                   r = reallyPoll();
  137                   if (r != null) return r;
  138                   if (timeout != 0) return null;
  139               }
  140           }
  141       }
  142   
  143       /**
  144        * Removes the next reference object in this queue, blocking until one
  145        * becomes available.
  146        *
  147        * @return A reference object, blocking until one becomes available
  148        * @throws  InterruptedException  If the wait is interrupted
  149        */
  150       public Reference<? extends T> remove() throws InterruptedException {
  151           return remove(0);
  152       }
  153   
  154   }

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