1 /*
   2  * Copyright (c) 1997, 2013, 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<S> extends ReferenceQueue<S> {
  44         boolean enqueue(Reference<? extends S> r) {
  45             return false;
  46         }
  47     }
  48 
  49     static ReferenceQueue<Object> NULL = new Null<>();
  50     static ReferenceQueue<Object> 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 (lock) {
  59             // Check that since getting the lock this reference hasn't already been
  60             // enqueued (and even then removed)
  61             ReferenceQueue<?> queue = r.queue;
  62             if ((queue == NULL) || (queue == ENQUEUED)) {
  63                 return false;
  64             }
  65             assert queue == this;
  66             r.queue = ENQUEUED;
  67             r.next = (head == null) ? r : head;
  68             head = r;
  69             queueLength++;
  70             if (r instanceof FinalReference) {
  71                 sun.misc.VM.addFinalRefCount(1);
  72             }
  73             lock.notifyAll();
  74             return true;
  75         }
  76     }
  77 
  78     @SuppressWarnings("unchecked")
  79     private Reference<? extends T> reallyPoll() {       /* Must hold lock */
  80         Reference<? extends T> r = head;
  81         if (r != null) {
  82             head = (r.next == r) ?
  83                 null :
  84                 r.next; // Unchecked due to the next field having a raw type in Reference
  85             r.queue = NULL;




  86             r.next = r;
  87             queueLength--;
  88             if (r instanceof FinalReference) {
  89                 sun.misc.VM.addFinalRefCount(-1);
  90             }
  91             return r;
  92         }
  93         return null;
  94     }
  95 
  96     /**
  97      * Polls this queue to see if a reference object is available.  If one is
  98      * available without further delay then it is removed from the queue and
  99      * returned.  Otherwise this method immediately returns <tt>null</tt>.
 100      *
 101      * @return  A reference object, if one was immediately available,
 102      *          otherwise <code>null</code>
 103      */
 104     public Reference<? extends T> poll() {
 105         if (head == null)
 106             return null;
 107         synchronized (lock) {
 108             return reallyPoll();
 109         }
 110     }
 111 
 112     /**
 113      * Removes the next reference object in this queue, blocking until either
 114      * one becomes available or the given timeout period expires.
 115      *
 116      * <p> This method does not offer real-time guarantees: It schedules the
 117      * timeout as if by invoking the {@link Object#wait(long)} method.
 118      *
 119      * @param  timeout  If positive, block for up to <code>timeout</code>
 120      *                  milliseconds while waiting for a reference to be
 121      *                  added to this queue.  If zero, block indefinitely.
 122      *
 123      * @return  A reference object, if one was available within the specified
 124      *          timeout period, otherwise <code>null</code>
 125      *
 126      * @throws  IllegalArgumentException
 127      *          If the value of the timeout argument is negative
 128      *
 129      * @throws  InterruptedException
 130      *          If the timeout wait is interrupted
 131      */
 132     public Reference<? extends T> remove(long timeout)
 133         throws IllegalArgumentException, InterruptedException
 134     {
 135         if (timeout < 0) {
 136             throw new IllegalArgumentException("Negative timeout value");
 137         }
 138         synchronized (lock) {
 139             Reference<? extends T> r = reallyPoll();
 140             if (r != null) return r;
 141             long start = (timeout == 0) ? 0 : System.nanoTime();
 142             for (;;) {
 143                 lock.wait(timeout);
 144                 r = reallyPoll();
 145                 if (r != null) return r;
 146                 if (timeout != 0) {
 147                     long end = System.nanoTime();
 148                     timeout -= (end - start) / 1000_000;
 149                     if (timeout <= 0) return null;
 150                     start = end;
 151                 }
 152             }
 153         }
 154     }
 155 
 156     /**
 157      * Removes the next reference object in this queue, blocking until one
 158      * becomes available.
 159      *
 160      * @return A reference object, blocking until one becomes available
 161      * @throws  InterruptedException  If the wait is interrupted
 162      */
 163     public Reference<? extends T> remove() throws InterruptedException {
 164         return remove(0);
 165     }
 166 


































 167 }
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