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 import sun.misc.Unsafe;
  29 import java.util.function.Consumer;
  30 
  31 /**
  32  * Reference queues, to which registered reference objects are appended by the
  33  * garbage collector after the appropriate reachability changes are detected.
  34  *
  35  * @author   Mark Reinhold
  36  * @since    1.2
  37  */
  38 
  39 public class ReferenceQueue<T> {
  40 
  41     /**
  42      * Constructs a new reference-object queue.
  43      */
  44     public ReferenceQueue() { }
  45 
  46     private static class Null<S> extends ReferenceQueue<S> {
  47         boolean enqueue(Reference<? extends S> r) {
  48             return false;
  49         }
  50     }
  51 
  52     static ReferenceQueue<Object> NULL = new Null<>();
  53     static ReferenceQueue<Object> ENQUEUED = new Null<>();
  54 
  55     static private class Lock { };
  56     private Lock lock = new Lock();
  57     private volatile Reference<? extends T> head = null;
  58     private long queueLength = 0;
  59 
  60     boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
  61         synchronized (lock) {
  62             // Check that since getting the lock this reference hasn't already been
  63             // enqueued (and even then removed)
  64             ReferenceQueue<?> queue = r.queue;
  65             if ((queue == NULL) || (queue == ENQUEUED)) {
  66                 return false;
  67             }
  68             assert queue == this;
  69             r.queue = ENQUEUED;
  70             r.next = (head == null) ? r : head;
  71             head = r;
  72             queueLength++;
  73             if (r instanceof FinalReference) {
  74                 sun.misc.VM.addFinalRefCount(1);
  75             }
  76             lock.notifyAll();
  77             return true;
  78         }
  79     }
  80 
  81     @SuppressWarnings("unchecked")
  82     private Reference<? extends T> reallyPoll() {       /* Must hold lock */
  83         Reference<? extends T> r = head;
  84         if (r != null) {
  85             head = (r.next == r) ?
  86                 null :
  87                 r.next; // Unchecked due to the next field having a raw type in Reference
  88             r.queue = NULL;
  89             // make sure the store to r.next below doesn't float up
  90             // before the store to r.queue above because we need this
  91             // for correct lock-less iteration in forEach()
  92             U.storeFence();
  93             r.next = r;
  94             queueLength--;
  95             if (r instanceof FinalReference) {
  96                 sun.misc.VM.addFinalRefCount(-1);
  97             }
  98             return r;
  99         }
 100         return null;
 101     }
 102 
 103     /**
 104      * Polls this queue to see if a reference object is available.  If one is
 105      * available without further delay then it is removed from the queue and
 106      * returned.  Otherwise this method immediately returns <tt>null</tt>.
 107      *
 108      * @return  A reference object, if one was immediately available,
 109      *          otherwise <code>null</code>
 110      */
 111     public Reference<? extends T> poll() {
 112         if (head == null)
 113             return null;
 114         synchronized (lock) {
 115             return reallyPoll();
 116         }
 117     }
 118 
 119     /**
 120      * Removes the next reference object in this queue, blocking until either
 121      * one becomes available or the given timeout period expires.
 122      *
 123      * <p> This method does not offer real-time guarantees: It schedules the
 124      * timeout as if by invoking the {@link Object#wait(long)} method.
 125      *
 126      * @param  timeout  If positive, block for up to <code>timeout</code>
 127      *                  milliseconds while waiting for a reference to be
 128      *                  added to this queue.  If zero, block indefinitely.
 129      *
 130      * @return  A reference object, if one was available within the specified
 131      *          timeout period, otherwise <code>null</code>
 132      *
 133      * @throws  IllegalArgumentException
 134      *          If the value of the timeout argument is negative
 135      *
 136      * @throws  InterruptedException
 137      *          If the timeout wait is interrupted
 138      */
 139     public Reference<? extends T> remove(long timeout)
 140         throws IllegalArgumentException, InterruptedException
 141     {
 142         if (timeout < 0) {
 143             throw new IllegalArgumentException("Negative timeout value");
 144         }
 145         synchronized (lock) {
 146             Reference<? extends T> r = reallyPoll();
 147             if (r != null) return r;
 148             long start = (timeout == 0) ? 0 : System.nanoTime();
 149             for (;;) {
 150                 lock.wait(timeout);
 151                 r = reallyPoll();
 152                 if (r != null) return r;
 153                 if (timeout != 0) {
 154                     long end = System.nanoTime();
 155                     timeout -= (end - start) / 1000_000;
 156                     if (timeout <= 0) return null;
 157                     start = end;
 158                 }
 159             }
 160         }
 161     }
 162 
 163     /**
 164      * Removes the next reference object in this queue, blocking until one
 165      * becomes available.
 166      *
 167      * @return A reference object, blocking until one becomes available
 168      * @throws  InterruptedException  If the wait is interrupted
 169      */
 170     public Reference<? extends T> remove() throws InterruptedException {
 171         return remove(0);
 172     }
 173 
 174     /**
 175      * Iterate queue and invoke given action with each Reference.
 176      * Suitable for diagnostic purposes.
 177      * WARNING: any use of this method should make sure to not
 178      * retain the referents of iterated references (in case of
 179      * FinalReference(s)) so that their life is not prolonged more
 180      * than necessary.
 181      */
 182     void forEach(Consumer<? super Reference<? extends T>> action) {
 183         for (Reference<? extends T> r = head; r != null;) {
 184             action.accept(r);
 185             Reference rn = r.next;
 186             // make sure load of r.queue below doesn't float up before
 187             // the load of r.next above. The corresponding stores are
 188             // performed in reallyPoll() in reverse order.
 189             U.loadFence();
 190             if (rn == r) {
 191                 if (r.queue == ENQUEUED) {
 192                     // still enqueued -> we reached end of chain
 193                     r = null;
 194                 } else {
 195                     // already dequeued: r.queue == NULL; ->
 196                     // restart from head when overtaken by queue poller(s)
 197                     r = head;
 198                 }
 199             } else {
 200                 // next in chain
 201                 r = rn;
 202             }
 203         }
 204     }
 205 
 206     /* we need a fence */
 207     private static final Unsafe U = Unsafe.getUnsafe();
 208 }