J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 1 | /* |
| 2 | * Copyright 1997-2005 Sun Microsystems, Inc. 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. Sun designates this |
| 8 | * particular file as subject to the "Classpath" exception as provided |
| 9 | * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| 22 | * CA 95054 USA or visit www.sun.com if you need additional information or |
| 23 | * have any 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 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 | synchronized (lock) { |
| 99 | return reallyPoll(); |
| 100 | } |
| 101 | } |
| 102 | |
| 103 | /** |
| 104 | * Removes the next reference object in this queue, blocking until either |
| 105 | * one becomes available or the given timeout period expires. |
| 106 | * |
| 107 | * <p> This method does not offer real-time guarantees: It schedules the |
| 108 | * timeout as if by invoking the {@link Object#wait(long)} method. |
| 109 | * |
| 110 | * @param timeout If positive, block for up to <code>timeout</code> |
| 111 | * milliseconds while waiting for a reference to be |
| 112 | * added to this queue. If zero, block indefinitely. |
| 113 | * |
| 114 | * @return A reference object, if one was available within the specified |
| 115 | * timeout period, otherwise <code>null</code> |
| 116 | * |
| 117 | * @throws IllegalArgumentException |
| 118 | * If the value of the timeout argument is negative |
| 119 | * |
| 120 | * @throws InterruptedException |
| 121 | * If the timeout wait is interrupted |
| 122 | */ |
| 123 | public Reference<? extends T> remove(long timeout) |
| 124 | throws IllegalArgumentException, InterruptedException |
| 125 | { |
| 126 | if (timeout < 0) { |
| 127 | throw new IllegalArgumentException("Negative timeout value"); |
| 128 | } |
| 129 | synchronized (lock) { |
| 130 | Reference<? extends T> r = reallyPoll(); |
| 131 | if (r != null) return r; |
| 132 | for (;;) { |
| 133 | lock.wait(timeout); |
| 134 | r = reallyPoll(); |
| 135 | if (r != null) return r; |
| 136 | if (timeout != 0) return null; |
| 137 | } |
| 138 | } |
| 139 | } |
| 140 | |
| 141 | /** |
| 142 | * Removes the next reference object in this queue, blocking until one |
| 143 | * becomes available. |
| 144 | * |
| 145 | * @return A reference object, blocking until one becomes available |
| 146 | * @throws InterruptedException If the wait is interrupted |
| 147 | */ |
| 148 | public Reference<? extends T> remove() throws InterruptedException { |
| 149 | return remove(0); |
| 150 | } |
| 151 | |
| 152 | } |