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 }