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 static java.util.concurrent.TimeUnit.*; 29 30 /** 31 * Reference queues, to which registered reference objects are appended by the 32 * garbage collector after the appropriate reachability changes are detected. 33 * 34 * @author Mark Reinhold 35 * @since 1.2 36 */ 37 38 public class ReferenceQueue<T> { 39 40 /** 41 * Constructs a new reference-object queue. 42 */ 43 public ReferenceQueue() { } 44 45 private static class Null<S> extends ReferenceQueue<S> { 46 boolean enqueue(Reference<? extends S> r) { 47 return false; 48 } 49 } 50 51 static ReferenceQueue<Object> NULL = new Null<>(); 52 static ReferenceQueue<Object> ENQUEUED = new Null<>(); 53 54 static private class Lock { }; 55 private Lock lock = new Lock(); 56 private volatile Reference<? extends T> head = null; 57 private long queueLength = 0; 58 59 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */ 60 synchronized (lock) { 61 // Check that since getting the lock this reference hasn't already been 62 // enqueued (and even then removed) 63 ReferenceQueue<?> queue = r.queue; 64 if ((queue == NULL) || (queue == ENQUEUED)) { 65 return false; 66 } 67 assert queue == this; 68 r.queue = ENQUEUED; 69 r.next = (head == null) ? r : head; 70 head = r; 71 queueLength++; 72 if (r instanceof FinalReference) { 73 sun.misc.VM.addFinalRefCount(1); 74 } 75 lock.notifyAll(); 76 return true; 77 } 78 } 79 80 @SuppressWarnings("unchecked") 81 private Reference<? extends T> reallyPoll() { /* Must hold lock */ 82 Reference<? extends T> r = head; 83 if (r != null) { 84 head = (r.next == r) ? 85 null : 86 r.next; // Unchecked due to the next field having a raw type in Reference 87 r.queue = NULL; 88 r.next = r; 89 queueLength--; 90 if (r instanceof FinalReference) { 91 sun.misc.VM.addFinalRefCount(-1); 92 } 93 return r; 94 } 95 return null; 96 } 97 98 /** 99 * Polls this queue to see if a reference object is available. If one is 100 * available without further delay then it is removed from the queue and 101 * returned. Otherwise this method immediately returns <tt>null</tt>. 102 * 103 * @return A reference object, if one was immediately available, 104 * otherwise <code>null</code> 105 */ 106 public Reference<? extends T> poll() { 107 if (head == null) 108 return null; 109 synchronized (lock) { 110 return reallyPoll(); 111 } 112 } 113 114 /** 115 * Removes the next reference object in this queue, blocking until either 116 * one becomes available or the given timeout period expires. 117 * 118 * <p> This method does not offer real-time guarantees: It schedules the 119 * timeout as if by invoking the {@link Object#wait(long)} method. 120 * 121 * @param timeout If positive, block for up to <code>timeout</code> 122 * milliseconds while waiting for a reference to be 123 * added to this queue. If zero, block indefinitely. 124 * 125 * @return A reference object, if one was available within the specified 126 * timeout period, otherwise <code>null</code> 127 * 128 * @throws IllegalArgumentException 129 * If the value of the timeout argument is negative 130 * 131 * @throws InterruptedException 132 * If the timeout wait is interrupted 133 */ 134 public Reference<? extends T> remove(long timeout) 135 throws IllegalArgumentException, InterruptedException 136 { 137 if (timeout < 0) { 138 throw new IllegalArgumentException("Negative timeout value"); 139 } 140 synchronized (lock) { 141 Reference<? extends T> r = reallyPoll(); 142 if (r != null) return r; 143 long start = (timeout == 0) ? 0 : System.nanoTime(); 144 for (;;) { 145 lock.wait(timeout); 146 r = reallyPoll(); 147 if (r != null) return r; 148 if (timeout != 0) { 149 long end = System.nanoTime(); 150 timeout -= NANOSECONDS.toMillis(end - start); 151 if (timeout <= 0) return null; 152 start = end; 153 } 154 } 155 } 156 } 157 158 /** 159 * Removes the next reference object in this queue, blocking until one 160 * becomes available. 161 * 162 * @return A reference object, blocking until one becomes available 163 * @throws InterruptedException If the wait is interrupted 164 */ 165 public Reference<? extends T> remove() throws InterruptedException { 166 return remove(0); 167 } 168 169 }