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 for (;;) { 142 lock.wait(timeout); 143 r = reallyPoll(); 144 if (r != null) return r; 145 if (timeout != 0) return null; 146 } 147 } 148 } 149 150 /** 151 * Removes the next reference object in this queue, blocking until one 152 * becomes available. 153 * 154 * @return A reference object, blocking until one becomes available 155 * @throws InterruptedException If the wait is interrupted 156 */ 157 public Reference<? extends T> remove() throws InterruptedException { 158 return remove(0); 159 } 160 161 }