1 /* 2 * Copyright (c) 1997, 2015, 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 java.util.function.Consumer; 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 private Reference<? extends T> reallyPoll() { /* Must hold lock */ 81 Reference<? extends T> r = head; 82 if (r != null) { 83 @SuppressWarnings("unchecked") 84 Reference<? extends T> rn = r.next; 85 head = (rn == r) ? null : rn; 86 r.queue = NULL; 87 r.next = r; 88 queueLength--; 89 if (r instanceof FinalReference) { 90 sun.misc.VM.addFinalRefCount(-1); 91 } 92 return r; 93 } 94 return null; 95 } 96 97 /** 98 * Polls this queue to see if a reference object is available. If one is 99 * available without further delay then it is removed from the queue and 100 * returned. Otherwise this method immediately returns <tt>null</tt>. 101 * 102 * @return A reference object, if one was immediately available, 103 * otherwise <code>null</code> 104 */ 105 public Reference<? extends T> poll() { 106 if (head == null) 107 return null; 108 synchronized (lock) { 109 return reallyPoll(); 110 } 111 } 112 113 /** 114 * Removes the next reference object in this queue, blocking until either 115 * one becomes available or the given timeout period expires. 116 * 117 * <p> This method does not offer real-time guarantees: It schedules the 118 * timeout as if by invoking the {@link Object#wait(long)} method. 119 * 120 * @param timeout If positive, block for up to <code>timeout</code> 121 * milliseconds while waiting for a reference to be 122 * added to this queue. If zero, block indefinitely. 123 * 124 * @return A reference object, if one was available within the specified 125 * timeout period, otherwise <code>null</code> 126 * 127 * @throws IllegalArgumentException 128 * If the value of the timeout argument is negative 129 * 130 * @throws InterruptedException 131 * If the timeout wait is interrupted 132 */ 133 public Reference<? extends T> remove(long timeout) 134 throws IllegalArgumentException, InterruptedException 135 { 136 if (timeout < 0) { 137 throw new IllegalArgumentException("Negative timeout value"); 138 } 139 synchronized (lock) { 140 Reference<? extends T> r = reallyPoll(); 141 if (r != null) return r; 142 long start = (timeout == 0) ? 0 : System.nanoTime(); 143 for (;;) { 144 lock.wait(timeout); 145 r = reallyPoll(); 146 if (r != null) return r; 147 if (timeout != 0) { 148 long end = System.nanoTime(); 149 timeout -= (end - start) / 1000_000; 150 if (timeout <= 0) return null; 151 start = end; 152 } 153 } 154 } 155 } 156 157 /** 158 * Removes the next reference object in this queue, blocking until one 159 * becomes available. 160 * 161 * @return A reference object, blocking until one becomes available 162 * @throws InterruptedException If the wait is interrupted 163 */ 164 public Reference<? extends T> remove() throws InterruptedException { 165 return remove(0); 166 } 167 168 /** 169 * Iterate queue and invoke given action with each Reference. 170 * Suitable for diagnostic purposes. 171 * WARNING: any use of this method should make sure to not 172 * retain the referents of iterated references (in case of 173 * FinalReference(s)) so that their life is not prolonged more 174 * than necessary. 175 */ 176 void forEach(Consumer<? super Reference<? extends T>> action) { 177 for (Reference<? extends T> r = head; r != null;) { 178 action.accept(r); 179 @SuppressWarnings("unchecked") 180 Reference<? extends T> rn = r.next; 181 if (rn == r) { 182 if (r.queue == ENQUEUED) { 183 // still enqueued -> we reached end of chain 184 r = null; 185 } else { 186 // already dequeued: r.queue == NULL; -> 187 // restart from head when overtaken by queue poller(s) 188 r = head; 189 } 190 } else { 191 // next in chain 192 r = rn; 193 } 194 } 195 } 196 }