ReferenceQueue.java revision 12745:f068a4ffddd2
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 26package java.lang.ref; 27 28import 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 38public 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 private static 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.next = (head == null) ? r : head; 69 head = r; 70 queueLength++; 71 // Update r.queue *after* adding to list, to avoid race 72 // with concurrent enqueued checks and fast-path poll(). 73 // Volatiles ensure ordering. 74 r.queue = ENQUEUED; 75 if (r instanceof FinalReference) { 76 sun.misc.VM.addFinalRefCount(1); 77 } 78 lock.notifyAll(); 79 return true; 80 } 81 } 82 83 private Reference<? extends T> reallyPoll() { /* Must hold lock */ 84 Reference<? extends T> r = head; 85 if (r != null) { 86 r.queue = NULL; 87 // Update r.queue *before* removing from list, to avoid 88 // race with concurrent enqueued checks and fast-path 89 // poll(). Volatiles ensure ordering. 90 @SuppressWarnings("unchecked") 91 Reference<? extends T> rn = r.next; 92 head = (rn == r) ? null : rn; 93 r.next = r; 94 queueLength--; 95 if (r instanceof FinalReference) { 96 sun.misc.VM.addFinalRefCount(-1); 97 } 98 return r; 99 } 100 return null; 101 } 102 103 /** 104 * Polls this queue to see if a reference object is available. If one is 105 * available without further delay then it is removed from the queue and 106 * returned. Otherwise this method immediately returns {@code null}. 107 * 108 * @return A reference object, if one was immediately available, 109 * otherwise {@code null} 110 */ 111 public Reference<? extends T> poll() { 112 if (head == null) 113 return null; 114 synchronized (lock) { 115 return reallyPoll(); 116 } 117 } 118 119 /** 120 * Removes the next reference object in this queue, blocking until either 121 * one becomes available or the given timeout period expires. 122 * 123 * <p> This method does not offer real-time guarantees: It schedules the 124 * timeout as if by invoking the {@link Object#wait(long)} method. 125 * 126 * @param timeout If positive, block for up to {@code timeout} 127 * milliseconds while waiting for a reference to be 128 * added to this queue. If zero, block indefinitely. 129 * 130 * @return A reference object, if one was available within the specified 131 * timeout period, otherwise {@code null} 132 * 133 * @throws IllegalArgumentException 134 * If the value of the timeout argument is negative 135 * 136 * @throws InterruptedException 137 * If the timeout wait is interrupted 138 */ 139 public Reference<? extends T> remove(long timeout) 140 throws IllegalArgumentException, InterruptedException 141 { 142 if (timeout < 0) { 143 throw new IllegalArgumentException("Negative timeout value"); 144 } 145 synchronized (lock) { 146 Reference<? extends T> r = reallyPoll(); 147 if (r != null) return r; 148 long start = (timeout == 0) ? 0 : System.nanoTime(); 149 for (;;) { 150 lock.wait(timeout); 151 r = reallyPoll(); 152 if (r != null) return r; 153 if (timeout != 0) { 154 long end = System.nanoTime(); 155 timeout -= (end - start) / 1000_000; 156 if (timeout <= 0) return null; 157 start = end; 158 } 159 } 160 } 161 } 162 163 /** 164 * Removes the next reference object in this queue, blocking until one 165 * becomes available. 166 * 167 * @return A reference object, blocking until one becomes available 168 * @throws InterruptedException If the wait is interrupted 169 */ 170 public Reference<? extends T> remove() throws InterruptedException { 171 return remove(0); 172 } 173 174 /** 175 * Iterate queue and invoke given action with each Reference. 176 * Suitable for diagnostic purposes. 177 * WARNING: any use of this method should make sure to not 178 * retain the referents of iterated references (in case of 179 * FinalReference(s)) so that their life is not prolonged more 180 * than necessary. 181 */ 182 void forEach(Consumer<? super Reference<? extends T>> action) { 183 for (Reference<? extends T> r = head; r != null;) { 184 action.accept(r); 185 @SuppressWarnings("unchecked") 186 Reference<? extends T> rn = r.next; 187 if (rn == r) { 188 if (r.queue == ENQUEUED) { 189 // still enqueued -> we reached end of chain 190 r = null; 191 } else { 192 // already dequeued: r.queue == NULL; -> 193 // restart from head when overtaken by queue poller(s) 194 r = head; 195 } 196 } else { 197 // next in chain 198 r = rn; 199 } 200 } 201 } 202} 203