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| 1 | /* | ||
| 2 | * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | ||
| 3 | * | ||
| 4 | * This code is free software; you can redistribute it and/or modify it | ||
| 5 | * under the terms of the GNU General Public License version 2 only, as | ||
| 6 | * published by the Free Software Foundation. Oracle designates this | ||
| 7 | * particular file as subject to the "Classpath" exception as provided | ||
| 8 | * by Oracle in the LICENSE file that accompanied this code. | ||
| 9 | * | ||
| 10 | * This code is distributed in the hope that it will be useful, but WITHOUT | ||
| 11 | * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | ||
| 12 | * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | ||
| 13 | * version 2 for more details (a copy is included in the LICENSE file that | ||
| 14 | * accompanied this code). | ||
| 15 | * | ||
| 16 | * You should have received a copy of the GNU General Public License version | ||
| 17 | * 2 along with this work; if not, write to the Free Software Foundation, | ||
| 18 | * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. | ||
| 19 | * | ||
| 20 | * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | ||
| 21 | * or visit www.oracle.com if you need additional information or have any | ||
| 22 | * questions. | ||
| 23 | */ | ||
| 24 | |||
| 25 | /* | ||
| 26 | * This file is available under and governed by the GNU General Public | ||
| 27 | * License version 2 only, as published by the Free Software Foundation. | ||
| 28 | * However, the following notice accompanied the original version of this | ||
| 29 | * file: | ||
| 30 | * | ||
| 31 | * Written by Doug Lea with assistance from members of JCP JSR-166 | ||
| 32 | * Expert Group and released to the public domain, as explained at | ||
| 33 | * http://creativecommons.org/publicdomain/zero/1.0/ | ||
| 34 | */ | ||
| 35 | |||
| 36 | package java.util.concurrent; | ||
| 37 | import java.util.concurrent.locks.*; | ||
| 38 | import java.util.concurrent.atomic.*; | ||
| 39 | |||
| 40 | /** | ||
| 41 | * A synchronization aid that allows one or more threads to wait until | ||
| 42 | * a set of operations being performed in other threads completes. | ||
| 43 | * | ||
| 44 | * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>. | ||
| 45 | * The {@link #await await} methods block until the current count reaches | ||
| 46 | * zero due to invocations of the {@link #countDown} method, after which | ||
| 47 | * all waiting threads are released and any subsequent invocations of | ||
| 48 | * {@link #await await} return immediately. This is a one-shot phenomenon | ||
| 49 | * -- the count cannot be reset. If you need a version that resets the | ||
| 50 | * count, consider using a {@link CyclicBarrier}. | ||
| 51 | * | ||
| 52 | * <p>A {@code CountDownLatch} is a versatile synchronization tool | ||
| 53 | * and can be used for a number of purposes. A | ||
| 54 | * {@code CountDownLatch} initialized with a count of one serves as a | ||
| 55 | * simple on/off latch, or gate: all threads invoking {@link #await await} | ||
| 56 | * wait at the gate until it is opened by a thread invoking {@link | ||
| 57 | * #countDown}. A {@code CountDownLatch} initialized to <em>N</em> | ||
| 58 | * can be used to make one thread wait until <em>N</em> threads have | ||
| 59 | * completed some action, or some action has been completed N times. | ||
| 60 | * | ||
| 61 | * <p>A useful property of a {@code CountDownLatch} is that it | ||
| 62 | * doesn't require that threads calling {@code countDown} wait for | ||
| 63 | * the count to reach zero before proceeding, it simply prevents any | ||
| 64 | * thread from proceeding past an {@link #await await} until all | ||
| 65 | * threads could pass. | ||
| 66 | * | ||
| 67 | * <p><b>Sample usage:</b> Here is a pair of classes in which a group | ||
| 68 | * of worker threads use two countdown latches: | ||
| 69 | * <ul> | ||
| 70 | * <li>The first is a start signal that prevents any worker from proceeding | ||
| 71 | * until the driver is ready for them to proceed; | ||
| 72 | * <li>The second is a completion signal that allows the driver to wait | ||
| 73 | * until all workers have completed. | ||
| 74 | * </ul> | ||
| 75 | * | ||
| 76 | * <pre> | ||
| 77 | * class Driver { // ... | ||
| 78 | * void main() throws InterruptedException { | ||
| 79 | * CountDownLatch startSignal = new CountDownLatch(1); | ||
| 80 | * CountDownLatch doneSignal = new CountDownLatch(N); | ||
| 81 | * | ||
| 82 | * for (int i = 0; i < N; ++i) // create and start threads | ||
| 83 | * new Thread(new Worker(startSignal, doneSignal)).start(); | ||
| 84 | * | ||
| 85 | * doSomethingElse(); // don't let run yet | ||
| 86 | * startSignal.countDown(); // let all threads proceed | ||
| 87 | * doSomethingElse(); | ||
| 88 | * doneSignal.await(); // wait for all to finish | ||
| 89 | * } | ||
| 90 | * } | ||
| 91 | * | ||
| 92 | * class Worker implements Runnable { | ||
| 93 | * private final CountDownLatch startSignal; | ||
| 94 | * private final CountDownLatch doneSignal; | ||
| 95 | * Worker(CountDownLatch startSignal, CountDownLatch doneSignal) { | ||
| 96 | * this.startSignal = startSignal; | ||
| 97 | * this.doneSignal = doneSignal; | ||
| 98 | * } | ||
| 99 | * public void run() { | ||
| 100 | * try { | ||
| 101 | * startSignal.await(); | ||
| 102 | * doWork(); | ||
| 103 | * doneSignal.countDown(); | ||
| 104 | * } catch (InterruptedException ex) {} // return; | ||
| 105 | * } | ||
| 106 | * | ||
| 107 | * void doWork() { ... } | ||
| 108 | * } | ||
| 109 | * | ||
| 110 | * </pre> | ||
| 111 | * | ||
| 112 | * <p>Another typical usage would be to divide a problem into N parts, | ||
| 113 | * describe each part with a Runnable that executes that portion and | ||
| 114 | * counts down on the latch, and queue all the Runnables to an | ||
| 115 | * Executor. When all sub-parts are complete, the coordinating thread | ||
| 116 | * will be able to pass through await. (When threads must repeatedly | ||
| 117 | * count down in this way, instead use a {@link CyclicBarrier}.) | ||
| 118 | * | ||
| 119 | * <pre> | ||
| 120 | * class Driver2 { // ... | ||
| 121 | * void main() throws InterruptedException { | ||
| 122 | * CountDownLatch doneSignal = new CountDownLatch(N); | ||
| 123 | * Executor e = ... | ||
| 124 | * | ||
| 125 | * for (int i = 0; i < N; ++i) // create and start threads | ||
| 126 | * e.execute(new WorkerRunnable(doneSignal, i)); | ||
| 127 | * | ||
| 128 | * doneSignal.await(); // wait for all to finish | ||
| 129 | * } | ||
| 130 | * } | ||
| 131 | * | ||
| 132 | * class WorkerRunnable implements Runnable { | ||
| 133 | * private final CountDownLatch doneSignal; | ||
| 134 | * private final int i; | ||
| 135 | * WorkerRunnable(CountDownLatch doneSignal, int i) { | ||
| 136 | * this.doneSignal = doneSignal; | ||
| 137 | * this.i = i; | ||
| 138 | * } | ||
| 139 | * public void run() { | ||
| 140 | * try { | ||
| 141 | * doWork(i); | ||
| 142 | * doneSignal.countDown(); | ||
| 143 | * } catch (InterruptedException ex) {} // return; | ||
| 144 | * } | ||
| 145 | * | ||
| 146 | * void doWork() { ... } | ||
| 147 | * } | ||
| 148 | * | ||
| 149 | * </pre> | ||
| 150 | * | ||
| 151 | * <p>Memory consistency effects: Until the count reaches | ||
| 152 | * zero, actions in a thread prior to calling | ||
| 153 | * {@code countDown()} | ||
| 154 | * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> | ||
| 155 | * actions following a successful return from a corresponding | ||
| 156 | * {@code await()} in another thread. | ||
| 157 | * | ||
| 158 | * @since 1.5 | ||
| 159 | * @author Doug Lea | ||
| 160 | */ | ||
| 161 | public class CountDownLatch { | ||
| 162 | /** | ||
| 163 | * Synchronization control For CountDownLatch. | ||
| 164 | * Uses AQS state to represent count. | ||
| 165 | */ | ||
| 166 | private static final class Sync extends AbstractQueuedSynchronizer { | ||
| 167 | private static final long serialVersionUID = 4982264981922014374L; | ||
| 168 | |||
| 169 | Sync(int count) { | ||
| 170 | setState(count); | ||
| 171 | } | ||
| 172 | |||
| 173 | int getCount() { | ||
| 174 | return getState(); | ||
| 175 | } | ||
| 176 | |||
| 177 | protected int tryAcquireShared(int acquires) { | ||
| 178 | return (getState() == 0) ? 1 : -1; | ||
| 179 | } | ||
| 180 | |||
| 181 | protected boolean tryReleaseShared(int releases) { | ||
| 182 | // Decrement count; signal when transition to zero | ||
| 183 | for (;;) { | ||
| 184 | int c = getState(); | ||
| 185 | if (c == 0) | ||
| 186 | return false; | ||
| 187 | int nextc = c-1; | ||
| 188 | if (compareAndSetState(c, nextc)) | ||
| 189 | return nextc == 0; | ||
| 190 | } | ||
| 191 | } | ||
| 192 | } | ||
| 193 | |||
| 194 | private final Sync sync; | ||
| 195 | |||
| 196 | /** | ||
| 197 | * Constructs a {@code CountDownLatch} initialized with the given count. | ||
| 198 | * | ||
| 199 | * @param count the number of times {@link #countDown} must be invoked | ||
| 200 | * before threads can pass through {@link #await} | ||
| 201 | * @throws IllegalArgumentException if {@code count} is negative | ||
| 202 | */ | ||
| 203 | public CountDownLatch(int count) { | ||
| 204 | if (count < 0) throw new IllegalArgumentException("count < 0"); | ||
| 205 | this.sync = new Sync(count); | ||
| 206 | } | ||
| 207 | |||
| 208 | /** | ||
| 209 | * Causes the current thread to wait until the latch has counted down to | ||
| 210 | * zero, unless the thread is {@linkplain Thread#interrupt interrupted}. | ||
| 211 | * | ||
| 212 | * <p>If the current count is zero then this method returns immediately. | ||
| 213 | * | ||
| 214 | * <p>If the current count is greater than zero then the current | ||
| 215 | * thread becomes disabled for thread scheduling purposes and lies | ||
| 216 | * dormant until one of two things happen: | ||
| 217 | * <ul> | ||
| 218 | * <li>The count reaches zero due to invocations of the | ||
| 219 | * {@link #countDown} method; or | ||
| 220 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} | ||
| 221 | * the current thread. | ||
| 222 | * </ul> | ||
| 223 | * | ||
| 224 | * <p>If the current thread: | ||
| 225 | * <ul> | ||
| 226 | * <li>has its interrupted status set on entry to this method; or | ||
| 227 | * <li>is {@linkplain Thread#interrupt interrupted} while waiting, | ||
| 228 | * </ul> | ||
| 229 | * then {@link InterruptedException} is thrown and the current thread's | ||
| 230 | * interrupted status is cleared. | ||
| 231 | * | ||
| 232 | * @throws InterruptedException if the current thread is interrupted | ||
| 233 | * while waiting | ||
| 234 | */ | ||
| 235 | public void await() throws InterruptedException { | ||
| 236 | sync.acquireSharedInterruptibly(1); | ||
| 237 | } | ||
| 238 | |||
| 239 | /** | ||
| 240 | * Causes the current thread to wait until the latch has counted down to | ||
| 241 | * zero, unless the thread is {@linkplain Thread#interrupt interrupted}, | ||
| 242 | * or the specified waiting time elapses. | ||
| 243 | * | ||
| 244 | * <p>If the current count is zero then this method returns immediately | ||
| 245 | * with the value {@code true}. | ||
| 246 | * | ||
| 247 | * <p>If the current count is greater than zero then the current | ||
| 248 | * thread becomes disabled for thread scheduling purposes and lies | ||
| 249 | * dormant until one of three things happen: | ||
| 250 | * <ul> | ||
| 251 | * <li>The count reaches zero due to invocations of the | ||
| 252 | * {@link #countDown} method; or | ||
| 253 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} | ||
| 254 | * the current thread; or | ||
| 255 | * <li>The specified waiting time elapses. | ||
| 256 | * </ul> | ||
| 257 | * | ||
| 258 | * <p>If the count reaches zero then the method returns with the | ||
| 259 | * value {@code true}. | ||
| 260 | * | ||
| 261 | * <p>If the current thread: | ||
| 262 | * <ul> | ||
| 263 | * <li>has its interrupted status set on entry to this method; or | ||
| 264 | * <li>is {@linkplain Thread#interrupt interrupted} while waiting, | ||
| 265 | * </ul> | ||
| 266 | * then {@link InterruptedException} is thrown and the current thread's | ||
| 267 | * interrupted status is cleared. | ||
| 268 | * | ||
| 269 | * <p>If the specified waiting time elapses then the value {@code false} | ||
| 270 | * is returned. If the time is less than or equal to zero, the method | ||
| 271 | * will not wait at all. | ||
| 272 | * | ||
| 273 | * @param timeout the maximum time to wait | ||
| 274 | * @param unit the time unit of the {@code timeout} argument | ||
| 275 | * @return {@code true} if the count reached zero and {@code false} | ||
| 276 | * if the waiting time elapsed before the count reached zero | ||
| 277 | * @throws InterruptedException if the current thread is interrupted | ||
| 278 | * while waiting | ||
| 279 | */ | ||
| 280 | public boolean await(long timeout, TimeUnit unit) | ||
| 281 | throws InterruptedException { | ||
| 282 | return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); | ||
| 283 | } | ||
| 284 | |||
| 285 | /** | ||
| 286 | * Decrements the count of the latch, releasing all waiting threads if | ||
| 287 | * the count reaches zero. | ||
| 288 | * | ||
| 289 | * <p>If the current count is greater than zero then it is decremented. | ||
| 290 | * If the new count is zero then all waiting threads are re-enabled for | ||
| 291 | * thread scheduling purposes. | ||
| 292 | * | ||
| 293 | * <p>If the current count equals zero then nothing happens. | ||
| 294 | */ | ||
| 295 | public void countDown() { | ||
| 296 | sync.releaseShared(1); | ||
| 297 | } | ||
| 298 | |||
| 299 | /** | ||
| 300 | * Returns the current count. | ||
| 301 | * | ||
| 302 | * <p>This method is typically used for debugging and testing purposes. | ||
| 303 | * | ||
| 304 | * @return the current count | ||
| 305 | */ | ||
| 306 | public long getCount() { | ||
| 307 | return sync.getCount(); | ||
| 308 | } | ||
| 309 | |||
| 310 | /** | ||
| 311 | * Returns a string identifying this latch, as well as its state. | ||
| 312 | * The state, in brackets, includes the String {@code "Count ="} | ||
| 313 | * followed by the current count. | ||
| 314 | * | ||
| 315 | * @return a string identifying this latch, as well as its state | ||
| 316 | */ | ||
| 317 | public String toString() { | ||
| 318 | return super.toString() + "[Count = " + sync.getCount() + "]"; | ||
| 319 | } | ||
| 320 | } | ||