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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 | + | |
| 39 | +/** | |
| 40 | + * A synchronization aid that allows a set of threads to all wait for | |
| 41 | + * each other to reach a common barrier point. CyclicBarriers are | |
| 42 | + * useful in programs involving a fixed sized party of threads that | |
| 43 | + * must occasionally wait for each other. The barrier is called | |
| 44 | + * <em>cyclic</em> because it can be re-used after the waiting threads | |
| 45 | + * are released. | |
| 46 | + * | |
| 47 | + * <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command | |
| 48 | + * that is run once per barrier point, after the last thread in the party | |
| 49 | + * arrives, but before any threads are released. | |
| 50 | + * This <em>barrier action</em> is useful | |
| 51 | + * for updating shared-state before any of the parties continue. | |
| 52 | + * | |
| 53 | + * <p><b>Sample usage:</b> Here is an example of | |
| 54 | + * using a barrier in a parallel decomposition design: | |
| 55 | + * <pre> | |
| 56 | + * class Solver { | |
| 57 | + * final int N; | |
| 58 | + * final float[][] data; | |
| 59 | + * final CyclicBarrier barrier; | |
| 60 | + * | |
| 61 | + * class Worker implements Runnable { | |
| 62 | + * int myRow; | |
| 63 | + * Worker(int row) { myRow = row; } | |
| 64 | + * public void run() { | |
| 65 | + * while (!done()) { | |
| 66 | + * processRow(myRow); | |
| 67 | + * | |
| 68 | + * try { | |
| 69 | + * barrier.await(); | |
| 70 | + * } catch (InterruptedException ex) { | |
| 71 | + * return; | |
| 72 | + * } catch (BrokenBarrierException ex) { | |
| 73 | + * return; | |
| 74 | + * } | |
| 75 | + * } | |
| 76 | + * } | |
| 77 | + * } | |
| 78 | + * | |
| 79 | + * public Solver(float[][] matrix) { | |
| 80 | + * data = matrix; | |
| 81 | + * N = matrix.length; | |
| 82 | + * barrier = new CyclicBarrier(N, | |
| 83 | + * new Runnable() { | |
| 84 | + * public void run() { | |
| 85 | + * mergeRows(...); | |
| 86 | + * } | |
| 87 | + * }); | |
| 88 | + * for (int i = 0; i < N; ++i) | |
| 89 | + * new Thread(new Worker(i)).start(); | |
| 90 | + * | |
| 91 | + * waitUntilDone(); | |
| 92 | + * } | |
| 93 | + * } | |
| 94 | + * </pre> | |
| 95 | + * Here, each worker thread processes a row of the matrix then waits at the | |
| 96 | + * barrier until all rows have been processed. When all rows are processed | |
| 97 | + * the supplied {@link Runnable} barrier action is executed and merges the | |
| 98 | + * rows. If the merger | |
| 99 | + * determines that a solution has been found then <tt>done()</tt> will return | |
| 100 | + * <tt>true</tt> and each worker will terminate. | |
| 101 | + * | |
| 102 | + * <p>If the barrier action does not rely on the parties being suspended when | |
| 103 | + * it is executed, then any of the threads in the party could execute that | |
| 104 | + * action when it is released. To facilitate this, each invocation of | |
| 105 | + * {@link #await} returns the arrival index of that thread at the barrier. | |
| 106 | + * You can then choose which thread should execute the barrier action, for | |
| 107 | + * example: | |
| 108 | + * <pre> if (barrier.await() == 0) { | |
| 109 | + * // log the completion of this iteration | |
| 110 | + * }</pre> | |
| 111 | + * | |
| 112 | + * <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model | |
| 113 | + * for failed synchronization attempts: If a thread leaves a barrier | |
| 114 | + * point prematurely because of interruption, failure, or timeout, all | |
| 115 | + * other threads waiting at that barrier point will also leave | |
| 116 | + * abnormally via {@link BrokenBarrierException} (or | |
| 117 | + * {@link InterruptedException} if they too were interrupted at about | |
| 118 | + * the same time). | |
| 119 | + * | |
| 120 | + * <p>Memory consistency effects: Actions in a thread prior to calling | |
| 121 | + * {@code await()} | |
| 122 | + * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> | |
| 123 | + * actions that are part of the barrier action, which in turn | |
| 124 | + * <i>happen-before</i> actions following a successful return from the | |
| 125 | + * corresponding {@code await()} in other threads. | |
| 126 | + * | |
| 127 | + * @since 1.5 | |
| 128 | + * @see CountDownLatch | |
| 129 | + * | |
| 130 | + * @author Doug Lea | |
| 131 | + */ | |
| 132 | +public class CyclicBarrier { | |
| 133 | + /** | |
| 134 | + * Each use of the barrier is represented as a generation instance. | |
| 135 | + * The generation changes whenever the barrier is tripped, or | |
| 136 | + * is reset. There can be many generations associated with threads | |
| 137 | + * using the barrier - due to the non-deterministic way the lock | |
| 138 | + * may be allocated to waiting threads - but only one of these | |
| 139 | + * can be active at a time (the one to which <tt>count</tt> applies) | |
| 140 | + * and all the rest are either broken or tripped. | |
| 141 | + * There need not be an active generation if there has been a break | |
| 142 | + * but no subsequent reset. | |
| 143 | + */ | |
| 144 | + private static class Generation { | |
| 145 | + boolean broken = false; | |
| 146 | + } | |
| 147 | + | |
| 148 | + /** The lock for guarding barrier entry */ | |
| 149 | + private final ReentrantLock lock = new ReentrantLock(); | |
| 150 | + /** Condition to wait on until tripped */ | |
| 151 | + private final Condition trip = lock.newCondition(); | |
| 152 | + /** The number of parties */ | |
| 153 | + private final int parties; | |
| 154 | + /* The command to run when tripped */ | |
| 155 | + private final Runnable barrierCommand; | |
| 156 | + /** The current generation */ | |
| 157 | + private Generation generation = new Generation(); | |
| 158 | + | |
| 159 | + /** | |
| 160 | + * Number of parties still waiting. Counts down from parties to 0 | |
| 161 | + * on each generation. It is reset to parties on each new | |
| 162 | + * generation or when broken. | |
| 163 | + */ | |
| 164 | + private int count; | |
| 165 | + | |
| 166 | + /** | |
| 167 | + * Updates state on barrier trip and wakes up everyone. | |
| 168 | + * Called only while holding lock. | |
| 169 | + */ | |
| 170 | + private void nextGeneration() { | |
| 171 | + // signal completion of last generation | |
| 172 | + trip.signalAll(); | |
| 173 | + // set up next generation | |
| 174 | + count = parties; | |
| 175 | + generation = new Generation(); | |
| 176 | + } | |
| 177 | + | |
| 178 | + /** | |
| 179 | + * Sets current barrier generation as broken and wakes up everyone. | |
| 180 | + * Called only while holding lock. | |
| 181 | + */ | |
| 182 | + private void breakBarrier() { | |
| 183 | + generation.broken = true; | |
| 184 | + count = parties; | |
| 185 | + trip.signalAll(); | |
| 186 | + } | |
| 187 | + | |
| 188 | + /** | |
| 189 | + * Main barrier code, covering the various policies. | |
| 190 | + */ | |
| 191 | + private int dowait(boolean timed, long nanos) | |
| 192 | + throws InterruptedException, BrokenBarrierException, | |
| 193 | + TimeoutException { | |
| 194 | + final ReentrantLock lock = this.lock; | |
| 195 | + lock.lock(); | |
| 196 | + try { | |
| 197 | + final Generation g = generation; | |
| 198 | + | |
| 199 | + if (g.broken) | |
| 200 | + throw new BrokenBarrierException(); | |
| 201 | + | |
| 202 | + if (Thread.interrupted()) { | |
| 203 | + breakBarrier(); | |
| 204 | + throw new InterruptedException(); | |
| 205 | + } | |
| 206 | + | |
| 207 | + int index = --count; | |
| 208 | + if (index == 0) { // tripped | |
| 209 | + boolean ranAction = false; | |
| 210 | + try { | |
| 211 | + final Runnable command = barrierCommand; | |
| 212 | + if (command != null) | |
| 213 | + command.run(); | |
| 214 | + ranAction = true; | |
| 215 | + nextGeneration(); | |
| 216 | + return 0; | |
| 217 | + } finally { | |
| 218 | + if (!ranAction) | |
| 219 | + breakBarrier(); | |
| 220 | + } | |
| 221 | + } | |
| 222 | + | |
| 223 | + // loop until tripped, broken, interrupted, or timed out | |
| 224 | + for (;;) { | |
| 225 | + try { | |
| 226 | + if (!timed) | |
| 227 | + trip.await(); | |
| 228 | + else if (nanos > 0L) | |
| 229 | + nanos = trip.awaitNanos(nanos); | |
| 230 | + } catch (InterruptedException ie) { | |
| 231 | + if (g == generation && ! g.broken) { | |
| 232 | + breakBarrier(); | |
| 233 | + throw ie; | |
| 234 | + } else { | |
| 235 | + // We're about to finish waiting even if we had not | |
| 236 | + // been interrupted, so this interrupt is deemed to | |
| 237 | + // "belong" to subsequent execution. | |
| 238 | + Thread.currentThread().interrupt(); | |
| 239 | + } | |
| 240 | + } | |
| 241 | + | |
| 242 | + if (g.broken) | |
| 243 | + throw new BrokenBarrierException(); | |
| 244 | + | |
| 245 | + if (g != generation) | |
| 246 | + return index; | |
| 247 | + | |
| 248 | + if (timed && nanos <= 0L) { | |
| 249 | + breakBarrier(); | |
| 250 | + throw new TimeoutException(); | |
| 251 | + } | |
| 252 | + } | |
| 253 | + } finally { | |
| 254 | + lock.unlock(); | |
| 255 | + } | |
| 256 | + } | |
| 257 | + | |
| 258 | + /** | |
| 259 | + * Creates a new <tt>CyclicBarrier</tt> that will trip when the | |
| 260 | + * given number of parties (threads) are waiting upon it, and which | |
| 261 | + * will execute the given barrier action when the barrier is tripped, | |
| 262 | + * performed by the last thread entering the barrier. | |
| 263 | + * | |
| 264 | + * @param parties the number of threads that must invoke {@link #await} | |
| 265 | + * before the barrier is tripped | |
| 266 | + * @param barrierAction the command to execute when the barrier is | |
| 267 | + * tripped, or {@code null} if there is no action | |
| 268 | + * @throws IllegalArgumentException if {@code parties} is less than 1 | |
| 269 | + */ | |
| 270 | + public CyclicBarrier(int parties, Runnable barrierAction) { | |
| 271 | + if (parties <= 0) throw new IllegalArgumentException(); | |
| 272 | + this.parties = parties; | |
| 273 | + this.count = parties; | |
| 274 | + this.barrierCommand = barrierAction; | |
| 275 | + } | |
| 276 | + | |
| 277 | + /** | |
| 278 | + * Creates a new <tt>CyclicBarrier</tt> that will trip when the | |
| 279 | + * given number of parties (threads) are waiting upon it, and | |
| 280 | + * does not perform a predefined action when the barrier is tripped. | |
| 281 | + * | |
| 282 | + * @param parties the number of threads that must invoke {@link #await} | |
| 283 | + * before the barrier is tripped | |
| 284 | + * @throws IllegalArgumentException if {@code parties} is less than 1 | |
| 285 | + */ | |
| 286 | + public CyclicBarrier(int parties) { | |
| 287 | + this(parties, null); | |
| 288 | + } | |
| 289 | + | |
| 290 | + /** | |
| 291 | + * Returns the number of parties required to trip this barrier. | |
| 292 | + * | |
| 293 | + * @return the number of parties required to trip this barrier | |
| 294 | + */ | |
| 295 | + public int getParties() { | |
| 296 | + return parties; | |
| 297 | + } | |
| 298 | + | |
| 299 | + /** | |
| 300 | + * Waits until all {@linkplain #getParties parties} have invoked | |
| 301 | + * <tt>await</tt> on this barrier. | |
| 302 | + * | |
| 303 | + * <p>If the current thread is not the last to arrive then it is | |
| 304 | + * disabled for thread scheduling purposes and lies dormant until | |
| 305 | + * one of the following things happens: | |
| 306 | + * <ul> | |
| 307 | + * <li>The last thread arrives; or | |
| 308 | + * <li>Some other thread {@linkplain Thread#interrupt interrupts} | |
| 309 | + * the current thread; or | |
| 310 | + * <li>Some other thread {@linkplain Thread#interrupt interrupts} | |
| 311 | + * one of the other waiting threads; or | |
| 312 | + * <li>Some other thread times out while waiting for barrier; or | |
| 313 | + * <li>Some other thread invokes {@link #reset} on this barrier. | |
| 314 | + * </ul> | |
| 315 | + * | |
| 316 | + * <p>If the current thread: | |
| 317 | + * <ul> | |
| 318 | + * <li>has its interrupted status set on entry to this method; or | |
| 319 | + * <li>is {@linkplain Thread#interrupt interrupted} while waiting | |
| 320 | + * </ul> | |
| 321 | + * then {@link InterruptedException} is thrown and the current thread's | |
| 322 | + * interrupted status is cleared. | |
| 323 | + * | |
| 324 | + * <p>If the barrier is {@link #reset} while any thread is waiting, | |
| 325 | + * or if the barrier {@linkplain #isBroken is broken} when | |
| 326 | + * <tt>await</tt> is invoked, or while any thread is waiting, then | |
| 327 | + * {@link BrokenBarrierException} is thrown. | |
| 328 | + * | |
| 329 | + * <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting, | |
| 330 | + * then all other waiting threads will throw | |
| 331 | + * {@link BrokenBarrierException} and the barrier is placed in the broken | |
| 332 | + * state. | |
| 333 | + * | |
| 334 | + * <p>If the current thread is the last thread to arrive, and a | |
| 335 | + * non-null barrier action was supplied in the constructor, then the | |
| 336 | + * current thread runs the action before allowing the other threads to | |
| 337 | + * continue. | |
| 338 | + * If an exception occurs during the barrier action then that exception | |
| 339 | + * will be propagated in the current thread and the barrier is placed in | |
| 340 | + * the broken state. | |
| 341 | + * | |
| 342 | + * @return the arrival index of the current thread, where index | |
| 343 | + * <tt>{@link #getParties()} - 1</tt> indicates the first | |
| 344 | + * to arrive and zero indicates the last to arrive | |
| 345 | + * @throws InterruptedException if the current thread was interrupted | |
| 346 | + * while waiting | |
| 347 | + * @throws BrokenBarrierException if <em>another</em> thread was | |
| 348 | + * interrupted or timed out while the current thread was | |
| 349 | + * waiting, or the barrier was reset, or the barrier was | |
| 350 | + * broken when {@code await} was called, or the barrier | |
| 351 | + * action (if present) failed due an exception. | |
| 352 | + */ | |
| 353 | + public int await() throws InterruptedException, BrokenBarrierException { | |
| 354 | + try { | |
| 355 | + return dowait(false, 0L); | |
| 356 | + } catch (TimeoutException toe) { | |
| 357 | + throw new Error(toe); // cannot happen; | |
| 358 | + } | |
| 359 | + } | |
| 360 | + | |
| 361 | + /** | |
| 362 | + * Waits until all {@linkplain #getParties parties} have invoked | |
| 363 | + * <tt>await</tt> on this barrier, or the specified waiting time elapses. | |
| 364 | + * | |
| 365 | + * <p>If the current thread is not the last to arrive then it is | |
| 366 | + * disabled for thread scheduling purposes and lies dormant until | |
| 367 | + * one of the following things happens: | |
| 368 | + * <ul> | |
| 369 | + * <li>The last thread arrives; or | |
| 370 | + * <li>The specified timeout elapses; or | |
| 371 | + * <li>Some other thread {@linkplain Thread#interrupt interrupts} | |
| 372 | + * the current thread; or | |
| 373 | + * <li>Some other thread {@linkplain Thread#interrupt interrupts} | |
| 374 | + * one of the other waiting threads; or | |
| 375 | + * <li>Some other thread times out while waiting for barrier; or | |
| 376 | + * <li>Some other thread invokes {@link #reset} on this barrier. | |
| 377 | + * </ul> | |
| 378 | + * | |
| 379 | + * <p>If the current thread: | |
| 380 | + * <ul> | |
| 381 | + * <li>has its interrupted status set on entry to this method; or | |
| 382 | + * <li>is {@linkplain Thread#interrupt interrupted} while waiting | |
| 383 | + * </ul> | |
| 384 | + * then {@link InterruptedException} is thrown and the current thread's | |
| 385 | + * interrupted status is cleared. | |
| 386 | + * | |
| 387 | + * <p>If the specified waiting time elapses then {@link TimeoutException} | |
| 388 | + * is thrown. If the time is less than or equal to zero, the | |
| 389 | + * method will not wait at all. | |
| 390 | + * | |
| 391 | + * <p>If the barrier is {@link #reset} while any thread is waiting, | |
| 392 | + * or if the barrier {@linkplain #isBroken is broken} when | |
| 393 | + * <tt>await</tt> is invoked, or while any thread is waiting, then | |
| 394 | + * {@link BrokenBarrierException} is thrown. | |
| 395 | + * | |
| 396 | + * <p>If any thread is {@linkplain Thread#interrupt interrupted} while | |
| 397 | + * waiting, then all other waiting threads will throw {@link | |
| 398 | + * BrokenBarrierException} and the barrier is placed in the broken | |
| 399 | + * state. | |
| 400 | + * | |
| 401 | + * <p>If the current thread is the last thread to arrive, and a | |
| 402 | + * non-null barrier action was supplied in the constructor, then the | |
| 403 | + * current thread runs the action before allowing the other threads to | |
| 404 | + * continue. | |
| 405 | + * If an exception occurs during the barrier action then that exception | |
| 406 | + * will be propagated in the current thread and the barrier is placed in | |
| 407 | + * the broken state. | |
| 408 | + * | |
| 409 | + * @param timeout the time to wait for the barrier | |
| 410 | + * @param unit the time unit of the timeout parameter | |
| 411 | + * @return the arrival index of the current thread, where index | |
| 412 | + * <tt>{@link #getParties()} - 1</tt> indicates the first | |
| 413 | + * to arrive and zero indicates the last to arrive | |
| 414 | + * @throws InterruptedException if the current thread was interrupted | |
| 415 | + * while waiting | |
| 416 | + * @throws TimeoutException if the specified timeout elapses | |
| 417 | + * @throws BrokenBarrierException if <em>another</em> thread was | |
| 418 | + * interrupted or timed out while the current thread was | |
| 419 | + * waiting, or the barrier was reset, or the barrier was broken | |
| 420 | + * when {@code await} was called, or the barrier action (if | |
| 421 | + * present) failed due an exception | |
| 422 | + */ | |
| 423 | + public int await(long timeout, TimeUnit unit) | |
| 424 | + throws InterruptedException, | |
| 425 | + BrokenBarrierException, | |
| 426 | + TimeoutException { | |
| 427 | + return dowait(true, unit.toNanos(timeout)); | |
| 428 | + } | |
| 429 | + | |
| 430 | + /** | |
| 431 | + * Queries if this barrier is in a broken state. | |
| 432 | + * | |
| 433 | + * @return {@code true} if one or more parties broke out of this | |
| 434 | + * barrier due to interruption or timeout since | |
| 435 | + * construction or the last reset, or a barrier action | |
| 436 | + * failed due to an exception; {@code false} otherwise. | |
| 437 | + */ | |
| 438 | + public boolean isBroken() { | |
| 439 | + final ReentrantLock lock = this.lock; | |
| 440 | + lock.lock(); | |
| 441 | + try { | |
| 442 | + return generation.broken; | |
| 443 | + } finally { | |
| 444 | + lock.unlock(); | |
| 445 | + } | |
| 446 | + } | |
| 447 | + | |
| 448 | + /** | |
| 449 | + * Resets the barrier to its initial state. If any parties are | |
| 450 | + * currently waiting at the barrier, they will return with a | |
| 451 | + * {@link BrokenBarrierException}. Note that resets <em>after</em> | |
| 452 | + * a breakage has occurred for other reasons can be complicated to | |
| 453 | + * carry out; threads need to re-synchronize in some other way, | |
| 454 | + * and choose one to perform the reset. It may be preferable to | |
| 455 | + * instead create a new barrier for subsequent use. | |
| 456 | + */ | |
| 457 | + public void reset() { | |
| 458 | + final ReentrantLock lock = this.lock; | |
| 459 | + lock.lock(); | |
| 460 | + try { | |
| 461 | + breakBarrier(); // break the current generation | |
| 462 | + nextGeneration(); // start a new generation | |
| 463 | + } finally { | |
| 464 | + lock.unlock(); | |
| 465 | + } | |
| 466 | + } | |
| 467 | + | |
| 468 | + /** | |
| 469 | + * Returns the number of parties currently waiting at the barrier. | |
| 470 | + * This method is primarily useful for debugging and assertions. | |
| 471 | + * | |
| 472 | + * @return the number of parties currently blocked in {@link #await} | |
| 473 | + */ | |
| 474 | + public int getNumberWaiting() { | |
| 475 | + final ReentrantLock lock = this.lock; | |
| 476 | + lock.lock(); | |
| 477 | + try { | |
| 478 | + return parties - count; | |
| 479 | + } finally { | |
| 480 | + lock.unlock(); | |
| 481 | + } | |
| 482 | + } | |
| 483 | +} | |