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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 | } | ||