1 /* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/publicdomain/zero/1.0/ 5 */ 6 7 package java.util.concurrent; 8 9 import java.util.concurrent.locks.Condition; 10 import java.util.concurrent.locks.ReentrantLock; 11 12 /** 13 * A synchronization aid that allows a set of threads to all wait for 14 * each other to reach a common barrier point. CyclicBarriers are 15 * useful in programs involving a fixed sized party of threads that 16 * must occasionally wait for each other. The barrier is called 17 * <em>cyclic</em> because it can be re-used after the waiting threads 18 * are released. 19 * 20 * <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command 21 * that is run once per barrier point, after the last thread in the party 22 * arrives, but before any threads are released. 23 * This <em>barrier action</em> is useful 24 * for updating shared-state before any of the parties continue. 25 * 26 * <p><b>Sample usage:</b> Here is an example of using a barrier in a 27 * parallel decomposition design: 28 * 29 * <pre> {@code 30 * class Solver { 31 * final int N; 32 * final float[][] data; 33 * final CyclicBarrier barrier; 34 * 35 * class Worker implements Runnable { 36 * int myRow; 37 * Worker(int row) { myRow = row; } 38 * public void run() { 39 * while (!done()) { 40 * processRow(myRow); 41 * 42 * try { 43 * barrier.await(); 44 * } catch (InterruptedException ex) { 45 * return; 46 * } catch (BrokenBarrierException ex) { 47 * return; 48 * } 49 * } 50 * } 51 * } 52 * 53 * public Solver(float[][] matrix) { 54 * data = matrix; 55 * N = matrix.length; 56 * Runnable barrierAction = 57 * new Runnable() { public void run() { mergeRows(...); }}; 58 * barrier = new CyclicBarrier(N, barrierAction); 59 * 60 * List<Thread> threads = new ArrayList<>(N); 61 * for (int i = 0; i < N; i++) { 62 * Thread thread = new Thread(new Worker(i)); 63 * threads.add(thread); 64 * thread.start(); 65 * } 66 * 67 * // wait until done 68 * for (Thread thread : threads) 69 * thread.join(); 70 * } 71 * }}</pre> 72 * 73 * Here, each worker thread processes a row of the matrix then waits at the 74 * barrier until all rows have been processed. When all rows are processed 75 * the supplied {@link Runnable} barrier action is executed and merges the 76 * rows. If the merger 77 * determines that a solution has been found then {@code done()} will return 78 * {@code true} and each worker will terminate. 79 * 80 * <p>If the barrier action does not rely on the parties being suspended when 81 * it is executed, then any of the threads in the party could execute that 82 * action when it is released. To facilitate this, each invocation of 83 * {@link #await} returns the arrival index of that thread at the barrier. 84 * You can then choose which thread should execute the barrier action, for 85 * example: 86 * <pre> {@code 87 * if (barrier.await() == 0) { 88 * // log the completion of this iteration 89 * }}</pre> 90 * 91 * <p>The {@code CyclicBarrier} uses an all-or-none breakage model 92 * for failed synchronization attempts: If a thread leaves a barrier 93 * point prematurely because of interruption, failure, or timeout, all 94 * other threads waiting at that barrier point will also leave 95 * abnormally via {@link BrokenBarrierException} (or 96 * {@link InterruptedException} if they too were interrupted at about 97 * the same time). 98 * 99 * <p>Memory consistency effects: Actions in a thread prior to calling 100 * {@code await()} 101 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 102 * actions that are part of the barrier action, which in turn 103 * <i>happen-before</i> actions following a successful return from the 104 * corresponding {@code await()} in other threads. 105 * 106 * @since 1.5 107 * @see CountDownLatch 108 * 109 * @author Doug Lea 110 */ 111 public class CyclicBarrier { 112 /** 113 * Each use of the barrier is represented as a generation instance. 114 * The generation changes whenever the barrier is tripped, or 115 * is reset. There can be many generations associated with threads 116 * using the barrier - due to the non-deterministic way the lock 117 * may be allocated to waiting threads - but only one of these 118 * can be active at a time (the one to which {@code count} applies) 119 * and all the rest are either broken or tripped. 120 * There need not be an active generation if there has been a break 121 * but no subsequent reset. 122 */ 123 private static class Generation { 124 boolean broken; // initially false 125 } 126 127 /** The lock for guarding barrier entry */ 128 private final ReentrantLock lock = new ReentrantLock(); 129 /** Condition to wait on until tripped */ 130 private final Condition trip = lock.newCondition(); 131 /** The number of parties */ 132 private final int parties; 133 /** The command to run when tripped */ 134 private final Runnable barrierCommand; 135 /** The current generation */ 136 private Generation generation = new Generation(); 137 138 /** 139 * Number of parties still waiting. Counts down from parties to 0 140 * on each generation. It is reset to parties on each new 141 * generation or when broken. 142 */ 143 private int count; 144 145 /** 146 * Updates state on barrier trip and wakes up everyone. 147 * Called only while holding lock. 148 */ nextGeneration()149 private void nextGeneration() { 150 // signal completion of last generation 151 trip.signalAll(); 152 // set up next generation 153 count = parties; 154 generation = new Generation(); 155 } 156 157 /** 158 * Sets current barrier generation as broken and wakes up everyone. 159 * Called only while holding lock. 160 */ breakBarrier()161 private void breakBarrier() { 162 generation.broken = true; 163 count = parties; 164 trip.signalAll(); 165 } 166 167 /** 168 * Main barrier code, covering the various policies. 169 */ dowait(boolean timed, long nanos)170 private int dowait(boolean timed, long nanos) 171 throws InterruptedException, BrokenBarrierException, 172 TimeoutException { 173 final ReentrantLock lock = this.lock; 174 lock.lock(); 175 try { 176 final Generation g = generation; 177 178 if (g.broken) 179 throw new BrokenBarrierException(); 180 181 if (Thread.interrupted()) { 182 breakBarrier(); 183 throw new InterruptedException(); 184 } 185 186 int index = --count; 187 if (index == 0) { // tripped 188 boolean ranAction = false; 189 try { 190 final Runnable command = barrierCommand; 191 if (command != null) 192 command.run(); 193 ranAction = true; 194 nextGeneration(); 195 return 0; 196 } finally { 197 if (!ranAction) 198 breakBarrier(); 199 } 200 } 201 202 // loop until tripped, broken, interrupted, or timed out 203 for (;;) { 204 try { 205 if (!timed) 206 trip.await(); 207 else if (nanos > 0L) 208 nanos = trip.awaitNanos(nanos); 209 } catch (InterruptedException ie) { 210 if (g == generation && ! g.broken) { 211 breakBarrier(); 212 throw ie; 213 } else { 214 // We're about to finish waiting even if we had not 215 // been interrupted, so this interrupt is deemed to 216 // "belong" to subsequent execution. 217 Thread.currentThread().interrupt(); 218 } 219 } 220 221 if (g.broken) 222 throw new BrokenBarrierException(); 223 224 if (g != generation) 225 return index; 226 227 if (timed && nanos <= 0L) { 228 breakBarrier(); 229 throw new TimeoutException(); 230 } 231 } 232 } finally { 233 lock.unlock(); 234 } 235 } 236 237 /** 238 * Creates a new {@code CyclicBarrier} that will trip when the 239 * given number of parties (threads) are waiting upon it, and which 240 * will execute the given barrier action when the barrier is tripped, 241 * performed by the last thread entering the barrier. 242 * 243 * @param parties the number of threads that must invoke {@link #await} 244 * before the barrier is tripped 245 * @param barrierAction the command to execute when the barrier is 246 * tripped, or {@code null} if there is no action 247 * @throws IllegalArgumentException if {@code parties} is less than 1 248 */ CyclicBarrier(int parties, Runnable barrierAction)249 public CyclicBarrier(int parties, Runnable barrierAction) { 250 if (parties <= 0) throw new IllegalArgumentException(); 251 this.parties = parties; 252 this.count = parties; 253 this.barrierCommand = barrierAction; 254 } 255 256 /** 257 * Creates a new {@code CyclicBarrier} that will trip when the 258 * given number of parties (threads) are waiting upon it, and 259 * does not perform a predefined action when the barrier is tripped. 260 * 261 * @param parties the number of threads that must invoke {@link #await} 262 * before the barrier is tripped 263 * @throws IllegalArgumentException if {@code parties} is less than 1 264 */ CyclicBarrier(int parties)265 public CyclicBarrier(int parties) { 266 this(parties, null); 267 } 268 269 /** 270 * Returns the number of parties required to trip this barrier. 271 * 272 * @return the number of parties required to trip this barrier 273 */ getParties()274 public int getParties() { 275 return parties; 276 } 277 278 /** 279 * Waits until all {@linkplain #getParties parties} have invoked 280 * {@code await} on this barrier. 281 * 282 * <p>If the current thread is not the last to arrive then it is 283 * disabled for thread scheduling purposes and lies dormant until 284 * one of the following things happens: 285 * <ul> 286 * <li>The last thread arrives; or 287 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 288 * the current thread; or 289 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 290 * one of the other waiting threads; or 291 * <li>Some other thread times out while waiting for barrier; or 292 * <li>Some other thread invokes {@link #reset} on this barrier. 293 * </ul> 294 * 295 * <p>If the current thread: 296 * <ul> 297 * <li>has its interrupted status set on entry to this method; or 298 * <li>is {@linkplain Thread#interrupt interrupted} while waiting 299 * </ul> 300 * then {@link InterruptedException} is thrown and the current thread's 301 * interrupted status is cleared. 302 * 303 * <p>If the barrier is {@link #reset} while any thread is waiting, 304 * or if the barrier {@linkplain #isBroken is broken} when 305 * {@code await} is invoked, or while any thread is waiting, then 306 * {@link BrokenBarrierException} is thrown. 307 * 308 * <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting, 309 * then all other waiting threads will throw 310 * {@link BrokenBarrierException} and the barrier is placed in the broken 311 * state. 312 * 313 * <p>If the current thread is the last thread to arrive, and a 314 * non-null barrier action was supplied in the constructor, then the 315 * current thread runs the action before allowing the other threads to 316 * continue. 317 * If an exception occurs during the barrier action then that exception 318 * will be propagated in the current thread and the barrier is placed in 319 * the broken state. 320 * 321 * @return the arrival index of the current thread, where index 322 * {@code getParties() - 1} indicates the first 323 * to arrive and zero indicates the last to arrive 324 * @throws InterruptedException if the current thread was interrupted 325 * while waiting 326 * @throws BrokenBarrierException if <em>another</em> thread was 327 * interrupted or timed out while the current thread was 328 * waiting, or the barrier was reset, or the barrier was 329 * broken when {@code await} was called, or the barrier 330 * action (if present) failed due to an exception 331 */ await()332 public int await() throws InterruptedException, BrokenBarrierException { 333 try { 334 return dowait(false, 0L); 335 } catch (TimeoutException toe) { 336 throw new Error(toe); // cannot happen 337 } 338 } 339 340 /** 341 * Waits until all {@linkplain #getParties parties} have invoked 342 * {@code await} on this barrier, or the specified waiting time elapses. 343 * 344 * <p>If the current thread is not the last to arrive then it is 345 * disabled for thread scheduling purposes and lies dormant until 346 * one of the following things happens: 347 * <ul> 348 * <li>The last thread arrives; or 349 * <li>The specified timeout elapses; or 350 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 351 * the current thread; or 352 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 353 * one of the other waiting threads; or 354 * <li>Some other thread times out while waiting for barrier; or 355 * <li>Some other thread invokes {@link #reset} on this barrier. 356 * </ul> 357 * 358 * <p>If the current thread: 359 * <ul> 360 * <li>has its interrupted status set on entry to this method; or 361 * <li>is {@linkplain Thread#interrupt interrupted} while waiting 362 * </ul> 363 * then {@link InterruptedException} is thrown and the current thread's 364 * interrupted status is cleared. 365 * 366 * <p>If the specified waiting time elapses then {@link TimeoutException} 367 * is thrown. If the time is less than or equal to zero, the 368 * method will not wait at all. 369 * 370 * <p>If the barrier is {@link #reset} while any thread is waiting, 371 * or if the barrier {@linkplain #isBroken is broken} when 372 * {@code await} is invoked, or while any thread is waiting, then 373 * {@link BrokenBarrierException} is thrown. 374 * 375 * <p>If any thread is {@linkplain Thread#interrupt interrupted} while 376 * waiting, then all other waiting threads will throw {@link 377 * BrokenBarrierException} and the barrier is placed in the broken 378 * state. 379 * 380 * <p>If the current thread is the last thread to arrive, and a 381 * non-null barrier action was supplied in the constructor, then the 382 * current thread runs the action before allowing the other threads to 383 * continue. 384 * If an exception occurs during the barrier action then that exception 385 * will be propagated in the current thread and the barrier is placed in 386 * the broken state. 387 * 388 * @param timeout the time to wait for the barrier 389 * @param unit the time unit of the timeout parameter 390 * @return the arrival index of the current thread, where index 391 * {@code getParties() - 1} indicates the first 392 * to arrive and zero indicates the last to arrive 393 * @throws InterruptedException if the current thread was interrupted 394 * while waiting 395 * @throws TimeoutException if the specified timeout elapses. 396 * In this case the barrier will be broken. 397 * @throws BrokenBarrierException if <em>another</em> thread was 398 * interrupted or timed out while the current thread was 399 * waiting, or the barrier was reset, or the barrier was broken 400 * when {@code await} was called, or the barrier action (if 401 * present) failed due to an exception 402 */ await(long timeout, TimeUnit unit)403 public int await(long timeout, TimeUnit unit) 404 throws InterruptedException, 405 BrokenBarrierException, 406 TimeoutException { 407 return dowait(true, unit.toNanos(timeout)); 408 } 409 410 /** 411 * Queries if this barrier is in a broken state. 412 * 413 * @return {@code true} if one or more parties broke out of this 414 * barrier due to interruption or timeout since 415 * construction or the last reset, or a barrier action 416 * failed due to an exception; {@code false} otherwise. 417 */ isBroken()418 public boolean isBroken() { 419 final ReentrantLock lock = this.lock; 420 lock.lock(); 421 try { 422 return generation.broken; 423 } finally { 424 lock.unlock(); 425 } 426 } 427 428 /** 429 * Resets the barrier to its initial state. If any parties are 430 * currently waiting at the barrier, they will return with a 431 * {@link BrokenBarrierException}. Note that resets <em>after</em> 432 * a breakage has occurred for other reasons can be complicated to 433 * carry out; threads need to re-synchronize in some other way, 434 * and choose one to perform the reset. It may be preferable to 435 * instead create a new barrier for subsequent use. 436 */ reset()437 public void reset() { 438 final ReentrantLock lock = this.lock; 439 lock.lock(); 440 try { 441 breakBarrier(); // break the current generation 442 nextGeneration(); // start a new generation 443 } finally { 444 lock.unlock(); 445 } 446 } 447 448 /** 449 * Returns the number of parties currently waiting at the barrier. 450 * This method is primarily useful for debugging and assertions. 451 * 452 * @return the number of parties currently blocked in {@link #await} 453 */ getNumberWaiting()454 public int getNumberWaiting() { 455 final ReentrantLock lock = this.lock; 456 lock.lock(); 457 try { 458 return parties - count; 459 } finally { 460 lock.unlock(); 461 } 462 } 463 } 464