| /* |
| * Written by Doug Lea with assistance from members of JCP JSR-166 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/publicdomain/zero/1.0/ |
| */ |
| |
| package java.util.concurrent; |
| import java.util.concurrent.locks.*; |
| |
| /** |
| * A synchronization aid that allows a set of threads to all wait for |
| * each other to reach a common barrier point. CyclicBarriers are |
| * useful in programs involving a fixed sized party of threads that |
| * must occasionally wait for each other. The barrier is called |
| * <em>cyclic</em> because it can be re-used after the waiting threads |
| * are released. |
| * |
| * <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command |
| * that is run once per barrier point, after the last thread in the party |
| * arrives, but before any threads are released. |
| * This <em>barrier action</em> is useful |
| * for updating shared-state before any of the parties continue. |
| * |
| * <p><b>Sample usage:</b> Here is an example of |
| * using a barrier in a parallel decomposition design: |
| * |
| * <pre> {@code |
| * class Solver { |
| * final int N; |
| * final float[][] data; |
| * final CyclicBarrier barrier; |
| * |
| * class Worker implements Runnable { |
| * int myRow; |
| * Worker(int row) { myRow = row; } |
| * public void run() { |
| * while (!done()) { |
| * processRow(myRow); |
| * |
| * try { |
| * barrier.await(); |
| * } catch (InterruptedException ex) { |
| * return; |
| * } catch (BrokenBarrierException ex) { |
| * return; |
| * } |
| * } |
| * } |
| * } |
| * |
| * public Solver(float[][] matrix) { |
| * data = matrix; |
| * N = matrix.length; |
| * barrier = new CyclicBarrier(N, |
| * new Runnable() { |
| * public void run() { |
| * mergeRows(...); |
| * } |
| * }); |
| * for (int i = 0; i < N; ++i) |
| * new Thread(new Worker(i)).start(); |
| * |
| * waitUntilDone(); |
| * } |
| * }}</pre> |
| * |
| * Here, each worker thread processes a row of the matrix then waits at the |
| * barrier until all rows have been processed. When all rows are processed |
| * the supplied {@link Runnable} barrier action is executed and merges the |
| * rows. If the merger |
| * determines that a solution has been found then <tt>done()</tt> will return |
| * <tt>true</tt> and each worker will terminate. |
| * |
| * <p>If the barrier action does not rely on the parties being suspended when |
| * it is executed, then any of the threads in the party could execute that |
| * action when it is released. To facilitate this, each invocation of |
| * {@link #await} returns the arrival index of that thread at the barrier. |
| * You can then choose which thread should execute the barrier action, for |
| * example: |
| * <pre> {@code |
| * if (barrier.await() == 0) { |
| * // log the completion of this iteration |
| * }}</pre> |
| * |
| * <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model |
| * for failed synchronization attempts: If a thread leaves a barrier |
| * point prematurely because of interruption, failure, or timeout, all |
| * other threads waiting at that barrier point will also leave |
| * abnormally via {@link BrokenBarrierException} (or |
| * {@link InterruptedException} if they too were interrupted at about |
| * the same time). |
| * |
| * <p>Memory consistency effects: Actions in a thread prior to calling |
| * {@code await()} |
| * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
| * actions that are part of the barrier action, which in turn |
| * <i>happen-before</i> actions following a successful return from the |
| * corresponding {@code await()} in other threads. |
| * |
| * @since 1.5 |
| * @see CountDownLatch |
| * |
| * @author Doug Lea |
| */ |
| public class CyclicBarrier { |
| /** |
| * Each use of the barrier is represented as a generation instance. |
| * The generation changes whenever the barrier is tripped, or |
| * is reset. There can be many generations associated with threads |
| * using the barrier - due to the non-deterministic way the lock |
| * may be allocated to waiting threads - but only one of these |
| * can be active at a time (the one to which <tt>count</tt> applies) |
| * and all the rest are either broken or tripped. |
| * There need not be an active generation if there has been a break |
| * but no subsequent reset. |
| */ |
| private static class Generation { |
| boolean broken = false; |
| } |
| |
| /** The lock for guarding barrier entry */ |
| private final ReentrantLock lock = new ReentrantLock(); |
| /** Condition to wait on until tripped */ |
| private final Condition trip = lock.newCondition(); |
| /** The number of parties */ |
| private final int parties; |
| /* The command to run when tripped */ |
| private final Runnable barrierCommand; |
| /** The current generation */ |
| private Generation generation = new Generation(); |
| |
| /** |
| * Number of parties still waiting. Counts down from parties to 0 |
| * on each generation. It is reset to parties on each new |
| * generation or when broken. |
| */ |
| private int count; |
| |
| /** |
| * Updates state on barrier trip and wakes up everyone. |
| * Called only while holding lock. |
| */ |
| private void nextGeneration() { |
| // signal completion of last generation |
| trip.signalAll(); |
| // set up next generation |
| count = parties; |
| generation = new Generation(); |
| } |
| |
| /** |
| * Sets current barrier generation as broken and wakes up everyone. |
| * Called only while holding lock. |
| */ |
| private void breakBarrier() { |
| generation.broken = true; |
| count = parties; |
| trip.signalAll(); |
| } |
| |
| /** |
| * Main barrier code, covering the various policies. |
| */ |
| private int dowait(boolean timed, long nanos) |
| throws InterruptedException, BrokenBarrierException, |
| TimeoutException { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| final Generation g = generation; |
| |
| if (g.broken) |
| throw new BrokenBarrierException(); |
| |
| if (Thread.interrupted()) { |
| breakBarrier(); |
| throw new InterruptedException(); |
| } |
| |
| int index = --count; |
| if (index == 0) { // tripped |
| boolean ranAction = false; |
| try { |
| final Runnable command = barrierCommand; |
| if (command != null) |
| command.run(); |
| ranAction = true; |
| nextGeneration(); |
| return 0; |
| } finally { |
| if (!ranAction) |
| breakBarrier(); |
| } |
| } |
| |
| // loop until tripped, broken, interrupted, or timed out |
| for (;;) { |
| try { |
| if (!timed) |
| trip.await(); |
| else if (nanos > 0L) |
| nanos = trip.awaitNanos(nanos); |
| } catch (InterruptedException ie) { |
| if (g == generation && ! g.broken) { |
| breakBarrier(); |
| throw ie; |
| } else { |
| // We're about to finish waiting even if we had not |
| // been interrupted, so this interrupt is deemed to |
| // "belong" to subsequent execution. |
| Thread.currentThread().interrupt(); |
| } |
| } |
| |
| if (g.broken) |
| throw new BrokenBarrierException(); |
| |
| if (g != generation) |
| return index; |
| |
| if (timed && nanos <= 0L) { |
| breakBarrier(); |
| throw new TimeoutException(); |
| } |
| } |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Creates a new <tt>CyclicBarrier</tt> that will trip when the |
| * given number of parties (threads) are waiting upon it, and which |
| * will execute the given barrier action when the barrier is tripped, |
| * performed by the last thread entering the barrier. |
| * |
| * @param parties the number of threads that must invoke {@link #await} |
| * before the barrier is tripped |
| * @param barrierAction the command to execute when the barrier is |
| * tripped, or {@code null} if there is no action |
| * @throws IllegalArgumentException if {@code parties} is less than 1 |
| */ |
| public CyclicBarrier(int parties, Runnable barrierAction) { |
| if (parties <= 0) throw new IllegalArgumentException(); |
| this.parties = parties; |
| this.count = parties; |
| this.barrierCommand = barrierAction; |
| } |
| |
| /** |
| * Creates a new <tt>CyclicBarrier</tt> that will trip when the |
| * given number of parties (threads) are waiting upon it, and |
| * does not perform a predefined action when the barrier is tripped. |
| * |
| * @param parties the number of threads that must invoke {@link #await} |
| * before the barrier is tripped |
| * @throws IllegalArgumentException if {@code parties} is less than 1 |
| */ |
| public CyclicBarrier(int parties) { |
| this(parties, null); |
| } |
| |
| /** |
| * Returns the number of parties required to trip this barrier. |
| * |
| * @return the number of parties required to trip this barrier |
| */ |
| public int getParties() { |
| return parties; |
| } |
| |
| /** |
| * Waits until all {@linkplain #getParties parties} have invoked |
| * <tt>await</tt> on this barrier. |
| * |
| * <p>If the current thread is not the last to arrive then it is |
| * disabled for thread scheduling purposes and lies dormant until |
| * one of the following things happens: |
| * <ul> |
| * <li>The last thread arrives; or |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * one of the other waiting threads; or |
| * <li>Some other thread times out while waiting for barrier; or |
| * <li>Some other thread invokes {@link #reset} on this barrier. |
| * </ul> |
| * |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| * </ul> |
| * then {@link InterruptedException} is thrown and the current thread's |
| * interrupted status is cleared. |
| * |
| * <p>If the barrier is {@link #reset} while any thread is waiting, |
| * or if the barrier {@linkplain #isBroken is broken} when |
| * <tt>await</tt> is invoked, or while any thread is waiting, then |
| * {@link BrokenBarrierException} is thrown. |
| * |
| * <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting, |
| * then all other waiting threads will throw |
| * {@link BrokenBarrierException} and the barrier is placed in the broken |
| * state. |
| * |
| * <p>If the current thread is the last thread to arrive, and a |
| * non-null barrier action was supplied in the constructor, then the |
| * current thread runs the action before allowing the other threads to |
| * continue. |
| * If an exception occurs during the barrier action then that exception |
| * will be propagated in the current thread and the barrier is placed in |
| * the broken state. |
| * |
| * @return the arrival index of the current thread, where index |
| * <tt>{@link #getParties()} - 1</tt> indicates the first |
| * to arrive and zero indicates the last to arrive |
| * @throws InterruptedException if the current thread was interrupted |
| * while waiting |
| * @throws BrokenBarrierException if <em>another</em> thread was |
| * interrupted or timed out while the current thread was |
| * waiting, or the barrier was reset, or the barrier was |
| * broken when {@code await} was called, or the barrier |
| * action (if present) failed due an exception. |
| */ |
| public int await() throws InterruptedException, BrokenBarrierException { |
| try { |
| return dowait(false, 0L); |
| } catch (TimeoutException toe) { |
| throw new Error(toe); // cannot happen |
| } |
| } |
| |
| /** |
| * Waits until all {@linkplain #getParties parties} have invoked |
| * <tt>await</tt> on this barrier, or the specified waiting time elapses. |
| * |
| * <p>If the current thread is not the last to arrive then it is |
| * disabled for thread scheduling purposes and lies dormant until |
| * one of the following things happens: |
| * <ul> |
| * <li>The last thread arrives; or |
| * <li>The specified timeout elapses; or |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * one of the other waiting threads; or |
| * <li>Some other thread times out while waiting for barrier; or |
| * <li>Some other thread invokes {@link #reset} on this barrier. |
| * </ul> |
| * |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| * </ul> |
| * then {@link InterruptedException} is thrown and the current thread's |
| * interrupted status is cleared. |
| * |
| * <p>If the specified waiting time elapses then {@link TimeoutException} |
| * is thrown. If the time is less than or equal to zero, the |
| * method will not wait at all. |
| * |
| * <p>If the barrier is {@link #reset} while any thread is waiting, |
| * or if the barrier {@linkplain #isBroken is broken} when |
| * <tt>await</tt> is invoked, or while any thread is waiting, then |
| * {@link BrokenBarrierException} is thrown. |
| * |
| * <p>If any thread is {@linkplain Thread#interrupt interrupted} while |
| * waiting, then all other waiting threads will throw {@link |
| * BrokenBarrierException} and the barrier is placed in the broken |
| * state. |
| * |
| * <p>If the current thread is the last thread to arrive, and a |
| * non-null barrier action was supplied in the constructor, then the |
| * current thread runs the action before allowing the other threads to |
| * continue. |
| * If an exception occurs during the barrier action then that exception |
| * will be propagated in the current thread and the barrier is placed in |
| * the broken state. |
| * |
| * @param timeout the time to wait for the barrier |
| * @param unit the time unit of the timeout parameter |
| * @return the arrival index of the current thread, where index |
| * <tt>{@link #getParties()} - 1</tt> indicates the first |
| * to arrive and zero indicates the last to arrive |
| * @throws InterruptedException if the current thread was interrupted |
| * while waiting |
| * @throws TimeoutException if the specified timeout elapses |
| * @throws BrokenBarrierException if <em>another</em> thread was |
| * interrupted or timed out while the current thread was |
| * waiting, or the barrier was reset, or the barrier was broken |
| * when {@code await} was called, or the barrier action (if |
| * present) failed due an exception |
| */ |
| public int await(long timeout, TimeUnit unit) |
| throws InterruptedException, |
| BrokenBarrierException, |
| TimeoutException { |
| return dowait(true, unit.toNanos(timeout)); |
| } |
| |
| /** |
| * Queries if this barrier is in a broken state. |
| * |
| * @return {@code true} if one or more parties broke out of this |
| * barrier due to interruption or timeout since |
| * construction or the last reset, or a barrier action |
| * failed due to an exception; {@code false} otherwise. |
| */ |
| public boolean isBroken() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| return generation.broken; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Resets the barrier to its initial state. If any parties are |
| * currently waiting at the barrier, they will return with a |
| * {@link BrokenBarrierException}. Note that resets <em>after</em> |
| * a breakage has occurred for other reasons can be complicated to |
| * carry out; threads need to re-synchronize in some other way, |
| * and choose one to perform the reset. It may be preferable to |
| * instead create a new barrier for subsequent use. |
| */ |
| public void reset() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| breakBarrier(); // break the current generation |
| nextGeneration(); // start a new generation |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Returns the number of parties currently waiting at the barrier. |
| * This method is primarily useful for debugging and assertions. |
| * |
| * @return the number of parties currently blocked in {@link #await} |
| */ |
| public int getNumberWaiting() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| return parties - count; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| } |