Java 并发编程之 AQS CyclicBarrier 源码解析

说明

CyclicBarrier与CountDownLatch的差别在于：CyclicBarrier可重复使用，而CountDownLatch在计数归0后就不能再用。

await

使用ReentrantLock和Condition保证同步和等待。

// java.util.concurrent.CyclicBarrier#await()
public int await() throws InterruptedException, BrokenBarrierException {
    try {
        return dowait(false, 0L);
    } catch (TimeoutException toe) {
        throw new Error(toe); // cannot happen
    }
}

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();
    }
}

private void nextGeneration() {
    // signal completion of last generation
    trip.signalAll();
    // set up next generation
    count = parties;
    generation = new Generation();
}

private void breakBarrier() {
    generation.broken = true;
    count = parties;
    trip.signalAll();
}

nextGeneration：将计数重置，且唤醒所有await的线程。

count为0时，调用CyclicBarrier中构造方法传入的Runnable，Runnable中有异常未捕获时，会重置计数，且唤醒所有await线程，并标识已经被break了，之后再执行await会抛出BrokenBarrierException异常。举例：

public class CyclicBarrierRunnableExceptionDemo {

    public static void main(String[] args) throws BrokenBarrierException, InterruptedException {
        CyclicBarrier cyclicBarrier = null;
        try {
            cyclicBarrier = new CyclicBarrier(1, () -> {
                int i = 1 / 0;
            });
            cyclicBarrier.await();
        } catch (Exception e) {
            System.out.println(e);
            cyclicBarrier.await();
        }
    }

}