并发编程（12）CyclicBarrier

如果你的多个线程来执行一些任务，等任务全部完成后，才可以继续往下走，此时可以用CyclicBarrier。

await()方法

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();
        }
	    //进来一个线程 count - 1
        int index = --count;
        //count == 0 表示所有线程均已到位
        if (index == 0) {  // tripped
            boolean ranAction = false;
            try {
                final Runnable command = barrierCommand;
                if (command != null)
                    command.run();
                ranAction = true;
                //唤醒所有等待线程，重置count，generation。
                nextGeneration();
                return 0;
            } finally {
                if (!ranAction)
                    breakBarrier();
            }
        }

        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 {
                    Thread.currentThread().interrupt();
                }
            }

            if (g.broken)
                throw new BrokenBarrierException();
		   //generation已经更新，返回index
            if (g != generation)
                return index;

            if (timed && nanos <= 0L) {
                breakBarrier();
                throw new TimeoutException();
            }
        }
    } finally {
        lock.unlock();
    }
}

应用示例：

public class CyclicBarrierTest {
    private static CyclicBarrier cyclicBarrier;

    static class CyclicBarrierThread extends Thread{
        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + "准备好了");
            //等待
            try {
                cyclicBarrier.await();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args){
        cyclicBarrier = new CyclicBarrier(5, new Runnable() {
            @Override
            public void run() {
                System.out.println("全部线程准备好，开始任务....");
            }
        });
        for(int i = 0 ; i < 5 ; i++){
            new CyclicBarrierThread().start();
        }
    }
}