CyclicBarrier 源码解析

public class CyclicBarries {

    static class TaskThread extends Thread {

        CyclicBarrier barrier;

        public TaskThread(CyclicBarrier barrier) {
            this.barrier = barrier;
        }

        @Override
        public void run() {
            try {
                Thread.sleep(1000);
                System.out.println(getName() + " 到达栅栏 A");
                barrier.await();
                System.out.println(getName() + " 冲破栅栏 A");

                Thread.sleep(2000);
                System.out.println(getName() + " 到达栅栏 B");
                barrier.await();
                System.out.println(getName() + " 冲破栅栏 B");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {

        int threadNum = 5;

        CyclicBarrier barrier = new CyclicBarrier(threadNum, new Runnable() {
            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName() + " 完成最后任务");
            }

        });

        for(int i = 0; i < threadNum; i++) {
            new TaskThread(barrier).start();
        }

    }
}

 /**
     * Main barrier code, covering the various policies.
     */
    private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException
               //拿到重入锁
        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();
            }
          //每次调用await方法，数量就减一
            int index = --count;
            if (index == 0) {  // tripped
            //当index为0的时候，表示已经没有什么障碍了，可以突破了
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    //判断时候设置了回调函数，有得话运行一下
                    if (command != null)
                        command.run();
                    ranAction = true;
                    //设置下一个开闸标志
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                    //图片限制，激活线程
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                    //让自己先阻塞
                        trip.await();
                    else if (nanos > 0L)
                    //让自己在超时时间内休眠
                        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 {
        //释放锁
            lock.unlock();
        }
    }