// parties表示线程数，即游戏中的人数　barrierAction表示越过屏障之后所要执行的任务

 public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
   }

    // 没有任务的构造

    public CyclicBarrier(int parties) {
        this(parties, null);
    }

　// 通过ReentrantLock（可重入锁）实现同步，同步实现机制可先了解一下AQS　（AbstractQueuedSynchronizer）

    private final ReentrantLock lock = new ReentrantLock();

   // Condition 通过Condition实现线程等待，线程通信
    private final Condition trip = lock.newCondition();

   // 栅栏数量　（游戏房间内的人数）
    private final int parties;

   // 越过屏障之后需要执行的任务
    private final Runnable barrierCommand;

　// 设置屏障状态　如果运行中出现故障则状态broken设置为true 然后重置barrier
    private Generation generation = new Generation();

   // 当前某个屏障之前剩余的等待数量
    private int count;

dowait 方法为private 方法通过await方法进行调用，两个方式只是设置超时等待的区别

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

  public int await(long timeout, TimeUnit unit)
        throws InterruptedException,
               BrokenBarrierException,
               TimeoutException {

        //把超时时间转换为纳秒单位
        return dowait(true, unit.toNanos(timeout));
    }

    //timed 是否有设置超时　nanos 纳秒数

　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();
            }

　　　//每执行一次dowait  维护的当前的count数量– 如果等于0 表示所有线程都准备完毕，　可以进行下一步

            int index = –count;
            if (index == 0) {
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;

　　　　　  // 判断barrier 是否有任务　
                    if (command != null)
                        command.run();
                    ranAction = true;

　　　　　// 重置barrier　进行下一步
                    nextGeneration();
                    return 0;
                } finally {

                   // 如果任务没有执行成功,则设置broken
                    if (!ranAction)
                        breakBarrier();
                }
            }

           // 循环等待　直到唤醒　故障　中断　或者超时
            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();

                if (g != generation)
                    return index;

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