1. CyclicBarrier类似可重复使用的CountDownLatch，可以允许多个线程执行完后才可以执行某一操作，并且可以循环使用。第一个变量是允许线程数，第二个是所有线程执行完后最后需要执行的任务，线程执行时只会递减count值，循环使用时复原状态需要用parties重新设置count值。

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

2.线程执行await操作，当所有的线程执行完该操作后，整个流程才算是一次完整执行。

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 {
        return dowait(true, unit.toNanos(timeout));
    }

核心方法dowait，主要使用ReentrantLock锁，首先判断broken状态是否被强制打断。继而判断是否线程被中断，中断后则通知所有的线程，并且设置打断状态broken为true，它的初始值为false。然后给总数进行减一操作，当所有线程执行到该操作时，count值会为0，开始执行最后的任务，通知所有阻塞的线程，并且开始复原所有状态准备下一次循环使用。如果count不为0，也就是并不是所有线程都执行了dowait操作时，会进入等待操作trip.await()，当所有线程执行完时会通知等待的线程，唤醒后判断broken状态，最后g != generation为true，返回线程的执行顺序。如果是超时等待的话，会判断状态是否返回超时异常。

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();
        }
    }

3. 设置下一次循环的代码如下。

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