/** The lock for guarding barrier entry */
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    private final Condition trip = lock.newCondition();

 public CyclicBarrier(int parties, Runnable barrierAction) {  //这里运行添加一个Runnable任务，用于到底屏障点执行
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

    /**
     * Creates a new {@code CyclicBarrier} that will trip when the
     * given number of parties (threads) are waiting upon it, and
     * does not perform a predefined action when the barrier is tripped.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties) {  //parties代表在栅栏放开之前，parties个数量的线程必须被调用
        this(parties, null);
    }

    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L); //第一个参数代表是否有超时时间，第二个参数代表超时时间长
        } 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;   //调用Lock，CyclicBarrier并不能实现同步，所有需要通过lock来保证线程安全
        lock.lock();
        try {
            final Generation g = generation;  //当前的generation，generation的数据结构见下面

            if (g.broken)  //如果已损坏，则抛出异常
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {  //如果线程被中断
                breakBarrier();  //方法说明见下面，因为一旦线程被中断了，就没办法实现屏障点
                throw new InterruptedException();
            }

            int index = --count;  //将计算器parties减1
            if (index == 0) {  // tripped  //如果到底屏障点
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;  //获取要执行的Runnable任务
                    if (command != null)
                        command.run();  //如果不为空，则执行任务
                    ranAction = true;
                    nextGeneration(); //更新generation及重置屏障点，唤醒线程
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();  //重置屏障点，唤醒线程
                }
            }

            // loop until tripped, broken, interrupted, or timed out，未到底屏障点

            for (;;) {
                try {
                    if (!timed)  //如果没有设置超时时间，则使用condition将线程等待，为什么使用condition？  因为所有的await都是等待一个条件，即parties变为0
                        trip.await();
                    else if (nanos > 0L)  //否则设置超时时间
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {  //异常处理
                    if (g == generation && ! g.broken) {  //如果generation没有换代，且没有broken，则调用breakBarrier终止CyclicBarrier
                        breakBarrier();
                        throw ie;
                    } else {  //否则中断当前线程
                        // 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)  //当前generation 已经损坏
                    throw new BrokenBarrierException();

                if (g != generation)  //generation已经换代，返回index数
                    return index;

                if (timed && nanos <= 0L) { //等待超时
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    private static class Generation {  //同一批线程属于同一个generationboolean broken = false;

    }

CyclicBarrier的特点就是要么大家都执行完毕，要么大家都被异常中断，不会其中一个有中断而其他正常执行完毕的现象存在。有点像原子的概念，当然不是一回事。
所以这样的需要有一个状态来描述曾经是否有线程被中断过，这样后面执行的线程就知道是否需要继续等待了。Generation就是为了完成这个事的。
而多个线程同时竞争Generation，竞争CyclicBarrier的index，这样就需要通过lock来保证安全。

private void breakBarrier() {
        generation.broken = true;  //将generation职位True,代表generation已经损坏
        count = parties;  //重启count数为parties
        trip.signalAll();  //这里使用的condition的singalAll唤醒所有线程
    }

private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();  //产生一个新的generation
    }

public class CyclicBarrierTest1 {

    private static CyclicBarrier cyclicBarrier=new CyclicBarrier(5);  //定义一个CyclicBarrier，可以重复使用
    private static CountDownLatch countDownLatch=new CountDownLatch(5);  //定义一个CountDownLatch，用于最后每个人say bye后打印，bye bye。

    public static void main(String[] args){

        for(int i=1;i<=5;i++){

            new RunningMan("name"+i,Long.valueOf(i*1000),cyclicBarrier,countDownLatch).start();
        }
        try {
            countDownLatch.await();  //主线程等待最后bye bye
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("ByeBye");

    }
}

class RunningMan extends Thread{

    String name;
    long speed;
    CyclicBarrier cyclicBarrier;
    CountDownLatch countDownLatch;
    public RunningMan(String name,long speed,CyclicBarrier cyclicBarrier,CountDownLatch countDownLatch){

        this.name=name;
        this.speed=speed;
        this.cyclicBarrier=cyclicBarrier;
        this.countDownLatch=countDownLatch;
    }
    public void running(){  //奔跑

        System.out.println("Waiting "+name);  
        try {
            int index=cyclicBarrier.await(); //所有人开始等待人到期，最后一个人到期后就开始跑了

        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (BrokenBarrierException e) {
            e.printStackTrace();
        }
        System.out.println("Running "+speed);
        try {
            Thread.sleep(speed);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void celebrate(){  //庆祝一下

        System.out.println("Celebrate man comming "+name);  
        try {
            int index=cyclicBarrier.await();  //等待人到场后开始庆祝喝酒
//            System.out.println("index is "+index);

        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (BrokenBarrierException e) {
            e.printStackTrace();
        }
        System.out.println("Drinking "+speed);
        try {
            Thread.sleep(speed);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void sayGoodBy(){  //say good bye

        try {
            System.out.println(name+":say good bye");

            int index=cyclicBarrier.await();  //等待每个人say good bye
            countDownLatch.countDown();  //CountDownLatch减1
            
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (BrokenBarrierException e) {
            e.printStackTrace();
        }


    }

    public void run(){

        running();
        celebrate();
        sayGoodBy();
    }
}