1、循坏屏障可以用一句话来总结：集齐七颗龙珠，方能召唤神龙

/**
 *  CyclicBarrier
 *  的字面意思是可循环（Cyclic）使用屏障（Barrier）。它要做的事情是让
 *  一组线程到达一个屏障（也可以叫同步点）时被阻塞，直到最后一个线程到达屏障时，
 *  屏障才会开门，所有被屏障拦截的线程才会继续干活
 *
 *  线程进入屏障通过 CyclicBarrier 的 await（）方法。
 */
public class CyclicBarrierDemo {

    private static final int NUMBER = 7;

    public static void main(String[] args) {

        CyclicBarrier cyclicBarrier = new CyclicBarrier(NUMBER, () -> {
            System.out.println("集齐七颗龙珠，方能召唤神龙");
        });

        for (int i = 1; i <= NUMBER; i++) {
            final int temp = i;
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName()+"\t 收集到第；"+temp+"\t 颗龙珠");
                try {
                    cyclicBarrier.await();
                } catch (InterruptedException|BrokenBarrierException e) {
                    e.printStackTrace();
                }
            }, String.valueOf(i)).start();
        }
    }
}

2、循环屏障源码分析

/**
 * desc：循坏屏障的源码分析
 *
 * @author Leslie
 */
public class CyclicBarrier {


    /**
     * 每一个屏障代表一个Generation实例
     */
    private static class Generation {
        boolean broken = false;
    }

    /**
     * 可重用锁用于守护屏障
     */
    private final ReentrantLock lock = new ReentrantLock();

    /**
     * 守护屏障锁的通信用于通知屏障被推到
     */
    private final Condition trip = lock.newCondition();
    /**
     * 屏障的线程组成数量
     */
    private final int parties;

    /**
     * 屏障被推到要执行的任务
     */
    private final Runnable barrierCommand;
    /**
     * 当前屏障的 Generation 实例
     */
    private CyclicBarrier.Generation generation = new CyclicBarrier.Generation();

    /**
     * 屏障任然在等待的组成成员数，当成员数为0的时候，
     * 屏障被推到，开始执行屏障任务，同时会被重置成和初始parties值一样
     */
    private int count;

    /**
     * 更新状态，当屏障被推到的时候，同时唤醒所有的等待的线程，
     * 只有当持有锁的时候，才能被调用
     */
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new CyclicBarrier.Generation();
    }

    /**
     * 设置屏障被推到
     */
    private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }

    /**
     * 每一个线程执行完一个线程任务之后，就调用此方法，让屏障中的
     * count 数量减 1
     */
    private int dowait(boolean timed, long nanos)
            throws InterruptedException, BrokenBarrierException,
            TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final CyclicBarrier.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 {
                    // 当所有的线程都执行完任务之后，就开始执行屏障推到之后的任务
                    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)
                        //如果没有超时，就进行等待
                        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();
        }
    }

    /**
     * 屏障的构造器
     * @param parties 屏障的线程组成数量
     * @param barrierAction 屏障被推到时要执行的操作
     * @throws IllegalArgumentException 线程数量小于1 时抛出的异常
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

    /**
     * 构造一个没有任何动作执行的一个屏障
     *
     * @param parties 屏障线程组成数量
     * @throws IllegalArgumentException 参数异常
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }

    /**
     * 获取屏障的线程组成数量
     *
     * @return 屏障的线程组成数量
     */
    public int getParties() {
        return parties;
    }

    /**
     * 屏障中的线程成员，执行完线程任务之后就调用此方法
     * 这样可以使屏障统计当前线程成员中多少线程执行完了任务
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

    /**
     * 调用时 屏障线程执行任务数量减少 1
     */
    public int await(long timeout, TimeUnit unit)
            throws InterruptedException,
            BrokenBarrierException,
            TimeoutException {
        return dowait(true, unit.toNanos(timeout));
    }

    /**
     * 获取屏障的状态
     */
    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return generation.broken;
        } finally {
            lock.unlock();
        }
    }

    /**
     * 重置屏障中的状态
     */
    public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            breakBarrier();   // break the current generation
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }

    /**
     * 获取还有多少个线程还没有执行完其相应的任务
     */
    public int getNumberWaiting() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return parties - count;
        } finally {
            lock.unlock();
        }
    }

}

相应到此，你绝对掌握了CyclicBarrierl了。