CyclicBarrier的字面意思是循环屏障。允许一组线程达到一个屏障（或者同步点）时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被拦截的线程才会继续运行。

使用实例

public class CyclicBarrierDemo {    public static void main(String[] args) {        CyclicBarrier cyclicBarrier = new CyclicBarrier(2);        new Thread(new Action(cyclicBarrier)).start();        new Thread(new Action(cyclicBarrier)).start();    }   static class Action implements Runnable{        private CyclicBarrier cyclicBarrier;        public Action(CyclicBarrier cyclicBarrier){            this.cyclicBarrier = cyclicBarrier;        }        @Override        public void run() {            System.out.println(Thread.currentThread()+" get ready");            try {                cyclicBarrier.await();            } catch (InterruptedException | BrokenBarrierException e) {                e.printStackTrace();            }            System.out.println(Thread.currentThread()+" work done");        }    }}复制代码

实现原理

通过查看源码可知await()方法由dowait()方法完成的，源码如下：

private int dowait(boolean timed, long nanos)    throws InterruptedException, BrokenBarrierException,           TimeoutException {    // 1、获取锁并加锁    final ReentrantLock lock = this.lock;    lock.lock();    try {        // 2、获取代表屏障使用的generation实例        final Generation g = generation;        if (g.broken)            throw new BrokenBarrierException();        if (Thread.interrupted()) {            breakBarrier();            throw new InterruptedException();        }        // 3、被拦截的线程数的副本值 - 1        int index = --count;        if (index == 0) {  // tripped            boolean ranAction = false;            try {                final Runnable command = barrierCommand;                if (command != null)                    command.run();                ranAction = true;                // 5、唤醒被拦截的线程和初始化下一次屏障使用                nextGeneration();                return 0;            } finally {                if (!ranAction)                    breakBarrier();            }        }        // loop until tripped, broken, interrupted, or timed out        // 4、当前线程进入自旋过程，释放锁等待被唤醒        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 {        // 释放锁        lock.unlock();    }}复制代码

dowait方法的执行流程如下：

欢迎留言补充，共同交流。个人微信公众号求关注：