简单介绍
CyclicBarrier也是一个同步辅助类，它允许一组线程相互等待，直到到达某个公共的屏障点，只有当每个线程都准备就绪后，才能各自继续往下执行后续的操作。它和CountDownLatch有些相似的地方，都是通过计数器来实现的，当某个线程调用了await()方法之后呢，该线程就进入等待状态，而且计数器执行的是+1操作，当计数器的值达到我们设置的初始值的时候，因为调用await()方法进入等待状态的线程会被唤醒，继续执行它们后续的操作。由于CyclicBarrier在释放等待线程后可以重用，所以称它为循环屏障，可以循环使用。
应用场景
CyclicBarrier的使用场景和CountDownLatch很相似，它可以用于多线程计算数据最后合并计算结果的应用场景。
与CountDownLatch的区别
1、CountDownLatch的计数器只能使用一次；而CyclicBarrier的计数器可以使用reset()方法重置，循环使用。
2、CountDownLatch主要是实现1个或n个线程需要等待其它线程完成某项操作之后才能继续往下执行，它描述的是1个或n个线程等待其它线程的关系；而CyclicBarrier主要是实现了多个线程之间相互等待，直到所有线程都满足了条件之后才能继续执行后续的操作，它描述的是各个线程内部相互等待的关系。
使用举例
package com.yuxing.springbootdemo.juc;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import lombok.extern.slf4j.Slf4j;
@Slf4j
public class CyclicBarrierExample1 {
private static CyclicBarrier barrier = new CyclicBarrier(5);
public static void main(String[] args) throws InterruptedException {
ExecutorService executor = Executors.newCachedThreadPool();
for(int i=0;i<10;i++) {
final int threadNum = i;
Thread.sleep(1000);
executor.execute(() -> {
try {
race(threadNum);
} catch (Exception e) {
log.error("exception", e);
}
});
}
executor.shutdown();
}
private static void race(int threadNum) throws InterruptedException, BrokenBarrierException {
Thread.sleep(1000);
log.info("{} is ready", threadNum);
barrier.await();
log.info("{} continue", threadNum);
}
}
运行结果：
12:36:08.804 [pool-1-thread-1] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 0 is ready
12:36:09.802 [pool-1-thread-2] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 1 is ready
12:36:10.803 [pool-1-thread-3] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 2 is ready
12:36:11.803 [pool-1-thread-4] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 3 is ready
12:36:12.804 [pool-1-thread-5] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 4 is ready
12:36:12.804 [pool-1-thread-2] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 1 continue
12:36:12.804 [pool-1-thread-3] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 2 continue
12:36:12.804 [pool-1-thread-4] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 3 continue
12:36:12.804 [pool-1-thread-1] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 0 continue
12:36:12.804 [pool-1-thread-5] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 4 continue
12:36:13.804 [pool-1-thread-6] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 5 is ready
12:36:14.804 [pool-1-thread-3] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 6 is ready
12:36:15.805 [pool-1-thread-1] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 7 is ready
12:36:16.805 [pool-1-thread-4] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 8 is ready
12:36:17.806 [pool-1-thread-5] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 9 is ready
12:36:17.806 [pool-1-thread-3] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 6 continue
12:36:17.806 [pool-1-thread-6] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 5 continue
12:36:17.806 [pool-1-thread-4] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 8 continue
12:36:17.806 [pool-1-thread-1] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 7 continue
12:36:17.806 [pool-1-thread-5] INFO com.yuxing.springbootdemo.juc.CyclicBarrierExample1 - 9 continue