一：CyclicBarrier的工作原理
CyclicBarrier 和 CountDownLatch 非常类似，它也可以实现线程间的技术等待，但是它的功能比 CountDownLatch 更加复杂和强大。主要应用场景和 CountDownLatch 类似。
CyclicBarrier 的字面意思是可循环使用（Cyclic）的屏障（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。CyclicBarrier默认的构造方法是 CyclicBarrier(int parties)，其参数表示屏障拦截的线程数量，每个线程调用await方法告诉 CyclicBarrier 我已经到达了屏障，然后当前线程被阻塞。
构造函数：
cyclicBarrier.await(5000, TimeUnit.MILLISECONDS); //时间不能设置的太小，不然会报错
private int dowait(boolean timed, long nanos)核心方法如下： /*** Main barrier code, covering the various policies.*/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) { // trippedboolean 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 outfor (;;) {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();}}1：CyclicBarrier的使用场景
CyclicBarrier 可以用于多线程计算数据，最后合并计算结果的应用场景。比如我们用一个Excel保存了用户所有银行流水，每个Sheet保存一个帐户近一年的每笔银行流水，现在需要统计用户的日均银行流水，先用多线程处理每个sheet里的银行流水，都执行完之后，得到每个sheet的日均银行流水，最后，再用barrierAction用这些线程的计算结果，计算出整个Excel的日均银行流水。
二：CyclicBarrier的使用实例
package com.github.springbootdemo.demo;import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;public class CyclicBarrierExample {//请求的数量private static final int threatCount = 550;//需要同步的线程数量private static final CyclicBarrier cyclicBarrier = new CyclicBarrier(5);public static void main(String[] args) throws Exception {//创建固定数量的线程池ExecutorService threadPool = Executors.newFixedThreadPool(10);for(int i=0;i<threatCount;i++){final int threadNum = i;Thread.sleep(1000);threadPool.execute(()->{try {test(threadNum);}catch (Exception e){e.printStackTrace();}});}//关闭线程threadPool.shutdown();}public static void test(int threadNum) throws Exception{System.out.println("threadNum:" + threadNum + "is ready");try {cyclicBarrier.await(5000, TimeUnit.MILLISECONDS); //时间不能设置的太小，不然会报错}catch (Exception e){e.printStackTrace();}System.out.println("threadNum:" + threadNum + "is finish");}}
运行结果如下：
threadNum:0is ready
threadNum:1is ready
threadNum:2is ready
threadNum:3is ready
threadNum:4is ready
threadNum:4is finish
threadNum:0is finish
threadNum:1is finish
threadNum:3is finish
threadNum:2is finish
threadNum:5is ready
threadNum:6is ready三：CyclicBarrier和CountDownLatch的区别
CountDownLatch是计数器，只能使用一次，而CyclicBarrier的计数器提供reset功能，可以多次使用。
CountDownLatch是计数器，线程完成一个记录一个，只不过计数不是递增而是递减，而CyclicBarrier更像是一个阀门，需要所有线程都到达，阀门才能打开，然后继续执行。
