CyclicBarrier当计数减少到0时，会唤醒所有阻塞在同一个Condition上的线程，与CountDownLatch不同的是所有的线程必须同时被唤醒，就好比钓鱼比赛，所有人必须同时开始抛竿一样。CountDownLatch只要求主线程的动作在其他依赖的线程执行完之后执行就OK

测试代码：

package com.jv.parallel.feature;import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;/** 处理同一个动作有多个线程同时执行的场景*/
public class TestCyclicBarrier {public static void main(String[] args) {//1.得到一个CyclicBarrier实例CyclicBarrier cb = new CyclicBarrier(4);new Thread(new Fishing(cb),"1").start();new Thread(new Fishing(cb),"2").start();new Thread(new Fishing(cb),"3").start();new Thread(new Fishing(cb),"4").start();}static class Fishing implements Runnable{CyclicBarrier cb;public Fishing(CyclicBarrier cb) {this.cb = cb;}@Overridepublic void run() {try {cb.await();System.out.println("第（" + Thread.currentThread().getName() + "）个人开始钓鱼");} catch (InterruptedException e) {e.printStackTrace();} catch (BrokenBarrierException e) {e.printStackTrace();}}}
}

原理：

这里没有使用到什么设计模式，CyclicBarrier.await方法调用CyclicBarrier.dowait方法，每次调用await方法都会使计数器-1，当减少到0时就会唤醒所有的线程

    public int await() throws InterruptedException, BrokenBarrierException {try {return dowait(false, 0L);} catch (TimeoutException toe) {throw new Error(toe); // cannot happen}}

    public int await(long timeout, TimeUnit unit)throws InterruptedException,BrokenBarrierException,TimeoutException {return dowait(true, unit.toNanos(timeout));}

    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();}}

最核心的部分就是

每次调用await方法都会使内部的计数器临时变量-1，当减少到0时，就会调用nextGeneration方法

    private void nextGeneration() {// signal completion of last generationtrip.signalAll();// set up next generationcount = parties;generation = new Generation();}

在这里唤醒所有阻塞的线程

提醒：在声明CyclicBarrier的时候还可以传一个Runnable的实现类，当计数器减少到0时，会执行该实现类