Java编程思想中的例子
import javax.validation.constraints.Size;
import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

class TaskPortion implements Runnable{

    private static int count;
    private final int id=count++;
    private final CountDownLatch latch;
    private static Random random=new Random(47);

    public TaskPortion(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        try {
            doWork();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        latch.countDown();

    }

    public void doWork() throws InterruptedException {
        TimeUnit.MILLISECONDS.sleep(random.nextInt(1000));
        System.out.println(this+"complete");

    }
    @Override
    public String toString() {
        return "TaskPortion: "+String.format("%1$-2d",id);
    }
}
class WaitingTask implements Runnable{

    private final CountDownLatch latch;
    private static int count;
    private final int id=count++;

    public WaitingTask(CountDownLatch latch) {
        this.latch = latch;
    }
    public void run() {
        try {
            latch.await();
            System.out.println(this);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    @Override
    public String toString() {
        return "waiting task pass"+String.format("%1$-2d",id);
    }
}
public class CountDownLatchDemo {

    private static final int SIZE=20;
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        CountDownLatch latch=new CountDownLatch(SIZE);
        for (int i=0;i<SIZE;i++)
            executorService.execute(new TaskPortion(latch));     //
        for (int i = 0; i < SIZE; i++) {
            executorService.execute(new WaitingTask(latch));    

　　　　　executorService.shutdown();

　　　　} 
　　}
}

countdownlatch中有个计数器，当计数器减少到0的时候，释放所有等待的线程，coutDown()会让计数器的值减少，调用await()的线程将会进入等待状态，直到调用countdownlatch使得计数器数值减少到0，所有等待的线程都会开始执行。

CyclicBarrier

这个相对于上面的countdownlatch来说，这种可以重复的使用，而且await()已经具备countdownlatch中的countdown()和await()两种方法的作用。（前者await()被线程调用一次计数减一，后者不会，只能通过countdown()）

首先了解他的构造方法。

方法

 

 自己独立敲了一段书上的模拟赛马的demo（来自于Java编程思想）

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.*;

class Horse implements Runnable{
    private CyclicBarrier barrier;               //ByclicBarrier用来控制本类的在释放屏障之前的动作，由构造函数传入
    private int stride=0; 　　　　　　　　　　　　　　//马跑得轨迹
    private static int count;　　　　　　　　　　　　　//马的数量
    private final int id=count++;　　　　　　　　　　//设置马的id
    private Random random = new Random(47);　　　　//模拟赛马
    
    public Horse(CyclicBarrier barrier) {
        this.barrier = barrier;
    }
    @Override
    public void run() {
       try{
           while(!Thread.interrupted()){　　　　　　　　　　　　//这里模拟赛马的过程
               TimeUnit.MILLISECONDS.sleep(500);
            barrier.await();
            stride+=random.nextInt(3);
           }
       }catch(Exception e){
           e.printStackTrace();
        }
    }
    @Override
    public String toString() {
        return "Horse "+id+" ";
    }
    public String  getTrack(){
        StringBuilder s=new StringBuilder();//返回马当前到达屏障之后走的路径
        for (int i = 0; i < stride; i++) {
            s.append("=");
        }
        return s.toString();
    }
    public int getStride() {
        return stride;                    //马走了多少
    }

    public int getId() {
        return id;
    }
}
class HorseRace {
    private int nhorse;
    private ExecutorService exec;
    private final int FINISH_LINE = 75;
    private CyclicBarrier barrier;
    List<Horse> horses = new ArrayList<Horse>();
    private String track;

    public HorseRace(int nhorse, ExecutorService executorService) {
        exec = executorService;
        this.nhorse = nhorse;
        StringBuilder s = new StringBuilder();
        for (int i = 0; i < FINISH_LINE; i++) {
            s.append("=");
        }
        track = s.toString();
        System.out.println(s.toString());
        barrier = new CyclicBarrier(nhorse, () -> {        //表达式中定义了释放屏障之前的动作，打印当前所有马的路程并且判断是否有的码已经到达终点
            System.out.println(track);
            for (Horse horse : horses) {
                System.out.println(horse.getTrack());
                if (horse.getStride() > FINISH_LINE) {
                    System.out.println(horse.getId() + " won");
                    exec.shutdownNow();
                    return;
                }
            }
        });
        for (int i = 0; i < nhorse; i++) {
            Horse horse = new Horse(barrier);
            exec.execute(horse);
            horses.add(horse);
        }
    }
}
public class HorseRaceDeno {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        new HorseRace(7, executorService);
    }
}