让一些线程阻塞直到另一些线程完成一系列操作后才被唤醒。
 
 CountDownLatch主要有两个方法，当一个或多个线程调用await方法时，这些线程会阻塞。其它线程调用countDown方法会将计数器减1(调用countDown方法的线程不会阻塞)， 当计数器的值变为0时，因await方法阻塞的线程会被唤醒，继续执行。 解释：5个同学陆续离开教室后值班同学才可以关门。也即    秦灭6国，一统华夏main主线程必须要等前面5个线程完成全部工作后，自己才能开干

public class CountDownLatchDemo
{
    public static void main(String[] args) throws InterruptedException
    {
        //秦灭6国，一统华夏
        CountDownLatch cdl = new CountDownLatch(6);
        for (int i = 1; i <=6; i++) 
        {
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName()+"\t 国被灭");
                cdl.countDown();
            },CountryEnums.forEachCountryEnums(i).getRetMsg()).start();
        }
        cdl.await();
        System.out.println(Thread.currentThread().getName()+"*********秦灭6国，一统华夏");		
    }
}

public enum CountryEnums {
    ONE(1,"韩"),TWO(2,"魏"),THREE(3,"赵"),FOUR(4,"燕"),FIVE(5,"齐"),SIX(6,"楚");
    
    private Integer retCode;
    private String retMsg;
    
    
    private CountryEnums(Integer retCode, String retMsg) {
        this.retCode = retCode;
        this.retMsg = retMsg;
    }
    
    public Integer getRetCode() {
        return retCode;
    }
    public void setRetCode(Integer retCode) {
        this.retCode = retCode;
    }
    public String getRetMsg() {
        return retMsg;
    }
    public void setRetMsg(String retMsg) {
        this.retMsg = retMsg;
    }
    
    public static CountryEnums forEachCountryEnums(Integer index) {
        for (CountryEnums enums : values()) {
            if(enums.getRetCode() == index) {
                return enums;
            }
        }
        return null;
    }
    

}