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;
	}
	

}