想做的一个程序如题，主要是想统计子线程都执行完毕所用的时间，网上搜索到了CountDownLatch这个类，这个工具类可以理解为计数器。在这里用于表示正在运行的线程数，当一个子线程结束的时候，将这个计数器减一，最后在主线程的一个地方等待子线程全部执行完毕，再继续运行等待后面的程序。写了个Demo程序，如下：

//子线程
public class SubThread extends Thread{

	//子线程记数器,记载着运行的线程数
	private CountDownLatch runningThreadNum;

	public SubThread(CountDownLatch runningThreadNum){
		this.runningThreadNum = runningThreadNum;
	}
	
	@Override
	public void run() {
		System.out.println(Thread.currentThread().getName()+"-start");
		System.out.println(Thread.currentThread().getName()+"-do something");
		System.out.println(Thread.currentThread().getName()+"-end");
		runningThreadNum.countDown();//正在运行的线程数减一
	}
}

//Main主线程
public class MainThread {

	public static void main(String[] args) throws InterruptedException {
		long startTime = System.currentTimeMillis();
		int threadNum = 5; //线程数
		//定义正在运行的线程数
		CountDownLatch runningThreadNum = new CountDownLatch(threadNum);
		System.out.println(Thread.currentThread().getName()+"-start");
		//创建多个子线程
		for (int i = 0; i < threadNum; i++) {
			new SubThread(runningThreadNum).start();
		}
		//等待子线程都执行完了再执行主线程剩下的动作
		runningThreadNum.await();
		System.out.println(Thread.currentThread().getName()+"-end");
		long endTime = System.currentTimeMillis();
		System.out.println("runningTime:"+(endTime-startTime));
	}
}

main-start
Thread-0-start
Thread-0-do something
Thread-0-end
Thread-1-start
Thread-1-do something
Thread-1-end
Thread-2-start
Thread-2-do something
Thread-2-end
Thread-4-start
Thread-3-start
Thread-4-do something
Thread-3-do something
Thread-4-end
Thread-3-end
main-end
runningTime:16

看到打印的结果，满足我现在的要求。笔记下~~