CountDownLatch是一种闭锁，延迟线程的进度直到终止状态 包括一个计数器，初始化一个正数，表示需要等待的事件数量。

countDown:递减计数器 await:等待计数器直到0

import java.util.concurrent.CountDownLatch;

public class TestCountDownLatch {
	public static long timeTasks(int nThreads,final Runnable task) throws InterruptedException{
		final CountDownLatch startLatch=new CountDownLatch(1);
		final CountDownLatch endLatch = new CountDownLatch(nThreads);
		final StringBuffer stringBuffer=new StringBuffer();
		for(int i=0;i<nThreads;i++){
			 Thread t=new Thread(){
				public void run(){
					System.out.println("初始化线程资源");
					try{
						startLatch.await();
						try{
							//do something
							task.run();
						}finally{
							endLatch.countDown();
						}
						
					}catch(InterruptedException ignored){}
				}	
			};
			t.start();
		}
		long start = System.nanoTime();
		stringBuffer.append("startLatch await pre end\n");
		//初始化线程资源:如创建线程等，这些时间花费是不要算入任务时间的
		//调用countDown用 所有线程开始进行run方法
		startLatch.countDown();
		//等待所有线程结束
		endLatch.await();
		long end=System.nanoTime();
		System.out.println(stringBuffer.toString());
		return end-start;
	}
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Runnable runnable=new Runnable() {
			@Override
			public void run() {
				// TODO Auto-generated method stub
				try {
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
		};
		try {
			System.out.println("使用时间："+timeTasks(5,runnable));
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

}