有時候我們有這樣的一個需求，併發多個任務，需要等待一個訊號，然後他們都開始執行，所有任務執行完成後通知主執行緒。如果是類似這樣的需求你就可以考慮CountDownLatch這個類了。話不多說，直接上程式碼：

import java.util.concurrent.CountDownLatch;

/**
 * 開關鎖存器
 * @author li
 *
 */
public class LatchTest {
	
	private static final int N=5; 
	
	public static void main(String[] args) throws InterruptedException {
		
		CountDownLatch startSignal=new CountDownLatch(1);
		
		CountDownLatch allTaskEndSignal=new CountDownLatch(N);

		for(int i=0;i<N;i++){
			new Thread(new TaskLatch(startSignal,allTaskEndSignal)).start();
		}
		System.out.println("main:"+System.nanoTime());
		Thread.sleep(1000);
		startSignal.countDown();
		System.out.println("main:"+System.nanoTime());
		allTaskEndSignal.await();
		System.out.println("main:"+System.nanoTime());
	}
	
	static class TaskLatch implements Runnable{
		/*
		 * 用於等待啟動訊號
		 */
		private final CountDownLatch startSignal;
		/*
		 * 傳送結束訊號
		 */
		private final CountDownLatch taskEndSignal;
		
		public TaskLatch(CountDownLatch startSignal,CountDownLatch taskEndSignal){
			this.startSignal=startSignal;
			this.taskEndSignal=taskEndSignal;
		}

		@Override
		public void run() {
			try {
				startSignal.await();//阻塞
				doSomething();
			} catch (InterruptedException e) {
               //IG
			}
			taskEndSignal.countDown();
		}
		
		private void doSomething() throws InterruptedException{
			System.out.println(Thread.currentThread().getName()+":"+System.nanoTime());
			Thread.sleep(1000);
		}
		
	}

}
 

main:92926451620442
main:92927451389987
Thread-4:92927451744534
Thread-3:92927451637703
Thread-2:92927451625574
Thread-0:92927451480023
Thread-1:92927451480489
main:92928453290545