Java并发包-CountDownLatch

java.util.concurrent 
类 CountDownLatch

java.lang.Object
  ∟－java.util.concurrent.CountDownLatch

CountDownLatch

直译过来就是倒计数(CountDown)门闩(Latch)。倒计数不用说，门闩的意思顾名思义就是阻止前进。在这里就是指 CountDownLatch.await() 方法在倒计数为0以前会阻塞当前线程。

做用

CountDownLatch 的做用和 Thread.join() 方法相似，可用于一组线程和另一组线程的协做。例如，主线程在作一项工做以前须要一系列的准备工做，只有这些准备工做都完成，主线程才能继续它的工做。这些准备工做彼此独立，因此能够并发执行以提升速度。在这个场景下就能够使用 CountDownLatch 协调线程之间的调度了。在直接建立线程的年代（Java 5.0 以前），咱们能够使用 Thread.join()。在 JUC 出现后，由于线程池中的线程不能直接被引用，因此就必须使用 CountDownLatch 了。

package zy.test;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class CountDownLatchTest {

	public static void main(String[] args) {
		/**
		 * 需求：woker3 必须在 woker1和woker2执行完以后再执行
		 */
		CountDownLatch countDownLatch=new CountDownLatch(2);
		Thread woker1 = new Thread(new Worker("woker1", 5, countDownLatch));
		Thread woker2 = new Thread(new Worker("woker2", 5, countDownLatch));
		Thread woker3 = new Thread(new Worker("woker3", 3, countDownLatch));
		woker1.start();
		woker2.start();
		try {
			countDownLatch.await();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("worker1和worker2执行完成");
		woker3.start();
	}
}
class Worker implements Runnable{

	private String workName;
	
	private long time;
	
	private CountDownLatch countDownLatch;
	
	public Worker(String workName, long time,CountDownLatch countDownLatch) {
		super();
		this.workName = workName;
		this.time = time;
		this.countDownLatch=countDownLatch;
	}


	@Override
	public void run() {
		System.out.println(workName+" 开始工做。。。");
		try {
			TimeUnit.SECONDS.sleep(time);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println(workName+" 工做了 "+time+"秒后结束");
		countDownLatch.countDown();
		
	}
	
}

运行结果：