CountDownLatch的使用

适用场景：

一个任务的执行要等到所有任务都执行完后再做操作。

具体用法

以下面的场景为例：

任务	执行条件
A	无
B	无
C	无
D	A、B、C全部执行完

实现步骤：
1.在主线程处新建线程池，用于执行任务。
2.新建CountDownLatch实例latch，指定count值。
3.A、B、C三个任务共用同一个CountDownLatch实例latch，在A、B、C的每个任务执行方法的最后调用CountDownLatch的countDown()方法。
4.在要等待的任务D的执行位置的开始处调用CountDownLatch实例latch的await()方法来等待A、B、C三个任务的执行完成。

说明：
具体使用时，A、B、C、D可为独立于主线程的四个单独的线程，也可以是A、B、C是三个独立线程，而D做为主线程来等待A、B、C的完成。

下边以一个具体的实例来做一个示范。

import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
 * 
 * @author difeng
 * 
 */
public class CountDownLatchDemo {
	public static void main(String[] args) {
		ExecutorService executorService = Executors.newFixedThreadPool(3);
		CountDownLatch countDownLatch = new CountDownLatch(3);
		Producer worker1 = new Producer("producer1","partA",countDownLatch);	
		Producer worker2 = new Producer("producer2","partB",countDownLatch);	
		Producer worker3 = new Producer("producer3","partC",countDownLatch);
		executorService.execute(worker1);
		executorService.execute(worker2);
		executorService.execute(worker3);
	    executorService.execute(new AssemblyWorker(countDownLatch));
		executorService.shutdown();
	}
}
/**
 * 构件生产者
 * @author difeng
 *
 */
class Producer implements Runnable{
	String name;
	CountDownLatch countDownLatch;
	String partName;
	public Producer(String name,String partName,CountDownLatch countDownLatch){
		this.name = name;
		this.partName = partName;
		this.countDownLatch = countDownLatch;
	}
	public void run() {
		System.out.println("I am " + this.name + ",I am  making the " + this.partName);
		try {
			Thread.sleep(new Random().nextInt(10) * 1000);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		System.out.println("I am " + this.name + ",My work is done,I submit the " + this.partName + " to AssemblyWorker");
		this.countDownLatch.countDown();
	}
}
/**
 * 组装者
 *
 */
class AssemblyWorker implements Runnable{
	CountDownLatch countDownLatch;
	public AssemblyWorker(CountDownLatch countDownLatch){
		this.countDownLatch = countDownLatch;
	}
	public void run() {
		try {
			System.out.println("I am AssemblyWorker,I am waitting for the workers to submit their component");
			this.countDownLatch.await();
			System.out.println("all work has done,I am assembled");
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

执行结果如下:

I am producer2,I am  making the partB
I am producer3,I am  making the partC
I am producer1,I am  making the partA
I am producer3,My work is done,I submit the partC to AssemblyWorker
I am AssemblyWorker,I am waitting for the workers to submit their component
I am producer1,My work is done,I submit the partA to AssemblyWorker
I am producer2,My work is done,I submit the partB to AssemblyWorker
all work has done,I am assembled