下面请看一个应用场景：
有1个driver和5个worker，需要满足以下两点要求：
当driver完成了全部的工作之后才允许worker们开始工作；
当所有的worker都完成了自己的工作之后，driver主线程才能结束。
public class Driver {
public static void main(String[] args) throws InterruptedException {
CountDownLatch startSignal = new CountDownLatch(1);
CountDownLatch doneSignal = new CountDownLatch(5);
// 依次创建并启动5个worker线程
for (int i = 0; i < 5; ++i) {
new Thread(new Worker(startSignal, doneSignal)).start();
}
System.out.println("Driver is doing something...");
System.out.println("Driver is Finished, start all workers ...");
startSignal.countDown(); // Driver执行完毕，发出开始信号，使所有的worker线程开始执行
doneSignal.await(); // 等待所有的worker线程执行结束
System.out.println("Finished.");
}
}
class Worker implements Runnable{
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal;
Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
this.startSignal = startSignal;
this.doneSignal = doneSignal;
}
public void run() {
try {
startSignal.await(); // 等待Driver线程执行完毕，获得开始信号
System.out.println("Working now ...");
doneSignal.countDown(); // 当前worker执行完毕，释放一个完成信号
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
运行结果：
Driver is doing something...
Driver is Finished, start all workers ...
Working now ...
Working now ...
Working now ...
Working now ...
Working now ...
Finished.
此处修改CountDownLatch个数为一个代码如下：public class Driver {
public static void main(String[] args) throws Exception {
CountDownLatch latch = new CountDownLatch(5);
for (int i = 0; i < 5; ++i) {
new Thread(new Worker( latch),"t"+i).start();
}
latch.await();
System.out.println("结束");
}
static class Worker implements Runnable {
private final CountDownLatch latch1;
public Worker(CountDownLatch latch1) {
this.latch1=latch1;
}
public void run() {
System.out.println(Thread.currentThread().getName()+"开始干活 work"+"id是"+Thread.currentThread().getId()+" ");
latch1.countDown();
}
}
}
结果为：
t4开始干活 workid是14
t1开始干活 workid是11
t2开始干活 workid是12
t3开始干活 workid是13
t0开始干活 workid是10
结束