CountDownLatch,一个同步辅助类,在完成一组正在其他线程中执行的操作之前,它允许一个或多个线程一直等待。
主要方法
public CountDownLatch(int count);
public void countDown();
public void await() throws InterruptedException
public CountDownLatch(int count);在构造器中传入count值,表示需要等待被执行完成的线程/任务数目。
countDown() 当某一个线程执行完相应的操作后,调用countDown()标示这个操作已经完成。
await() ,通过调用await()声明当线程需要等待其他任务执行完成后才可以进行后续操作。
public class CountDownLatchDemo {
final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public static void main(String[] args) throws InterruptedException {
CountDownLatch latch=new CountDownLatch(2);//两个工人的协作
Worker worker1=new Worker("zhang san", 5000, latch);
Worker worker2=new Worker("li si", 8000, latch);
worker1.start();//
worker2.start();//
latch.await();//等待所有工人完成工作
System.out.println("all work done at "+sdf.format(new Date()));
} static class Worker extends Thread{
String workerName;
int workTime;
CountDownLatch latch;
public Worker(String workerName ,int workTime ,CountDownLatch latch){
this.workerName=workerName;
this.workTime=workTime;
this.latch=latch;
}
public void run(){
System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date()));
doWork();//工作了
System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));
latch.countDown();//工人完成工作,计数器减一
} private void doWork(){
try {
Thread.sleep(workTime);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} }
另外一个例子: 多个运动员在起点位置准备跑步,在全部准备好了之后才可以起跑;运动员全部到终点后,比赛才算结束。
// 一个CountDouwnLatch实例是不能重复使用的,也就是说它是一次性的,锁一经被打开就不能再关闭使用了,如果想重复使用,请考虑使用CyclicBarrier。
public class CountDownLatchTest {
// 模拟了100米赛跑,10名选手已经准备就绪,只等裁判一声令下。当所有人都到达终点时,比赛结束。
public static void main(String[] args) throws InterruptedException {
// 开始的倒数锁
final CountDownLatch begin = new CountDownLatch(1); // 结束的倒数锁
final CountDownLatch end = new CountDownLatch(10); // 十名选手
final ExecutorService exec = Executors.newFixedThreadPool(10); for (int index = 0; index < 10; index++) {
final int NO = index + 1;
Runnable run = new Runnable() {
public void run() {
try {
// 如果当前计数为零,则此方法立即返回。
// 等待
begin.await();
Thread.sleep((long) (Math.random() * 10000));
System.out.println("No." + NO + " arrived");
} catch (InterruptedException e) {
} finally {
// 每个选手到达终点时,end就减一
end.countDown();
}
}
};
exec.submit(run);
}
System.out.println("Game Start");
// begin减一,开始游戏
begin.countDown();
// 等待end变为0,即所有选手到达终点
end.await();
System.out.println("Game Over");
exec.shutdown();
}
}