Java 同步和协作工具类（三）：倒计时门栓 CountDownLatch

1. 怎样理解倒计时门栓 CountDownLatch

• 作用就相当于现实生活中的门栓，一开始是关闭的，所有希望通过该门的线程都需要等待，然后开始倒计时，倒计时变为 0 后，门栓打开，等待的所有线程都可以通过
• 不同的是，它是一次性的，打开后就不能再关上了

2. 门栓的两种应用场景

• 一种是同时开始，一种是主从协作
• 它们都有两类线程，互相需要同步

3. 使用 CountDownLatch 实现同时开始（运动员比赛）场景

public class RacerWithCountDownLatch {
    static class Racer extends Thread {
        CountDownLatch latch;
        
        public Racer(CountDownLatch latch) {
            this.latch = latch;
        }
        
        @Override
        public void run() {
            try {
                this.latch.await();
                System.out.println(getName() + " start run " + System.currentTimeMillis());
            } catch(InterruptedException e) {

            }
        }
    }
    
    public static void main(String[] args) throws InterruptedException {
        int num = 10;
        CountDownLatch latch = new CountDownLatch(1);
        Thread[] racers = new Thread[num];
        
        for(int i=0; i<num; i++) {
            racers[i] = new Racer(latch);
            racers[i].start();
        }
        
        Thread.sleep(1000);
        latch.countDown();
    }
}

4. 使用 CountDownLatch 实现主从协作（主线程等待工作线程结束）场景

public class MasterWorkerDemo {

    static class Worker extends Thread {
        CountDownLatch latch;
        
        public Worker(CountDownLatch latch) {
            this.latch = latch;
        }
        
        @Override
        public void run() {
            try {
                //模拟执行任务
                Thread.sleep((int) (Math.random() * 1000));
                //模拟异常情况
                if(Math.random() < 0.2) {
                    throw new RuntimeException("bad luck");
                }
            } catch(InterruptedException e) {

            } finally {
                this.latch.countDown(); //确保在工作线程发生异常的情况下也会被调用，使主线程能够从 await() 调用中返回
            }
        }
    }
    
    public static void main(String[] args) throws InterruptedException {
        int workerNum = 100;
        CountDownLatch latch = new CountDownLatch(workerNum);
        Worker[] workers = new Worker[workerNum];
        
        for(int i=0; i<workerNum; i++) {
            workers[i] = new Worker(latch);
            workers[i].start();
        }
        
        latch.await();
        System.out.println("collect worker results");
    }
}