1. 怎样理解倒计时门栓 CountDownLatch？
答：
- 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？
答：
- 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. 【笔试题】使用 CountDownLatch 实现同时开始（运动员比赛）场景？
答：
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. 【笔试题】使用 CountDownLatch 实现主从协作（主线程等待工作线程结束）场景？
答：
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");
}
}