CountDownLatch实例一


import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class CountDownLatchExample {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(5);
        Service service = new Service(latch);
        Runnable task = () -> service.exec();

        for (int i = 0; i < 5; i++) {
            Thread thread = new Thread(task);
            thread.start();
        }

        System.out.println("main thread await. ");
        latch.await();
        System.out.println("main thread finishes await. ");
    }
}

class Service {
    private CountDownLatch latch;

    public Service(CountDownLatch latch) {
        this.latch = latch;
    }

    public void exec() {
        try {
            System.out.println(Thread.currentThread().getName() + " execute task. ");
            sleep(2);
            System.out.println(Thread.currentThread().getName() + " finished task. ");
            //System.out.println(1/0);//即使发生异常，也会执行finally{}中的代码，但finally{}之后的代码不会执行
        } finally {
            latch.countDown();
            System.out.println(latch.getCount());
        }
        //System.out.println(111111111);//try中发生异常时此行代码不会执行，正常时才会执行
    }

    private void sleep(int seconds) {
        try {
            TimeUnit.SECONDS.sleep(seconds);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

输出：

Thread-0 execute task.
main thread await.
Thread-1 execute task.
Thread-3 execute task.
Thread-2 execute task.
Thread-4 execute task.
Thread-0 finished task.
4
Thread-1 finished task.
3
Thread-3 finished task.
2
Thread-4 finished task.
Thread-2 finished task.
1
0
main thread finishes await.

CountDownLatch中count down是倒数的意思，latch则是门闩的含义。整体含义可以理解为倒数的门栓，似乎有一点“三二一，芝麻开门”的感觉。CountDownLatch的作用也是如此，在构造CountDownLatch的时候需要传入一个整数n，在这个整数“倒数”到0之前，主线程需要等待在门口，而这个“倒数”过程则是由各个执行线程驱动的，每个线程执行完一个任务“倒数”一次。总结来说，CountDownLatch的作用就是等待其他的线程都执行完任务，必要时可以对各个任务的执行结果进行汇总，然后主线程才继续往下执行。

参考：https://www.jianshu.com/p/128476015902

CyclicBarrier和CountDownLatch区别

CountDownLatch实例一

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