写一个countdownlatch demo

下面是一个简单的countdownlatch demo，用于控制多个线程的同步：

import java.util.concurrent.CountDownLatch;

public class CountdownLatchDemo {

    public static void main(String[] args) throws InterruptedException {
        int threadNum = 5;
        CountDownLatch countDownLatch = new CountDownLatch(threadNum);

        for (int i = 0; i < threadNum; i++) {
            new Thread(new Task(countDownLatch)).start();
        }

        // 等待所有线程执行完毕
        countDownLatch.await();
        System.out.println("All threads have finished executing.");
    }

    static class Task implements Runnable {
        private CountDownLatch countDownLatch;

        public Task(CountDownLatch countDownLatch) {
            this.countDownLatch = countDownLatch;
        }

        @Override
        public void run() {
            try {
                System.out.println(Thread.currentThread().getName() + " is executing.");
                Thread.sleep(1000);
                System.out.println(Thread.currentThread().getName() + " has finished executing.");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                countDownLatch.countDown();
            }
        }
    }
}

在这个demo中，我们创建了5个线程，并且使用CountDownLatch来控制这些线程的同步。每个线程都会执行一个简单的任务，在任务执行完毕之后调用countDownLatch的countDown方法，表示线程已经执行完毕。主线程调用await方法，等待所有线程执行完毕，然后输出"All threads have finished executing."。