JAVA 多线程编程之CountDownLatch

CountDownLatch是Java中的一个同步工具类，它可以让一个或多个线程等待其他线程完成操作后再执行。CountDownLatch内部维护了一个计数器，初始值为线程数，每个线程完成任务后会将计数器减1，当计数器为0时，等待的线程就会被唤醒。

下面是一个简单的例子，其中有5个线程需要完成任务后主线程才能继续执行：

import java.util.concurrent.CountDownLatch;

public class CountDownLatchDemo {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(5);
        for (int i = 0; i < 5; i++) {
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName() + " is working");
                latch.countDown();
            }).start();
        }
        latch.await();
        System.out.println("All threads have finished working");
    }
}

输出结果为：

Thread-0 is working
Thread-1 is working
Thread-2 is working
Thread-3 is working
Thread-4 is working
All threads have finished working

在这个例子中，主线程调用了`latch.await()`方法，等待计数器变为0。每个线程完成任务后调用`latch.countDown()`方法，将计数器减1。当计数器变为0时，主线程被唤醒，继续执行。