Java中CountDownLatch工具类代码理解

CountDownLatch是Java中一个非常有用的同步工具类，它可以让一个或多个线程等待其他线程完成它们的操作后再执行。在Java中，CountDownLatch类是通过一个计数器来实现的，计数器的初始值可以设置为任何整数。当某个线程调用CountDownLatch的await()方法时，它会阻塞等待计数器的值变为0；而其他线程执行完它们的操作后会通过调用CountDownLatch的countDown()方法来减少计数器的值。当计数器的值变为0时，await()方法将返回，线程得以继续执行。

下面是一个使用CountDownLatch的示例代码：

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample {

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

        for (int i = 0; i < numThreads; i++) {
            new Thread(new Worker(i, latch)).start();
        }

        latch.await();
        System.out.println("All threads have finished!");
    }

    static class Worker implements Runnable {
        private int id;
        private CountDownLatch latch;

        public Worker(int id, CountDownLatch latch) {
            this.id = id;
            this.latch = latch;
        }

        public void run() {
            System.out.println("Worker " + id + " is working...");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Worker " + id + " has finished.");
            latch.countDown();
        }
    }
}

在这个例子中，我们创建了5个线程，并用CountDownLatch来等待它们全部执行完毕。每个线程都会休眠1秒钟，然后调用countDown()方法来减少计数器的值，最终当所有线程都执行完毕时，await()方法返回，打印出"All threads have finished!"。