java多线程使用CountDownLatch

在 Java 中，CountDownLatch 是一种同步工具，它可以用来控制线程的执行顺序或者等待其他线程完成某些操作。下面是一个使用 CountDownLatch 的示例，用于实现多线程的协调：

import java.util.concurrent.CountDownLatch;

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

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

        System.out.println("Main thread is waiting for workers to finish.");
        latch.await();
        System.out.println("All workers have finished. Main thread can continue.");
    }

    static class Worker implements Runnable {
        private final CountDownLatch latch;

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

        @Override
        public void run() {
            try {
                System.out.println("Worker is performing some task.");
                Thread.sleep(1000);
                System.out.println("Worker has finished its task.");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                latch.countDown();
            }
        }
    }
}

在上面的示例中，我们创建了一个 CountDownLatch 对象，并将其初始化为预期的线程数量（5）。然后，在主线程中创建了 5 个 Worker 线程，并启动它们。每个 Worker 线程都会执行一些任务，然后调用 `latch.countDown()` 来通知 CountDownLatch 已经完成了一部分工作。最后，主线程调用 `latch.await()` 来等待所有 Worker 线程完成任务。

当所有 Worker 线程完成任务后，主线程就会继续执行。这样，我们就通过 CountDownLatch 来协调多线程的执行顺序了。