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场景：需要启动多线程处理事情，而在所有事情做完之后，需要修改系统状态；那么如何判断所有线程（事情）都做完了呢？这就需要判断所有当前运行的线程状态了。

代码

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;/** 测试监控类 */
public class WatchThread {/** 测试函数 */
    public void testThread() throws InterruptedException {
        int threadNum = 10;
        // 初始化countDown
        CountDownLatch threadSignal = new CountDownLatch(threadNum);
        // 创建固定长度的线程池
        Executor executor = Executors.newFixedThreadPool(threadNum);
        for (int i = 0; i < threadNum; i++) { // 开threadNum个线程
            Runnable task = new TestThread(threadSignal);
            // 执行
            executor.execute(task);
        }
        threadSignal.await(); // 等待所有子线程执行完
        // do work
        System.out.println(Thread.currentThread().getName() + "+++++++结束.");
    }/** 测试函数 */
    public static void main(String[] args) throws InterruptedException {
        WatchThread test = new WatchThread();
        test.testThread();
    }

    private class TestThread implements Runnable {
        private CountDownLatch threadsSignal;public TestThread(CountDownLatch threadsSignal) {
            this.threadsSignal = threadsSignal;
        }public void run() {
            System.out.println(Thread.currentThread().getName() + "开始...");
            // do shomething
            System.out.println("开始了线程：：：：" + threadsSignal.getCount());
            // 线程结束时计数器减1
            threadsSignal.countDown();
            System.out.println(Thread.currentThread().getName() + "结束. 还有"
                    + threadsSignal.getCount() + " 个线程");
        }
    }

}