Java多线程(ExecutorService), 等待所有线程执行完毕.

Mr.Java.

已于 2023-11-18 11:15:47 修改

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分类专栏： # java 文章标签： ExecutorService CountDownLatch Java多线程

于 2018-08-23 12:33:05 首次发布

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本文介绍了使用CountDownLatch实现线程同步的方法，通过一个具体示例展示了如何在多线程环境中确保所有任务完成后继续执行主线程。文章对比了两种线程控制方式，推荐使用CountDownLatch以简化代码并提高效率。

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这篇文章非原创，来自两位大神的代码：

常用的两种方式:

第一种方式:来自大神cletus的回答, 原文链接

ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
while(...) {
  taskExecutor.execute(new MyTask());
}
taskExecutor.shutdown();
try {
  taskExecutor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
} catch (InterruptedException e) {
  ...
}

第二种方式:来自大神ChssPly76的回答, 原文链接

CountDownLatch latch = new CountDownLatch(totalNumberOfTasks);
ExecutorService taskExecutor = Executors.newFixedThreadPool(4);
while(...) {
  taskExecutor.execute(new MyTask());
}

try {
  latch.await();
} catch (InterruptedException E) {
   // handle
}

然后在线程方法中加入:

       try {
            ...
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            countDownLatch.countDown();
        }

作者建议用第二种方式，即用 CountDownLatch 来实现。

完整实例如下：

我们先定义一个 Runnable 类：CountDownLatchRunnable

关键点1：我们要通过 CountDownLatch 来计数，因此我需要把 CountDownLatch 对象传到这个 Runnable 类中。

关键点2：每次执行完后，一定要 countDownLatch.countDown()


@Slf4j
public class CountDownLatchRunnable implements Runnable {
    private final String command;
    private final CountDownLatch countDownLatch;

    public CountDownLatchRunnable(String command, CountDownLatch countDownLatch) {
        this.command = command;
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        log.info(Thread.currentThread().getName() + " start. Command = " + command);
        processCommand(command);
        log.info(Thread.currentThread().getName() + " finish.");
    }

    private void processCommand(String command) {
        log.info(Thread.currentThread().getName() + ":, doing work:" + command);
        try {
            Thread.sleep(1000);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            countDownLatch.countDown();
        }
    }
}

然后实现运行：


@Slf4j
public class Run {

    public static void main(String[] args) throws InterruptedException {

        log.info("Main thread start.");

        int theadNum = 3;
        // CountDownLatch 需要将对象传入需要'等待完成'的线程对象中
        CountDownLatch countDownLatch = new CountDownLatch(theadNum);
        ExecutorService executorService = Executors.newFixedThreadPool(theadNum);
        Runnable worker = new CountDownLatchRunnable("Task", countDownLatch);

        // 创建5个线程, 提交到线程池
        for (int i = 0; i < theadNum; i++) {
            executorService.execute(worker); // 同一个实例对象
        }
        // 等待所有线程执行完成(线程阻塞)
        countDownLatch.await();

        log.info("Main thread finish.");

        // 停止接受新任务,当已有任务将执行完,关闭线程池
        executorService.shutdown();
        // 等待线程池中所有的任务执行完成, 前提是执行了 shutdown
        while (!executorService.isTerminated()) {
            // 等待所有线程执行完成
        }
        log.info("exit");
    }
}

运行结果如下：

Connected to the target VM, address: '127.0.0.1:32229', transport: 'socket'
[zhang-console] INFO 2023-11-18 03:07:48 j.b.j.t.t.t2_wait_countdownlatch.Run: Main thread start.
[zhang-console] INFO 2023-11-18 03:07:48 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-1 start. Command = Task
[zhang-console] INFO 2023-11-18 03:07:48 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-1:, doing work:Task
[zhang-console] INFO 2023-11-18 03:07:48 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-2 start. Command = Task
[zhang-console] INFO 2023-11-18 03:07:48 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-2:, doing work:Task
[zhang-console] INFO 2023-11-18 03:07:48 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-3 start. Command = Task
[zhang-console] INFO 2023-11-18 03:07:48 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-3:, doing work:Task
[zhang-console] INFO 2023-11-18 03:07:49 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-3 finish.
[zhang-console] INFO 2023-11-18 03:07:49 j.b.j.t.t.t2_wait_countdownlatch.Run: Main thread finish.
[zhang-console] INFO 2023-11-18 03:07:49 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-1 finish.
[zhang-console] INFO 2023-11-18 03:07:49 j.b.j.t.t.t.CountDownLatchRunnable: pool-1-thread-2 finish.
[zhang-console] INFO 2023-11-18 03:07:49 j.b.j.t.t.t2_wait_countdownlatch.Run: exit
Disconnected from the target VM, address: '127.0.0.1:32229', transport: 'socket'