java - 如何在每个线程完成 Java 运行后运行任务？

Question

我有一个循环，它在每次迭代时创建一个新线程，如下所示:

for(int i = 0; i < REPEAT; i++) {
    new Thread(new MyTask(i)).start();
    Thread.sleep(1);
}

private void finalTask() {
    //Some code to be executed once every threads stopped running
}

其中 MyTask 是一个实现 Runnable 的类。我的目标是:我想在每个线程停止后运行 finalTask​​。为实现这一点，我尝试在每次线程完成运行时将变量递增 1，一旦该变量等于 REPEAT，最终任务就会运行。但这没有用。我在 Google 和 StackOverlow 上搜索了我的问题的答案，但是关于这个的信息很少，而且没有一个能正常工作。在最终任务之后总会有一个线程在运行。那我该怎么做呢？

Answer 1

您可以使用 CountDownLatch为了这。一个 CountDownLatch 是

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

CountDownLatch countDownLatch = new CountDownLatch(REPEAT);
for (int i = 0; i < REPEAT; i++) {
    new Thread(new MyTask(i, countDownLatch)).start();
    Thread.sleep(1);
}
finalTask(countDownLatch);

我创建了一个 CountDownLatch，其 count 被初始化为 REPEAT 的值。我将其传递给每个线程和 finalTask​​ 方法。

每个线程完成其工作后都应调用 countDownLatch 的 countDown 方法。

private static class MyTask implements Runnable {

    private int i;
    private CountDownLatch countDownLatch;

    private MyTask(int i, CountDownLatch countDownLatch) {
        this.i = i;
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        //Perform some task
        System.out.println("Running " + i);
        countDownLatch.countDown();
    }
}

finalTask​​ 方法的第一行应该调用 CountDownLatch 的 await 方法。这将导致运行 finalTask​​ 的线程等待，直到 CountDownLatch 的计数达到 0，即，直到所有线程(它们的 REPEAT 数量)完成并调用 countDown CountDownLatch。

 private static void finalTask(CountDownLatch countDownLatch) {
    try {
        countDownLatch.await(); //this will wait until the count becomes 0.
    } catch (InterruptedException e) {
        e.printStackTrace(); //handle it appropriately
    }
    //Some code to be executed once all threads stopped running
    System.out.println("All done");
}