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我正在学习如何在 java 中使用 countdownLatch，并在代码中创建了一个简单的示例，如下所示。

我对这种机制的了解是，它只是一种强制只有一个线程等待其他线程直到他们完成工作，然后等待的线程将在其他线程完成时启动它的工作.

我的问题是，如果我有 4 个线程“t1、t2、t3 和 t4”，它们应该按照说明的顺序开始，并且每个线程应该在前一个/前一个线程完成时开始。换句话说，t2 应该等待 t1 并在 t1 结束时开始，t3 应该等待 t2 并在 t2 结束时开始，t4 应该等待 t3 并在 t3 结束时开始。

1-如何使用 CountDownLatch 和循环障碍来做到这一点？

2-传递给CountDownLatch类的构造函数的countDown参数应该代表等待的线程数吗？

代码：

public class MainClass {

public static void main(String[] args) {

    CountDownLatch latch1 = new CountDownLatch(1);
    Thread t1 = new Thread(new AscendingOrder(latch1));
    Thread t2 = new Thread(new DescendingOrder(latch1));

    t1.start();
    t2.start();
}

static class AscendingOrder implements Runnable {

    private CountDownLatch latch;

    public AscendingOrder(CountDownLatch latch) {
        // TODO Auto-generated constructor stub
        this.latch = latch;
    }

    public void run() {
        // TODO Auto-generated method stub
        System.out.println("thread t1 started.");

        for (int i = 0; i < 10; i++)
            System.out.println(i); 

        this.latch.countDown();
    }

}

static class DescendingOrder implements Runnable {

    private CountDownLatch latch;

    public DescendingOrder(CountDownLatch latch1) {
        // TODO Auto-generated constructor stub
        this.latch = latch1;
    }

    public void run() {
        // TODO Auto-generated method stub
        System.out.println("thread t2 started and waiting");

        try {
            this.latch.await();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        for (int i = 10; i > 0; i--)
            System.out.println(i); 
    }

}

}

4

1 回答 1

0

1-如何使用 CountDownLatch 和循环障碍来做到这一点？

CyclicBarrier 不适合这个问题，我已经写过比较两者的帖子，请看这里

public class CountDownLatchTest {

    public static void main(String[] args) {
        CountDownLatch first = new CountDownLatch(1);
        CountDownLatch prev = first;
        for(int i=0;i<10;i++) {
            CountDownLatch next = new CountDownLatch(1);
            new TestThread(prev, next, i).start();
            prev = next;
        }       
        first.countDown();

    }

    public static class TestThread extends Thread {

        private CountDownLatch prev;
        private CountDownLatch next;
        private int id;

        public TestThread(CountDownLatch prev, CountDownLatch next, int id) {
            this.prev = prev;
            this.next = next;
            this.id = id;
        }

        public void run() {
            if (prev != null) {
                try {
                    prev.await();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            try {
                doWork();
            } finally {
                if (next != null) {
                    next.countDown();
                }
            }
        }

        public void doWork() {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            System.out.println("Work done in Thread : "
                    + id);
        }

    }
}

2-传递给CountDownLatch类的构造函数的countDown参数应该代表等待的线程数吗？

CountDownLatch.await() 会一直等到 CountDownLatch.countDown() 方法被调用 countDown 参数次。

于 2015-04-03T08:36:05.360 回答