Java中的CountDownLatch

CountDownLatch的作用：

CountDownLatch类位于java.util.concurrent包下，利用它可以实现类似计数器的功能，当我们创建CountDownLatch的对象时，我们指定了它应等待的线程数，所有此类线程都需要在完成或准备好工作后调用CountDownLatch.countDown()来进行递减计数。一旦计数达到零，等待的任务就会开始运行。

CountDownLatch的示例：

import java.util.concurrent.CountDownLatch;

public class CountDownLatchDemo {
    public static void main(String args[])
            throws InterruptedException {
        // Let us create task that is going to
        // wait for four threads before it starts
        CountDownLatch latch = new CountDownLatch(4);

        // Let us create four worker
        // threads and start them.
        Worker first = new Worker(1000, latch,
                "WORKER-1");
        Worker second = new Worker(2000, latch,
                "WORKER-2");
        Worker third = new Worker(3000, latch,
                "WORKER-3");
        Worker fourth = new Worker(4000, latch,
                "WORKER-4");
        first.start();
        second.start();
        third.start();
        fourth.start();

        // The main task waits for four threads
        latch.await();

        // Main thread has started
        System.out.println(Thread.currentThread().getName() +
                "has finished");
    }
}

// A class to represent threads for which
// the main thread waits.
class Worker extends Thread {
    private int delay;
    private CountDownLatch latch;

    public Worker(int delay, CountDownLatch latch,
                  String name) {
        super(name);
        this.delay = delay;
        this.latch = latch;
    }
    @Override
    public void run() {
        try {
            Thread.sleep(delay);
            latch.countDown();
            System.out.println(Thread.currentThread().getName()
                    + " finished");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

输出：

Output:
WORKER-1 finished
WORKER-2 finished
WORKER-3 finished
WORKER-4 finished
main has finished

CountDownLatch类中最重要的3个方法：

1. public void await() throws InterruptedException { };   //调用await()方法的线程会被挂起，它会等待直到count值为0才继续执行
2. public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };  //和await()类似，只不过等待一定的时间后count值还没变为0的话就会继续执行
3. public void countDown() { };  //将count值减1

来源：oschina

链接：https://my.oschina.net/u/4227432/blog/3137408

标签

java

Count_Down