如何在Java多线程中使用CountDownLatch？

Question

有没有人能帮我理解一下Java CountDownLatch是什么，什么时候用？

我不是很清楚这个程序是如何工作的。据我所知，所有三个线程同时启动，每个线程将在3000ms后调用CountDownLatch。所以倒计时将一个接一个地递减。当latch变为零后，程序会打印“已完成”。也许我理解的方式是不正确的。

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class Processor implements Runnable {
    private CountDownLatch latch;

    public Processor(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        System.out.println("Started.");

        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        latch.countDown();
    }
}

//

public class App {

    public static void main(String[] args) {

        CountDownLatch latch = new CountDownLatch(3); // coundown from 3 to 0

        ExecutorService executor = Executors.newFixedThreadPool(3); // 3 Threads in pool

        for(int i=0; i < 3; i++) {
            executor.submit(new Processor(latch)); // ref to latch. each time call new Processes latch will count down by 1
        }

        try {
            latch.await();  // wait until latch counted down to 0
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Completed.");
    }

}

Answer 1

Java语言中的CountDownLatch是一种同步器，它允许一个Thread在开始处理之前等待一个或多个Thread。

CountDownLatch的工作原理是锁存，线程会一直等到门打开。一个线程等待创建CountDownLatch时指定的n线程数。

例如final CountDownLatch latch = new CountDownLatch(3);

这里我们将计数器设置为3。

任何调用CountDownLatch.await()的线程，通常是应用程序的主线程，都会一直等到计数为零，或者被另一个Thread中断。所有其他线程在完成或准备好执行作业后，都需要通过调用CountDownLatch.countDown()进行倒计时。一旦计数达到零，等待的Thread就开始运行。

在这里，计数是通过CountDownLatch.countDown()方法递减的。

调用await()方法的Thread将等待，直到初始计数为零。

要使计数为零，其他线程需要调用countDown()方法。一旦计数变为零，调用await()方法的线程将恢复(开始执行)。

CountDownLatch的缺点是它不可重用:一旦计数变为零，它就不再可用。

Answer 2

NikolaB很好地解释了这一点，但是示例有助于理解，所以这里有一个简单的示例……

 import java.util.concurrent.*;


  public class CountDownLatchExample {

  public static class ProcessThread implements Runnable {

    CountDownLatch latch;
    long workDuration;
    String name;

    public ProcessThread(String name, CountDownLatch latch, long duration){
        this.name= name;
        this.latch = latch;
        this.workDuration = duration;
    }


    public void run() {
        try {
            System.out.println(name +" Processing Something for "+ workDuration/1000 + " Seconds");
            Thread.sleep(workDuration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(name+ "completed its works");
        //when task finished.. count down the latch count...

        // basically this is same as calling lock object notify(), and object here is latch
        latch.countDown();
    }
}


public static void main(String[] args) {
    // Parent thread creating a latch object
    CountDownLatch latch = new CountDownLatch(3);

    new Thread(new ProcessThread("Worker1",latch, 2000)).start(); // time in millis.. 2 secs
    new Thread(new ProcessThread("Worker2",latch, 6000)).start();//6 secs
    new Thread(new ProcessThread("Worker3",latch, 4000)).start();//4 secs


    System.out.println("waiting for Children processes to complete....");
    try {
        //current thread will get notified if all chidren's are done 
        // and thread will resume from wait() mode.
        latch.await();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    System.out.println("All Process Completed....");

    System.out.println("Parent Thread Resuming work....");



     }
  }

Answer 3

CoundDownLatch使您能够让线程等待，直到所有其他线程执行完毕。

伪代码可以是：

// Main thread starts
// Create CountDownLatch for N threads
// Create and start N threads
// Main thread waits on latch
// N threads completes there tasks are returns
// Main thread resume execution