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Java Concurrency - CountDownLatch

24 Apr 2017

  • CountDownLatch is a very interesting class from java concurrent package. It allows one thread to wait for one or more threads to complete before it starts processing.

  • This can be super useful on java server side. The usage is pretty simple. You can initialize CountDownLatch with initial count and then decrement it in threads using countDown(). This will decrement the count by 1. Finally you can use await() in the thread which needs to await other threads processing before it starts its processing. It will wait until count becomes 0.

  • Below is the example of CountDownLatch. We have initialized the count at 3 and decrement the count in 3 other threads. The main thread will wait for all 3 threads to complete before it starts processing.

import java.util.concurrent.CountDownLatch;

public class TestCountDownLatch {

    public static void main(String[] args) {
        CountDownLatch latch = new CountDownLatch(3);
        Thread cacheService = new Thread(new Service(latch), "Cache Service");
        Thread namingService = new Thread(new Service(latch), "Naming Service");
        Thread validationService = new Thread(new Service(latch), "Validation Service");

        cacheService.start();
        namingService.start();
        validationService.start();

        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("All services are up!");
    }
}

class Service implements Runnable {

    private CountDownLatch latch;

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

    @Override
    public void run() {
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println(Thread.currentThread().getName() + " is up!");
        latch.countDown();
    }
}

Output:

Validation Service is up!
Cache Service is up!
Naming Service is up!
All services are up!

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