Java Concurrency Model - Part II - CountDownLatch

Note: Get the source code from

In this post i have tried to show an example using CountDownlatch, a synchronizer in Java concurrency framework.

Class CountDownLatchTest

package com.somitsolutions.training.java.ExperimentationWithCountdownLatch;

import java.util.concurrent.CountDownLatch;
import java.util.logging.Level;
import java.util.logging.Logger;

public class CountDownLatchTest {

    public static void main(String args[]) {
        final CountDownLatch latch = new CountDownLatch(3);
        Thread service1 = new Thread(new Service("Service1", 1000, latch));
        Thread service2 = new Thread(new Service("Service2", 1000, latch));
        Thread service3 = new Thread(new Service("Service3", 1000, latch));

        service1.start();
        service2.start();
        service3.start();

        // Application should not start processing any thread until all services are up
        // and ready to do their job.
        // CountDownLatch is an ideal choice here. The main thread will start with count 3
        // and wait until the count reaches zero. Each thread, once up and ready, will do
        // a countdown. This will ensure that the main thread does not start processing
        // until all services are up.

        // Count is 3 since we have 3 Threads (Services)
        try {
            latch.await(); // Main thread is waiting on CountDownLatch to finish
            System.out.println("All services are up, Application is starting now");
        } catch (InterruptedException ie) {
            ie.printStackTrace();
        }
    }
}

class Service implements Runnable {
    private final String name;
    private final int timeToStart;
    private final CountDownLatch latch;

    public Service(String name, int timeToStart, CountDownLatch latch) {
        this.name = name;
        this.timeToStart = timeToStart;
        this.latch = latch;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(timeToStart);
        } catch (InterruptedException ex) {
            Logger.getLogger(Service.class.getName()).log(Level.SEVERE, null, ex);
        }
        System.out.println(name + " is Up");
        latch.countDown(); // Reduce count of CountDownLatch by 1
    }
}