Friday, September 18, 2015

Java Concurrency Model - Part II - CountDownLatch

Note: Get the source code from https://github.com/sommukhopadhyay/CountDownLatche

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 service is up
       // and ready to do there job.
       // Countdown latch is idle choice here, main thread will start with count 3
       // and wait until count reaches zero. each thread once up and read will do
       // a count down. this will ensure that main thread is not started processing
       // until all services is 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();
       }
      
    }
  
}

/**
 * Service class which will be executed by Thread using CountDownLatch synchronizer.
 */
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
    }
  
}

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