Unable to determine why CountDownLatch is not functioning when multiple runnable interfaces are implemented in anonymous classes inside a class

Question

If I define a class Team and implement two runnable interfaces inside that class, I do not get to a point in the program where the tasks by team1 and team2 ends. However if I implement the runnable directly in the class as in WorkerOne, I get to line where it prints tasks by WorkerOne ended. I do not understand why the task by team1 and team2 is never complete and the application is not stopping. I have included the code along with the console output below. I will appreciate any idea or thought. Thank you.

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

class WorkerOne implements Runnable {
    private CountDownLatch latch;

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

    @Override
    public void run() {
        System.out
                .println("[Tasks by WorkerOne : ]" + " :: " + "[" + Thread.currentThread().getName() + "]" + " START");
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        latch.countDown();
        System.out.println("[Tasks by WorkerOne : ]" + " :: " + "[" + Thread.currentThread().getName() + "]" + " END");

    }

}

class Team {
    private CountDownLatch latch;

    Runnable team1 = new Runnable() {
        public void run() {
            System.out.println("[Tasks by team1: ]" + " :: " + "[" + Thread.currentThread().getName() + "]" + "START");
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            latch.countDown();
            System.out.println("[Tasks by team1 : ]" + " :: " + "[" + Thread.currentThread().getName() + "]" + " END");

        }
    };

    Runnable team2 = new Runnable() {
        public void run() {
            System.out.println("[Tasks by team2 : ]" + " :: " + "[" + Thread.currentThread().getName() + "]" + "START");
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            latch.countDown();
            System.out.println("[Tasks by team2 : ]" + " :: " + "[" + Thread.currentThread().getName() + "]" + " END");

        }
    };
}

public class Demo {

    public static void main(String[] args) {
        CountDownLatch latch = new CountDownLatch(3);
        ExecutorService service = Executors.newFixedThreadPool(3);
        service.submit(new WorkerOne(latch));
        service.submit(new Team().team1);
        service.submit(new Team().team2);
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Tasks completed......");
    }
}

The console output is:

[Tasks by WorkerOne : ] :: [pool-1-thread-1] START
[Tasks by team1: ] :: [pool-1-thread-2]START
[Tasks by team2 : ] :: [pool-1-thread-3]START
[Tasks by WorkerOne : ] :: [pool-1-thread-1] END

score 0 · Answer 1 · answered Jul 05 '16 at 23:52

The problem is because you do

    service.submit(new Team().team1);
    service.submit(new Team().team2);

The latch is a private member of Team, and you have created TWO instances of Team, each with its own latch.

I'm not sure why you're doing things this way, but I believe you want

    Team team = new Team();
    service.submit(team.team1);
    service.submit(team.team2);

score 0 · Accepted Answer · answered Jul 06 '16 at 05:41

The Team class' latch variable is never initialized. I suspect you intended, but forgot to, do this initialization, as you did in the WorkerOne class.

Executing the code as you posted it makes the Team runnables throw a NullPointerException when calling countDown() on the latch field. The main thread waits forever on its CountDownLatch as it will never be counted down to 0.

Unable to determine why CountDownLatch is not functioning when multiple runnable interfaces are implemented in anonymous classes inside a class

2 Answers2