Multithreading – JPA and threads in the playback frame

I'm trying to create a multithreaded server The problem is that I received the following error:

What I want to do is access dB from the new thread. Here is the code

package controllers;
import java.util.Iterator;
import java.util.List;
import models.Ball;


public class MainLoop extends Thread {

@Override
public void run() {
    List<Ball> balls;
    new Ball(5,5,2,10,15);
    while (true){
        balls = Ball.all().fetch(); //Here throws an exception 

        for (Iterator iterator = balls.iterator(); iterator.hasNext();) {
            Ball ball = (Ball) iterator.next();
            ball.applyForces();
        }
    }
}
}

Any ideas?

Solution

Instead of using normal threads, use jobs:

@OnApplicationStart 
public class MainLoop extends Job { 
       public void doJob() { 
               new BallJob().Now(); 
       }
}

And players:

public class BallJob extends Job {
public void doJob() {
    List<Ball> balls;
    new Ball(5,15);
    while (true){
        balls = Ball.all().fetch(); 
        for (Iterator iterator = balls.iterator(); iterator.hasNext();) {
            Ball ball = (Ball) iterator.next();
            ball.applyForces();
        }
    }
}