JPA and Threads in play framework

Question

I'm trying to create a multithreading server. The problem is that I get the following error: play.exceptions.JPAException: The JPA context is not initialized. JPA Entity Manager automatically start when one or more classes annotated with the @javax.persistence.Entity annotation are found in the application.

What I'm trying to do is access the db from the new thread here's 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(5,5,2,2,10,15);
    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?

Answer 1

Update

This is neater than what's below:

JPAPlugin.startTx(false);
// Do your stuff
JPAPlugin.endTx(false);

Had similar problem today.

You have to create new EntityManager and transaction for each thread and set it in JPA class.

Play uses ThreadLocal to keep it's EntityManager in JPA, so it's null for your created thread. Unfortunately you cannot use helper methods in JPA to do it (they are package private) and you have to use ThreadLocal directly. Here's how you can do this:

class Runner extends Runnable {
     @Override
     public void run() {
         if (JPA.local.get() == null) {
             EntityManager em = JPA.newEntityManager();
             final JPA jpa = new JPA();
             jpa.entityManager = em;
             JPA.local.set(jpa);
         }

         JPA.em().getTransaction().begin();
         ... DO YOUR STUFF HERE ...
         JPA.em().getTransaction().commit();
     }
}

I use it with single thread executor from java.util.concurrent without any problems.