Question

I am using Glassfish v3 server.

Usually the DB connection with EJB3 + JPA (Eclipselink) is done through injection, with @PersistenceUnit or @Persistencecontext.

However, there are 3 layers in my App :

• Core (contains business logic, entities, exception handling etc)

• an EJB on top of it, calling the right core objects and methods to do the job. This EJB is called by other internal modules of our ERP.

• a REST layer on top of it for uses by frontend web sites.

I do not want to get the entityManager, nor the EMF (EM factory) in the EJB, because I want my middle layer to be unaware that there is a DB used under it. I could after all, later, decide to change my core implementation for a non-DB-using one.

I see only two bad solutions :

• 1) Add an EM parameter every time i call a method of the core layer that needs DB connection. Very ugly and goes againt what I said above.

• 2) In every method of core needing DB connection, I create a factory, an EM, use them, and then close them both.

I tried to cut things in the middle, having one factory per class of the Core level, and EMs are created and closed in every method. But I still have memory leaks like this :

javax.servlet.ServletException: Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.0.0.v20091127-r5931): org.eclipse.persistence.exceptions.DatabaseException

Internal Exception: java.sql.SQLException: Error in allocating a connection. Cause: In-use connections equal max-pool-size and expired max-wait-time. Cannot allocate more connections.

I guess it’s because if one of my EJB methods uses 10 different objects, it creates 10 EM factories, and none of them is closed.

Example of typical use in a Core object:

 EntityManager em = emf.createEntityManager();
 em.getTransaction().begin();
 // do some stuff with em; for example persist, etc
 em.flush();
 em.close();

Should I go for solution 2?
Is there a way to use a single EM factory at this core level ? I seems like the JPA spec assumes you’re going to use the entities at the EJB level only, which is bad in a multi layers apps.

EDIT : here is the current status after trying @Inject :

• Added an empty beans.xml file in the /META-INF directory on my CORE jar.

• The parent DAO class is now like this :

  public class ExampleBZL {

   public EntityManagerFactory emf;
   @Inject public Emf emfobject;
  
   public ExampleBZL()
   {
      this.emf = emfobject.emf;
   }
  

• The Emf class is very simple, and Stateless.

  @Stateless
  public class Emf implements EmfAbstract {

  @PersistenceUnit(unitName = Setup.persistenceUnitName)
  public EntityManagerFactory emf;
  
  public Emf()
  {
  }
  

  }

I must be doing something wrong, but the injection doesn’t work, altough in glassfish I see “[ejb, weld, web]” in the engines list, so the CDI is loaded.

Servlet.service() for servlet Jersey Web Application threw exception
java.lang.NullPointerException
    at com.blablabla.core.bizlogic.ExampleBZL.<init>(ExampleBZL.java:40)

Am I missing other annotations ?? Is it really working to do an inection in a JAR with those two small annotations (Stateless on one side, Inject on the other) ?

Answer 1

What if you had two session beans? One with the injected EntityManager that can leverage JTA, and the other is your current session bean.

I am currently putting together a series on my blog using a session bean as the REST service using EclipseLink & Glass Fish v3:

• Part 1 – The Database Model
• Part 2 – Mapping the Database Model to JPA Entities
• Part 3 – Mapping JPA Entities to XML using JAXB
• Part 4 – The RESTful Service

Below is how I inject the EntityManager on my session bean that is serving as my REST service:

package org.example.customer;

import javax.ejb.LocalBean;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.PersistenceContextType;
import javax.ws.rs.Path;

import org.eclipse.persistence.rest.JPASingleKeyResource;

@Stateless
@LocalBean
@Path("/customers")
public class CustomerService  {

    @PersistenceContext(unitName="CustomerService", type=PersistenceContextType.TRANSACTION)
    EntityManager entityManager;

}

You can link your session beans using the @EJB annotation:

package org.example;

import javax.ejb.EJB;
import javax.ejb.LocalBean;
import javax.ejb.Stateless;
import javax.naming.Context;
import javax.naming.InitialContext;

@Stateless
@LocalBean
@EJB(name = "someName", beanInterface = CustomerService.class)
public class OtherSessionBean {

    public Customer read(long id) {
        try {
            Context ctx = new InitialContext();
            CustomerService customerService = (CustomerService) ctx.lookup("java:comp/env/someName");
            return customerService.read(id);
        } catch(Exception e) {
            throw new RuntimeException(e);
        }
    }

}