JPA EntityManager Nullpointer Exception when splitting functionality in multiple classes (Glassfish, EclipseLink) - rest

Question

i know there are masses of questions concerning a similar problem, but i didn't manage to extract a solution for my problem.
I'm trying to build a RESTful webapplication that serves data, which is provided using JPA (EclipseLink 2.4) and runs on Glassfish 3.1.2.2.
For testing i set up a guestbook database in mysql and created this all-in-one solution:
// imports etc...
// Guestbook class has #Entity
#Stateless
#Path("guestbook")
public class GuestService {
#PersistenceContext
private EntityManager em;
#GET
public Response entry() {
Guestbook entry = em.find(Guestbook.class, someid);
return Response.ok(entry.getMsg()).build();
}
}
This works perfectly well, but i want to split it in to several layers like so: REST -> DAO -> Entity
Which would transform to:
#Stateless
#Path("guestbook")
public class GuestService {
private DAO dao;
#GET
public Response entry() {
Guestbook entry = dao.getEntry(someid);
return Response.ok(entry.getMsg()).build();
}
}
// ...
public class DAO {
#PersistenceContext
private EntityManager em;
public Guestbook getEntry(int someid) {
return em.find(Guestbook.class, someid);
}
}
So this would produce a nullpointer exception when accessing the em in getEntry() because it is null.
My persistence.xml looks like this:
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="..." xmlns:xsi="..." xsi:schemaLocation="...">
<persistence-unit name="guestbook" transaction-type="JTA">
<jta-data-source>jdbc/mysqldatasource</jta-data-source>
<class>model.Guestbook</class>
</persistence-unit>
</persistence>
Thanks in advance, i'm looking forward for your suggestions!

Answer 1

You've abstracted the DAO from GuestService just fine, but it seems that you forgot to make the DAO a #Stateless EJB and inject it in your GuestService using #EJB annotation.
#Stateless
public class DAO {
#PersistenceContext
private EntityManager em;
...
}
#Stateless
#Path("guestbook")
public class GuestService {
#EJB
private DAO dao;
...
}
I hope it helps!