[eclipselink-users] EJB 3.0 usage

[Bryan Hunt <bhunt@xxxxxxx>]

We are developing a client (RCP) / server (OSGi) application that uses  
EJB 3.0 for a portion of its persistence model.  We are using  
Hibernate with a layer of custom code that allows Hibernate to work in  
the OSGi plug-in architecture.  I originally designed the application  
to make use of EJB 3.0 in both the client and the server; however,  
Hibernate takes too long to startup so all use of the EJB 3.0  
EntityManager was pushed to the server.

The biggest issue with getting Hibernate to work with the OSGi plug-in  
architecture is, of course, the plug-in classloader.  Hibernate can't  
find your Entities defined in plug-ins without some help.  The problem  
was solved by a combination of the Eclipse buddy policy, and playing  
games with the context classloader and persistence.xml.

Another problem we had was with the persistence.xml file.  Part of our  
application is designed as a set of frameworks that are designed to be  
extensible, and some of these frameworks use the EntityManager.  We  
needed a way to make the persistence.xml file extensible.  The  
solution was to create the persistence.xml file on-the-fly at runtime  
when we initialize the EntityManagerFactory.  A side benefit of  
creating the persistence.xml at runtime is the ability to easily  
switch between different database vendors.  We typically use MySQL for  
debug / test and DB2 for production.  Switching databases is a simple  
matter of specifying which database plug-in to load in the launch  
configuration.

Our solution ties everything together through a persistence plugin  
that defines an extension point allowing other plug-ins to declare  
entities as an extension.  This allows multiple plug-ins to contribute  
entities (as extensions) to the same persistence context.  The plug-in  
also defines an OSGi service for obtaining an EntityManager.  It also  
gives us the ability to manage the same Entity in multiple persistence  
contexts.  We are not currently using this ability, but I can see a  
way we may use it in the future.

Since EclipseLink is developing a competing solution to Hibernate, I'm  
very interested in the work.  I'd really like to see an OSGi friendly  
EJB 3.0 solution.  I'll try to have a deeper look into EclipseLink in  
the next few weeks.

Bryan