Re: JPA in practice

Lew <>
Tue, 11 Aug 2009 21:19:46 -0400
Tom Anderson wrote:

Is there a convenient way to write code that gets an injected
EntityManager in a managed environment, but arranges its own provision
in an unmanaged one? Is that a meaningful thing to ask for?

I have a way, but I won't know if it works until I succeed at getting an
injected one. Then I can compare the two scenarios.

For the factory:

public class Persistuff
   public static final String PUNIT = "projectPU";

   @PersistenceUnit( unitName=PUNIT )
   private static EntityManagerFactory emf;

   private static final EntityManagerFactory EMFCANON =
     Persistence.createEntityManagerFactory( PUNIT );

   public static EntityManagerFactory getEmf()
     return (emf == null? EMFCANON : emf);

For the manager:

public class Bizzniss
     private EntityManager em;

   public void run()
     EntityManager mgr = (em != null? em
         : Persistuff.getEmf().getEntityManager());
     // use 'mgr' here

Unknown: resource consumption and packratting caused by injected values, if any.

Learning how to do this, I was running Glassfish 3 with a Postgres back end,
but my 4 GB RAM server box's power supply just gave up the ghost. Turns out
the combination of GF and PG with NetBeans was too much for my poor
single-core 64-bit workstation with only 1 GB RAM. Then I tried the
non-injective approach with Tomcat, Postgres and NetBeans. Turns out that
runs just great on the workstation.

That triggered a major "Hmmm." I may be on to a way to develop, deliver and
deploy full-blown custom apps very quickly with very low administrative and
hardware overhead.

Looks like JSF, JSP, JPA and servlets on Tomcat are a winning combination.
Once you factor in a few quirks.

(Bonus - Tomcat plays well with other environments like Apache Web Server and
many Java EE application servers.)


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