Re: JPA in practice
On Tue, 11 Aug 2009, Lew wrote:
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:
<http://java.sun.com/javaee/5/docs/tutorial/doc/bnbrm.html#bnbrp>
public class Bizzniss
{
@PersistenceContext
private EntityManager em;
public void run()
{
EntityManager mgr = (em != null? em
: Persistuff.getEmf().getEntityManager());
// use 'mgr' here
}
}
Do you need the injection bits in Persistuff? If injection is working,
Bizzniss will get an EntityManager of its own, and never have to call
Persistuff.getEmf(). The only time it will have to call it is when there
is no injection, in which case the injected in field in Persistuff will be
null anyway. I suppose this approach means you take advantage of the case
where a EntityManagerFactory is injected but a PersistenceContext isn't.
That seems like a bit of an unlikely case to me, but i am far from an
expert on this.
Unknown: resource consumption and packratting caused by injected values, if
any.
True that.
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.
I'm surprised GF took *so* much extra memory (and CPU?). What's it doing?
Are there any lightweight app servers that wouldn't be a big overhead over
Tomcat (or perhaps even better, Jetty)?
tom
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