Advice/best practises wanted : EntityManager/DAO/Transaction

ZV_GP · **Joined:** Tue Sep 12, 2006 3:19 am **Posts:** 6

hi there,

I'm looking for some best practises regarding the DAOs and their coöperation with Business Objects.

A lot of useful info is on
http://blog.hibernate.org/cgi-bin/blosx ... genericdao

but I'm missing info in the context of EntityManager/EJB3.

I'm developing a Swing application of which the data are persisted using Hibernate/EJB3.
- Data classes contain EJB annotations.
- Persistence is controlled by BusinessObjects and DAOs.
- BO's control beginning and end of a transaction

My DAOs are basically CRUD DAOs with some additional named queries.
The BOs'logic sometimes need to call the methods of different DAOs within a single transaction.
My current implementation features way too many duplicate code, so I'm interested to learn how others write similar code in a generic and elegant way.

I'm using an EntityManagerHelper, which instantiates the EntityManagerFactory and allows to create new EntityManagers.

Is it a good strategy, as featured on the website above, to create new DAOs for each call ?

What about exceptions ?
The BOs should do the rollback when an hibernate exception happens, the application should deal with the failure in the BOs.
I could rethrow the exceptions but I suppose it's clean to throw some kind of ApplicationException from within the BOs ?

I somewhere read about transaction annotations, but in my case these would have to be implemented in the BOs.
@TransactionAttribute(TransactionAttributeType.REQUIRED)
If I wouldn't use these annotations, my BOs would be filled with

EntityManagerHelper.createNewManager();
EntityManager em = EntityManagerHelper.getEntityManager();
EntityTransaction tx = em.getTransaction();
tx.begin();
log.debug("Start transaction");
try {
dao.insert(p);
} catch (Exception e) {
tx.rollback();
log.error("transaction rollback", e);
} finally {
em.getTransaction().commit();
em.close();
log.debug("Close transaction");
}

Currently my BOs are more or less duplactions of the methods in the DAOs.
It look kinda weird but I don't want to put transaction code in the DAOs.
Remarks on this ?

Geert[/url]

emmanuel · **Posted:** Thu Dec 07, 2006 1:49 pm

all our design effrot hase been put on EJB3 (PC propagation through session beans etc), check the EJB 3.0 design (or just use the Embeddable EJB 3 container)

marcelo.farinelli · **Joined:** Sun Dec 24, 2006 7:38 pm **Posts:** 2

ZV_GP

You made some interesting points here. Im going into the same problem and I have to design a rich application in SWT.

The approach Im going to use is just as you wrote. The very same one.

I was searching for a good material on the internet regarding Generic DAOs with BOs and EJB3 when I found you post.

I would like to know if you did any progress, and if you did, please post me some links or give me some advice.

I would like to stay in touch with you too, since we are using very similar development approaches.

Thanks in advance and happy coding...

greta · **Posted:** Tue May 01, 2007 11:40 pm

Just wondering if you ever got a good answer to this posting.

I'm facing the same problem -- I'm using Hibernate (via JPA interfaces) in a stand-alone (Java SE) environment. All the Java SE examples use the Hibernate interfaces. All the JPA examples are for Java EE.

I am using an abstract class to wrap my business methods (to handle the transaction and exception handling):

Code:

public abstract class Transaction<t> {
    /**
     * Logger for this class.
     */
    private static final Log LOG = LogFactory.getLog(Transaction.class);
    
    private EntityManager em;
    
    /**
     * Constructor.
     */
    public Transaction () {
    }
    
    /**
     * @return generic type
     */
    public t execute() {
        
        // Create a new EntityManager (using my helper class)
        em = PersistenceHelper.getEntityManager(false, true);        
        
        EntityTransaction tx = null;
        
        t returnValue = null;
                
        try {
            tx = em.getTransaction();
            tx.begin();
            
            returnValue = action();
            
            tx.commit();
        } catch (RuntimeException ex) {
            try {
                tx.rollback();
            } catch (RuntimeException rbEx) {
                LOG.error("Couldn't roll back transaction", rbEx);
            }
            throw ex;
        } finally {
            em.close();
            em = null;
        }
        
        return returnValue;
    }
    
    /**
     * Only call from inside action -- otherwise returns null.
     * 
     * @return EnityManager used by execute();
     */
    protected EntityManager getEntityManager() {
        return em;
    }
    
    /**
     * @return generic type
     */
    protected abstract t action();
    
}

Here is an example where I use this class:

Code:

    public Integer getWidgetCount() {
        
        Transaction<Integer> transaction = new Transaction<Integer>() {
            public Integer action() {
                WidgetDao dao = new WidgetDao(getEntityManager());
                
                return dao.countWidgets();
            }
        };
        
        return transaction.execute();
    }

I create a new DAO and EntityManager for each business operation (method).

Let me know what you've come up with. Thanks!

marcelo.farinelli · **Joined:** Sun Dec 24, 2006 7:38 pm **Posts:** 2

greta

The app I was developing has been under my desk for a while, since Im currently using JBoss Seam for a web project.

But at that time, I think I figured out how to deal with this situation.

I used a Generic DAO provided by Chris Bauer (AFAIR) which manages the entity manager, EJB3 entities (POJO) and a DAO for each entity.

I use JBoss 4.0.5 with EJB3 profile, so my SWT interface uses RMI to talk with the server. But thats quite simple to do it.

I ended up with a simple working example, and that structure gave me a nice clear separation of concerns.

Unfortunately, I did stop developing the app. But I still have that code. I can post here later if you want. Just need to go back to my office ;-)

greta · **Posted:** Thu May 03, 2007 4:33 pm

marcelo-

Please post your code - that would be great.

Do you generate any of your code? Do you generate your DB schema?

Thanks!