Design: should I add a 'DAO' layer between my Session bean and my Entities?

[Sol Mayer-Orn]

Hi,

I'm new to EJB3, and would like to ask a design question:

Is it customary to add a 'Data Access' layer between the Session beans and the Entities?
Or would your Session bean simply access the Entity API directly?

For example, say I have a 'Bank' session bean, and Account entities.

Then Bank can directly access the Entity API:

     // Session bean directly accessing Entity API:
     @Stateless
     @Remote(somePackage.Bank)
     public class BankBean implements Bank{
          @PersistenceContext
          private EntityManager em;
 
          public List<Account> findAccountsByOwnerName(String ownerName){
                Query query=em.createQuery("select a from Account a where a.ownerName= :n").setParameter("n", ownerName);
                return query.getResultList();
          }
     }

But I'm considering an alternative of writing a Data Access layer, something like the following.
It seems more work, but the session bean becomes more readable (especially if it has some additional complex logic and not just simple 'find'), and it can hide some sql implementation choices (e.g. if we find some way to optimize the query, or move to native sql, etc):

 
// Data Accesss helper:
public class AccountDao {
     public List<Account> findAccountsByOwnerName(String ownerName, EntityManager em){
           Query query=em.createQuery("select a from Account a where a.ownerName= :n").setParameter("n", ownerName);
           return query.getResultList();
     }
}
 
// Session bean uses the DataAccess helper:
@Stateless
@Remote(somePackage.Bank)
public class BankBean implements Bank{
     @PersistenceContext
     private EntityManager em;
     private AccountDao dao=new AccountDao();
 
     public List<Account> findAccountsByOwnerName(String ownerName){
         return dao.findAccountsByOwnerName(ownerName);
     }
}

Of course this is a simplified example... in real life, the code may be more complex (especially the Session logic), and a query may be re-used from several methods.

If anyone can share their views, and/or refer me to EBJ design tutorials, it will be appreciated.
Thanks
[ September 24, 2007: Message edited by: Sol Ma-Orn ]

[Darya Akbari]

Hi Sol,

I would use a DAOFactory in your Session Bean. See the links I gave in this thread why DAO? from JavaRanch's ORM forum for more info.

Regards,
Darya

[Sol Mayer-Orn]

Thanks for the excellent reply and the link.

Can I just ask: how do you make the DAO available to the Session bean?
I'm considering 3 alternatives (see below).
Which seems most reasonable, in your experience ? Or do you have a different, better approach?
Thanks again.

Options I was considering:

// Approach 1: Privately initialize the DAO inside the Session bean:
// This is simple, but goes against "inversion of control" philosophy (so 
// you can't easily re-configure the application to use a different DAO 
// implementation):
 
class MySessionBean{
  ...
  MyDao dao=new MyDAO(..);
}

// Approach 2: Make the DAO a local session bean, and have it injected into 
// the 'main' session bean:
 
class MySessionBean{
   @EJB
   MyDAO dao;  // MyDAO is a local session bean
}

 
// Approach 3: Register the DAO as a resource on JNDI - a simple resource, not any 
// type of EJB (I don't really know how to do it, but I hear it's possible):
 
class MySessionBean{
   @Resource
   MyDAO dao;  // MyDAO is registered on JNDI, but it's not an EJB
}

[ September 26, 2007: Message edited by: Sol Mayer-Orn ]

[Roger Chung-Wee]

What about a factory which returns the DAO instance. The factory could be configured to return the correct instance depending on a configuration which is set before the EJB container starts up.

[Sol Mayer-Orn]

Thanks, Roger Chung-Wee, for this reply.
The factory approach sounds good - neat and flexible.

But in that case, could I just shift my question a little: how do you make the *Factory* available to the Session bean ? Is your factory just a static method (or static singleton) ? Or is your factory a Local session bean ? etc.

Thanks again