Java EE Architecture - Are DAO's still recommended when using an ORM like JPA 2?

Question

If I'm using an ORM like JPA2 - where I have my entities that are mapped to my database, should I still be using a DAO? It seems like a lot more overhead.

For example, I would need to maintain three extra packages:

1. One that specifies my domain objects (which pretty much map my Entity objects):

   public class Employee {     private String firstName;     private String lastName;     ...     // Getters and setters } 

2. One that contains interfaces that specify my DAO methods

   public interface EmployeeDAO {     public void addEmployee(Employee employee);     public Employee getEmployeeById(long id);     ... } 

3. One that contains session beans that implement my DAO's

   public EmployeeJpaDAO implements EmployeeDAO {     interface method implementations here     ....     private method that transform my Employee entity into my Employee domain object } 

Now that's a lot of extra baggage to add every time I need to perform a new CRUD operation.

However the benefits I see from having a DAO is:

1. You can have an in memory implementation of the DAO for unit testing your service layer. This means you don't need to access the database to test business logic, and you can be assured that your objects will always contain the same values for properties

2. It separates business logic from database access logic

The option that doesn't involve implementing a DAO is to just use entity objects and EntityManager in the service layer:

@Stateless public class EmployeeEjb {     @PersistenceContext(unitName = "employee")     private EntityManager manager;      public Employee getEmployeeById(long id) {         return manager.createNamedQuery(Employee.GetEmployeeById).getResultList();     }     ... } 

Is there no middle ground here? Has anyone come across an architecture or implemented an architecture that meets some of the benefits of a DAO layer (most importantly the unit testability of business logic) that I mentioned above, but doesn't involve all the overhead involved to implement a DAO layer?

Thanks for any recommendations and/or suggestions! I'm really curious to see what some people have come up with in regards to this.

Answer 1

If I'm using an ORM like JPA2 - where I have my entities that are mapped to my database, should I still be using a DAO? It seems like a lot more overhead.

It is. And clearly, Java EE doesn't encourage using the DAO pattern when using JPA (JPA already provides a standardized implementation of the Domain Store pattern and there isn't much value at shielding it behind a DAO). I find the DAO to be anti-DRY in such situation.

So for simple cases (actually, most cases), I happily skip the DAO and I have no problem with that. For more complex cases (for example when using stored procedures, flat files), I'd use it. In other words, it depends, as summarized in Has JPA Killed the DAO?. See also the related questions below:

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(...) One that contains session beans that implement my DAO's

Noooo, you certainly don't want to implement a DAO as a Session Bean:

• You don't want to create as much (pooled) Session Bean as tables (big waste of resources)
• You don't want to chain Session Beans everywhere, don't reproduce errors from the past, this is a known bad practice that doesn't scale well.

So if you really want to go the DAO way and want the EM to be injected, either implement your DAOs as Spring beans (in Java EE 5) or CDI managed bean (in Java EE 6).

You can have an in memory implementation of the DAO for unit testing your service layer.

If you really want to do unit testing, mock the DAO/EntityManager, there is no difference. And if you want to do integration testing, you can configure JPA to use an in memory database. So at the end, I just don't buy this argument.

It separates business logic from database access logic

Honestly, I don't see a big difference between relying on a DAO vs an entity manager, I don't see how a DAO separate things "better". Again, I don't buy this argument.

And to my experience, changing the underlying persistence solution is a very exceptional event and I'm not going to introduce DAOs for something that is very likely not going to happen (YAGNI, KISS).

Is there no middle ground here? Has anyone come across an architecture or implemented an architecture that meets some of the benefits of a DAO layer (most importantly the unit testability of business logic) that I mentioned above, but doesn't involve all the overhead involved to implement a DAO layer?

I don't see much middle ground and, as strongly hinted, I don't use DAOs if I don't feel the need. And as I said, mock the EntityManager if you want to truly unit test the business logic. It works for me and I'm happy to write less code.

More resources

• JPA/EJB3 killed the DAO
• DAOs Aren't Dead - But They Either Collapsed Or Disappeared
• ...And Finally, You Should Introduce A DAO If: