How should EntityManager be used in a nicely decoupled service layer and data access layer?

First off, whether you should use a DAO layer or not is a debate that has been around since the appearance of JPA and the EntityManager which many people consider a DAO itself. The answer to this depends on the type of application you are developing, but in most of the cases you will want to.

First off, whether you should use a DAO layer or not is a debate that has been around since the appearance of JPA and the EntityManager which many people consider a DAO itself. The answer to this depends on the type of application you are developing, but in most of the cases you will want to: Use JPA criteria or custom queries. In this case you probably don't want to mix your business logic with your query creation.

This would lead to large methods and would violate the single responsibility principle. Reuse your JPA code as much as possible. Say you create a criteria query that retrieves a list of employees whose age is between 40 and 65 and have been working in the company for longer than 10 years.

You might want to reuse this type of query somewhere else in your service layer, and if that is the case, having it in a service would make this task difficult. That being said, if all you have in your application is CRUD operations and you don't think you might need to reuse any JPA code, a DAO layer is probably something overkill as it will act as a mere wrapper of the EntityManager, which doesn't sound right. Secondly, I would advise to use container managed transactions whenever possible.In case you are using an EJB container like TomEE or JBoss this would avoid a large amount of code dedicated to programmatically create and manage transactions.

In the case you are using en EJB container, you can take advantage of declarative transaction management. An example of this using DAOs would be to create your service layer components as EJBs and your DAOs too. @Stateless public class CustomerService { @EJB CustomerDao customerDao; public Long save(Customer customer) { // Business logic here return customerDao.

Save(customer); } } @Stateless public class CustomerDao { @PersistenceContext(unitName = "unit") EntityManager em; public Long save(Customer customer) { em. Persist(customer); return customer.getId(); } public Customer readCustomer(Long id) { // Criteria query built here } } In the example above, default transaction configuration is REQUIRED, which means that in absence of a transaction in the caller component, the EJB will create a new transaction. If the caller already creates a transaction (CustomerService) the component being called (CustomerDao) inherits the transaction.

This can be customized using the @TransactionAttribute annotation. If you are not using an EJB container, I think your example above would be probably equivalent. EDITED: for the sake of simplicity I have used no-interface EJBs above, but it would be a good practice to use an interface for those in order to make them e.g. More testable.

This will run in GlassFish, so I will edit the question to be explicit on that. Your example code assumes an EJB container, yes? – Pete Oct 22 at 17:29 Yes.

The code above assumes a JEE compliant container, which Glassfish actually is. – Gonzalo Garcia Lasurtegui Oct 22 at 17:39 1 I would opt for this solution then, so Gonzalo - you have my axe... vote I mean :-). You could modify it a bit to provide your service with CRUD operations (i.e.

Create GenericCRUDService). Then some of your services which are plainly CRUD-based won't have any specific DAO and if the services would need to reuse some complicated queries, than these can be defined in separated class, as Gonzalo shown, or treated as DDD Repository. – Piotr Nowicki Oct 22 at 22:36.

Typically, you would want to isolate any persistence code to your DAO layer. So service layer should not even know about EntityManager. I think it's ok if DAO layer returns annotated pojos since they remain pojos still.

For the transaction management, I suggest that you look at Spring ORM. But if you choose not to use Spring or other AOP solution, you can always expose transaction related methods via your DAO so you call them from the service layer. Doing so will make your life much harder but the choice is yours...

It gives you the nice CRUD interface and is database-agnostic. – Piotr Nowicki Oct 22 at 8:41 I would love to use Spring but not an option for this project. Exposing transaction logic as DAO methods seems just wrong - perhaps you could clarify with a code example.

– Pete Oct 22 at 17:37.

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