adam bien's blog

ServiceFacade In Greater Detail - and Some EJB 3 Code 📎

What is a ServiceFacade exactly? Yesterday we covered the difference between a ServiceFacade and a Service. ServiceFacade represents the boundary or an external component contract. Some aspects can be already derived from this fact:

  • It is the boundary between the (external) client and the realization of a component - so it is the only possible artifact with a remote interface.
  • ServiceFacade is stateless (only in exceptional cases stateful - why you would need state in procedures?)
  • Every method has to start a new transaction to be executed consistently
  • ServiceFacade is the single entry point into a component, so it can be easily decorated with aspects.

So in general case it is a:

  • @Stateless Session Bean
  • @Remote interface is optional, for web clients you should always use @Local interfaces
  • All methods are transactional - you will need @TransactionAttribute(TransactionAttributeType.REQUIRES_NEW) - it is more explicit
  • From the conceptual point of view - a ServiceFacade should only coordinate Services and should not implement any business logic.  
  • A ServiceFacade should be deployed with a business interface - its easier to mock. No-interface view would work with frameworks like mockito - but is not worth the optimization.
  • If it is needed, you can implement cross-cutting aspects - if you are doing so, provide the interceptors on class and not the method level - is more consistent. I try to avoid the usage of @Interceptors - the less magic, the better. James Gosling said once: "(AOP) It's like giving them a chainsaw without any safety instructions." :-)



@Stateless
@Local(BookOrderingService.class)
@TransactionAttribute(TransactionAttributeType.REQUIRES_NEW)
@Interceptors(PreconditionInterceptor.class) //optional
public class BookOrderingServiceBean implements BookOrderingService {

   @EJB
   DeliveryService deliveryService;

}

The business interface is just a POJI. It is not dependent on the EJB API at all. 
The code above (with several strategies, interceptors and ideas) was checked in (actually pushed :-)) into: http://kenai.com/projects/javaee-patterns