J2EE Design Patterns

Chapter 9. Business Tier Interfaces

If you’ve been reading this book straight through, you’ve been subjected to discussions of the top and bottom of a Java enterprise application: the presentation tier and the domain model. However, we’ve mostly talked around the interface between the two. That’s because it’s hard to talk about interfaces without knowing what’s being interfaced.

But here we are. Business tier interfaces connect the domain model with the client (in thick-client applications) or the server-side presentation layer (in web applications). They also implement some of the business logic for an application, controlling and limiting the actions that the presentation tier can perform on the domain model.

Integrating the client with the business logic for an application poses a few problems for the developer. The big one, of course, is organizational. If you’ve been doing use case-oriented design, you probably have a very good idea of the activities your users need to perform, and of the various business rules that will apply. But how do you represent these as Java objects? That’s a hard question. It’s equally difficult to decide where to put everything. EJBs? Client code? Database stored procedures? In Chapter 6, we introduced the ideas of process (or business) logic and domain logic. This split provides the basis for dividing logic between different components of the business tier: process logic for verbs (the transformations of the domain model), and domain logic for constraints ...