The service layer is the most valuable asset an enterprise application possesses. It is the core of all business logic processing and is the layer that holds the most detailed code. Our service layer has no coupling with the DAO implementation and is independent of how the data is used. It is purely focused on business logic operations, delivering data through a simple, generically typed value object using the data transfer object design pattern.

Our service layer implementation has clearly defined entry points for business logic operations. This is achieved through Java interfaces that define all publicly accessible methods. The use of interfaces also enables us to write test cases prior to coding the implementations—a core principle of test-driven development and extreme programming. In the following chapter, we will use these interfaces to define a request handling layer for web clients.