Organizing packages after domain concerns leads us to an architectural structure, where the actual business, rather than technical details are reflected.
The business use cases handle all logic required to fulfill the business purpose, using all our module contents. They act as a starting point into the application's domain. The use cases are exposed and invoked via the system boundaries. The enterprise systems offers communication interfaces to the outside world, mostly via web services or web-based frontends, that invoke the business functionalities.
When starting a new project, it makes sense to start with the domain logic first, indifferent to system boundaries or any technical implementation details. This contains constructing all contents of the domain, designing types, dependencies and responsibilities, and prototyping these into code. As we will see in this chapter, the actual domain logic is implemented primarily in plain Java. The initial model can be self-sufficient...