So far you have seen how to organize our code into layers so that we can avoid tight coupling between codes and improve reusability and separation of concerns. We just created one domain class, one repository class, and one service class to demonstrate a purpose, but a typical real-world MVC application may contain many domain, repository, and service classes. Each layer is usually connected through interfaces, and the controller always accesses domain objects from the repository via a service interface only.

So every typical enterprise-level Spring MVC application will logically have four layers, namely Presentation, Domain, Persistence, and Services. The Domain layer is sometimes called the model layer. The following block diagram will help you conceptualize this idea:

Layers of a Spring MVC application

So you have learned how to create a Service layer object and a repository layer object, but what you saw in the Service layer and repository...