So far so good, we have created a Presentation layer that contains a controller, a dispatcher servlet, view resolvers, and more. And then we created the Domain layer, which contains a single domain class Product. Finally, we created the Persistence layer, which contains a repository interface and an implementation to access our Product domain objects from an in-memory database.

But we are still missing one more concept called the Service layer. Why do we need the Service layer? We have seen a Persistence layer dealing with all data access (CRUD) related logic, a Presentation layer dealing with all web requests and view-related activities, and a Domain layer containing classes to hold information that is retrieved from database records/the Persistence layer. But where can we put the business operations code?

The Service layer exposes business operations that could be composed of multiple CRUD operations. Those CRUD operations are usually performed by the repository objects...