A Spring Boot REST API can become difficult to maintain when controllers, business rules, and database operations are mixed together. A simple layered structure keeps the code easier to test, extend, and understand.
Start with clear responsibilities
The controller should handle HTTP concerns: reading the request, validating input, selecting the correct status code, and returning a response. Business decisions belong in a service. Database access belongs in a repository. Entities represent stored data, while DTOs define the information exchanged through the API.
Use DTOs at the API boundary
Returning JPA entities directly may expose internal fields and tightly couple the API to the database model. Request and response DTOs make the public contract explicit. They also create a natural place for validation rules such as required fields, length limits, and date constraints.
Keep business rules in the service
A reservation controller should not decide whether a seat is available. It should ask the reservation service. The service can check ownership, availability, time conflicts, and other rules before saving anything. This makes the same logic reusable from another controller, a scheduled task, or a future messaging consumer.
Handle errors consistently
Use a global exception handler to convert domain errors into predictable JSON responses. A client should receive a useful status code, a clear message, and enough context to understand what failed. Consistent errors are easier for both frontend developers and API consumers.
Build for change
Good structure is not about creating unnecessary classes. It is about placing code where future changes will be least painful. A focused controller, a testable service, a small repository, and explicit DTOs provide a practical foundation for most Spring Boot projects.