How does the Single Responsibility Principle relate to the layered architecture used in a typical Spring Boot application?

A typical Spring Boot application's @Entity, @Repository, and @Service layers are SRP applied at the architectural level: the entity holds and describes data, the repository owns persistence, and the service owns business logic, and each layer changes for a completely different reason. This mirrors splitting an Invoice class into Invoice, InvoiceRepository, and EmailService — a database migration only touches the repository layer, a business rule change only touches the service layer, and a field rename only touches the entity, without any of those changes rippling into the other layers. This separation is also what makes each layer independently testable — a service can be unit tested with a mocked repository, without spinning up a real database, precisely because persistence logic was never mixed into the service in the first place. In short, SRP isn't just a rule for individual classes; it's the same reasoning, applied consistently, that produces a well-layered backend architecture.