Clean boundaries in a growing Spring Boot service
How a few intentional layers keep business logic testable while the codebase expands.
Organize around behavior
A small service can survive with controllers calling repositories directly. As rules accumulate, that shortcut makes every change touch transport, business logic, and persistence at once. The solution is not more layers for their own sake; it is a clear home for each kind of decision.
Controllers translate HTTP into application inputs. Use cases coordinate business behavior. Domain objects protect core rules. Adapters speak to databases and external systems. This keeps the important logic independent from Spring annotations and infrastructure details.
Dependencies should point inward
Business code should define the interfaces it needs. A persistence adapter can implement a domain-facing repository port, but the domain should not know which database or ORM is on the other side. That direction makes replacements and focused tests much easier.
Spring remains valuable at the application edge, where configuration assembles the pieces. Constructor injection makes those dependencies explicit and keeps objects straightforward to create in tests.
Test at the right seam
Most business rules deserve fast unit tests with simple fakes. A smaller number of integration tests should prove database mappings, serialization, and framework configuration. End-to-end tests then cover only the journeys that carry the most risk.
A clean boundary is useful when it changes how confidently the team can work. If an abstraction does not isolate volatility or make behavior clearer, it may not deserve to exist yet.