hkj-arch

To separate business logic from side effects in Java applications, implement the Functional Core, Imperative Shell pattern to isolate pure domain rules from effectful I/O boundaries. This enforces a clear separation, eliminating scattered validation and hidden state mutations.

You can test time-sensitive logic deterministically without mocks by using TimeSource and Effect Handlers. This pattern isolates time-based effects and I/O operations, allowing pure business logic to execute predictably in test environments.

The best way to model domain entities in Java 25 is using records, sealed interfaces, and pattern matching. This approach creates type-safe, immutable domain models with sealed error hierarchies for robust domain-oriented programming.

Yes, you can refactor a legacy Spring Boot service into a functional core architecture. This migration extracts mixed database calls, validation, and business rules into a testable pipeline where business rules become pure functions and I/O is pushed to the controller boundary.

Migrating to functional core architecture uses Higher-Kinded-J types to facilitate the transition from imperative services to pure, testable business logic. It supports isolating effectful boundaries and integrating domain-oriented programming within Java 25 applications.