Backend Implementation

REST API development with Kotlin and Spring Boot involves creating controllers, services, and repositories following layered architecture. Spring Boot handles dependency injection and lifecycle management, while Kotlin's concise syntax reduces boilerplate. Define endpoints in controllers, business logic in services, and data access in repositories, then validate inputs and handle errors consistently across all layers.

Input validation in Spring Boot uses annotations like @Valid and @NotNull on request objects, paired with exception handlers that map violations to standardized error responses. Validation occurs at the controller boundary before reaching service logic, preventing invalid data from corrupting business rules. Custom validators extend ConstraintValidator for domain-specific rules.

Testing Spring Boot APIs combines unit tests for services and repositories with integration tests using in-memory databases like H2. Use @SpringBootTest for full context loading and @WebMvcTest for isolated controller testing. In-memory databases enable reliable, repeatable end-to-end validation without external dependencies, ensuring proper build outcomes and error handling.

Spring Data repositories work seamlessly with Kotlin, enabling type-safe database access through JpaRepository and custom query methods. Kotlin's null-safety and extension functions integrate naturally with Spring's repository abstractions, reducing null-pointer exceptions and boilerplate code for common CRUD operations and domain queries.

REST error handling uses @ExceptionHandler to map domain exceptions to HTTP status codes and standardized error response objects. Return meaningful status codes (400 for validation, 404 for not found, 500 for server errors) with descriptive messages. Exception mapping ensures consistent error responses across all endpoints and improves client debugging.

In-memory databases like H2 are recommended for testing Spring Boot APIs to avoid external dependencies and ensure test isolation. They execute quickly, reset between tests, and validate database integration without requiring production infrastructure. This approach supports reliable end-to-end validation and faster build cycles during development.