spring-boot--kotlinlisted

# Spring Boot + Kotlin ## Project Structure Use feature-oriented MVC. One package per feature, not one package per layer. ``` src/main/kotlin/{base-package}/{feature}/ controller/ # HTTP layer only — no business logic service/ # Business logic repository/ # Data access — extends JpaRepository or CrudRepository domain/ # JPA entities dto/ # Request/response objects — no @Entity here src/test/kotlin/{base-package}/{feature}/ controller/ # MockMvc tests service/ # Unit tests with MockK repository/ # @DataJpaTest tests src/main/resources/ application.yml application-prod.yml # NOT in repo — use env vars db/migration/ # Flyway scripts ``` Never use `src/main/kotlin/controllers/`, `src/main/kotlin/services/`, etc. That is layer-first structure and it does not scale. ## Kotlin Idioms for Spring **Data classes for DTOs.** No setters, no mutable state. ```kotlin data class CreateUserRequest( @field:NotBlank val email: String, @field:Size(min = 2, max = 100) val name: String ) data class UserResponse( val id: UUID, val email: String, val name: String ) ``` **Sealed classes for domain results and errors.** ```kotlin sealed class UserResult { data class Found(val user: User) : UserResult() data class NotFound(val id: UUID) : UserResult() data class Conflict(val email: String) : UserResult() } ``` **Null safety.** Never use `!!` unless you have a