Spring Boot Kotlin Patterns

Description

Guidelines for writing idiomatic Spring Boot code with Kotlin following the patterns established in this project.

Use this skill when:

•Creating new controllers, services, or repositories
•Configuring Spring components
•Working with dependency injection
•Implementing data transfer objects (DTOs)

Core Principles

1. Constructor Injection (Never Field Injection)

Good:

kotlin

@RestController
class UserController(
    private val userService: UserService
)

Bad:

kotlin

@RestController
class UserController {
    @Autowired
    private lateinit var userService: UserService
}

2. Data Classes for DTOs and Responses

kotlin

data class UserResponse(
    val id: Long,
    val name: String,
    val email: String
)

3. Feature-Based Package Structure

code

src/main/kotlin/me/soenke/spring_demo/
├── config/           # Configuration classes
├── dadjoke/          # Dad jokes feature
│   ├── DadJokeController.kt
│   ├── model/
│   └── repository/
├── greeting/         # Greetings feature
├── name/             # Names feature
└── weather/          # Weather feature

Group by feature, not by layer (no separate controllers/, services/, repositories/ directories).

4. Structured Logging

Good:

kotlin

logger.info("Serving random dad joke. ID={}, Length={}", randomJoke.id, randomJoke.jokeText.length)

Bad:

kotlin

logger.info("Joke: $randomJoke")

Always use placeholders instead of string interpolation in log messages.

5. Spring Data JDBC with CrudRepository

kotlin

@Repository
interface DadJokeRepository : CrudRepository<DadJoke, Long> {
    @Query("SELECT * FROM dad_jokes ORDER BY RANDOM() LIMIT 1")
    fun findRandomJoke(): DadJoke?
}

6. Exception Handling with ResponseStatusException

kotlin

@GetMapping("/dad-jokes")
fun getRandomDadJoke(): ResponseEntity<DadJokeResponse> {
    val joke = repository.findRandomJoke()
        ?: throw ResponseStatusException(HttpStatus.NOT_FOUND, "No jokes found")

    return ResponseEntity.ok(DadJokeResponse(joke.text))
}

7. Kotlin Idioms

•Use data class for immutable DTOs
•Use ?.let or Elvis operator ?: for null handling
•Prefer val over var
•Use extension functions for utility logic
•Embrace null safety (no nullability annotations needed with proper types)

8. File Size Limit

Keep files under 300 lines. Split into multiple files when a class exceeds this limit.

9. Configuration as Code

Use @Configuration and @Bean for Spring configuration instead of XML or properties.

kotlin

@Configuration
class SecurityConfig {
    @Bean
    fun securityFilterChain(http: HttpSecurity): SecurityFilterChain {
        // Configuration here
    }
}

Common Patterns

RESTful Endpoints

kotlin

@RestController
@RequestMapping("/endpoint")
class MyController(private val service: MyService) {

    @GetMapping
    fun getAll(): ResponseEntity<List<ResponseDTO>> {
        return ResponseEntity.ok(service.getAll())
    }

    @PostMapping
    fun create(@RequestBody request: CreateRequest): ResponseEntity<ResponseDTO> {
        return ResponseEntity.status(HttpStatus.CREATED).body(service.create(request))
    }
}

Repository Pattern

kotlin

@Repository
interface UserRepository : CrudRepository<User, Long> {
    @Query("SELECT * FROM users WHERE email = :email")
    fun findByEmail(@Param("email") email: String): User?
}

Service Layer

kotlin

@Service
class UserService(private val repository: UserRepository) {

    fun getUserByEmail(email: String): UserResponse {
        val user = repository.findByEmail(email)
            ?: throw ResponseStatusException(HttpStatus.NOT_FOUND, "User not found")
        return user.toResponse()
    }
}

Import Organization

•Standard library imports
•Third-party imports
•Project imports (blank line between each)

kotlin

import java.time.LocalDateTime
import kotlin.math.max

import org.springframework.web.bind.annotation.GetMapping
import org.springframework.http.ResponseEntity

import me.soenke.spring_demo.dadjoke.DadJokeResponse