baks-spring-boot-best-practices

Spring Boot Best Practices (Kotlin)

Injection

Use constructor injection, not field injection.

// Bad
@Service
class PaymentService {
    @Autowired
    lateinit var repository: PaymentRepository
}

// Good
@Service
class PaymentService(
    private val repository: PaymentRepository,
)

Proxy Annotations

Proxy annotations (@Transactional, @Cacheable, @CacheEvict, @CachePut, @Async) must not be placed on private methods — the Spring proxy cannot intercept them.

// Bad
@Service
class InvoiceService {
    fun createInvoice() { persistInvoice() }

    @Transactional
    private fun persistInvoice() { /* proxy never intercepts this */ }
}

// Good
@Service
class InvoiceService {
    @Transactional
    fun createInvoice() { persistInvoice() }

    private fun persistInvoice() { /* transaction opened on the public method */ }
}

Avoid self-invocation between proxy-annotated methods. Calls via this bypass the proxy — inject a separate bean instead.

// Bad
@Service
class ReportService {
    @Transactional
    fun generate(id: Long) { archive(id) } // @Async never fires

    @Async
    fun archive(id: Long) { }
}

// Good
@Service
class ReportService(private val archiveService: ArchiveService) {
    @Transactional
    fun generate(id: Long) { archiveService.archive(id) }
}

@Service
class ArchiveService {
    @Async
    fun archive(id: Long) { }
}

Configuration

@Configuration classes must not hold mutable state (exceptions: @Value and @ConfigurationProperties fields).

// Bad
@Configuration
class MailConfiguration {
    var retries = 0
    @Bean fun mailClient(): MailClient = MailClient()
}

// Good
@Configuration
class MailConfiguration {
    @Bean
    fun mailClient(properties: MailProperties): MailClient =
        MailClient(properties.host, properties.port)
}

@Bean methods must live in @Configuration classes, not in @Component.

Exceptions

Custom exceptions must extend RuntimeException, not Exception.

// Bad
class PaymentFailedException(message: String) : Exception(message)

// Good
class PaymentFailedException(message: String) : RuntimeException(message)

JPA

Kotlin data class must not be used as JPA entities — data class is final and prevents lazy-loading proxies.

// Bad
@Entity
data class UserEntity(@Id val id: Long, val email: String)

// Good
@Entity
class UserEntity(
    @Id var id: Long? = null,
    var email: String,
)

All @Entity classes must declare an @Id field.

@Transactional belongs in the service layer, not in controllers.

Domain and entity packages must not import org.springframework.* — keep the domain model independent of Spring.

Naming

Annotation	Suffix
@Service	Service
@Repository	Repository
@Controller / @RestController	Controller
@ControllerAdvice / @RestControllerAdvice	ExceptionHandler or Advice
@ConfigurationProperties	Properties

REST API

Use specific HTTP method annotations — avoid @RequestMapping where @GetMapping, @PostMapping, etc. apply.

No trailing slash on mapping paths: /users, not /users/.

GET endpoints must return something — not Unit/void. If it is a command, use POST.

Do not expose JPA entities from REST — use dedicated DTOs.

Controllers must not inject repositories directly — go through a service.

// Bad
@RestController
class UserController(private val userRepository: UserRepository)

// Good
@RestController
class UserController(private val userService: UserService)