kotlin-spring-arch

name	kotlin-spring-arch
description	Architecture reference for this project — Controller/Service/Repository layer responsibilities, @Transactional strategy (readOnly optimization, N+1 prevention), ExpectedException usage (no subclasses), and Entity↔DTO conversion patterns.

Kotlin + Spring Boot Architecture Guide

Layer Structure

Controller

• Role: Request validation, DTO conversion, HTTP response
• Annotations: @RestController, @RequestMapping
• Validation: @Valid, @Validated
• Response: Return DTO directly — the framework auto-wraps it with CommonApiResponse

// CORRECT — return DTO directly
fun getStudent(): StudentResDto = queryStudentService.execute(id)

// WRONG — do not wrap manually
fun getStudent(): CommonApiResponse<StudentResDto> = CommonApiResponse(queryStudentService.execute(id))

Service

• Role: Business logic, transaction management
• Pattern: interface + implementation
• Transaction:
  • Read: @Transactional(readOnly = true)
  • Write: @Transactional
• Dependencies: Inject Repository via constructor injection

Repository

• Role: Data access
• JPA: Extend JpaRepository
• QueryDSL: Complex queries
• Avoid N+1: Fetch Join, @EntityGraph

Transaction Strategy

Read-only Optimization

@Transactional(readOnly = true)
fun findApiKeys(): List<ApiKeyResDto> {
    return repository.findAll()
        .map { it.toResDto() }
}

N+1 Problem Resolution

// ❌ N+1 occurs
repository.findAll() // 1 query
entity.relatedEntity // N queries

// ✅ Fetch Join
@Query("SELECT e FROM Entity e JOIN FETCH e.relatedEntity")
fun findAllWithRelated(): List<Entity>

Exception Handling

Use ExpectedException Directly

val student = studentRepository.findById(id).orElseThrow {
    ExpectedException("학생을 찾을 수 없습니다.", HttpStatus.NOT_FOUND)
}

Do not create custom exception subclasses extending ExpectedException.

Global Handler

Locate with: find . -name "GlobalExceptionHandler.kt" ! -path "*/build/*"

DTO Conversion Pattern

// Entity → ResDto
fun Entity.toResDto() = EntityResDto(
    id = this.id,
    name = this.name
)

// ReqDto → Entity
fun EntityReqDto.toEntity() = Entity(
    name = this.name
)