Kotlin Spring Skill

Idiomatic Spring Boot development with Kotlin.

Topics Covered

REST Controllers

@RestController
@RequestMapping("/api/v1/users")
class UserController(private val userService: UserService) {

    @GetMapping
    suspend fun findAll(): List<UserDTO> = userService.findAll()

    @GetMapping("/{id}")
    suspend fun findById(@PathVariable id: Long): UserDTO =
        userService.findById(id) ?: throw ResponseStatusException(NOT_FOUND)

    @PostMapping
    @ResponseStatus(CREATED)
    suspend fun create(@Valid @RequestBody request: CreateUserRequest) =
        userService.create(request)
}

Coroutines Integration

@Service
class UserService(private val repository: UserRepository) {
    suspend fun findAll() = withContext(Dispatchers.IO) {
        repository.findAll().map { it.toDTO() }
    }
}

Data JPA

@Entity
@Table(name = "users")
class User(
    @Id @GeneratedValue val id: Long = 0,
    @Column(nullable = false) val email: String,
    @Column(nullable = false) val name: String
)

interface UserRepository : JpaRepository<User, Long> {
    fun findByEmail(email: String): User?
}

Testing

@WebMvcTest(UserController::class)
class UserControllerTest {
    @Autowired lateinit var mockMvc: MockMvc
    @MockkBean lateinit var userService: UserService

    @Test
    fun `GET users returns list`() {
        every { userService.findAll() } returns listOf(user)
        mockMvc.get("/api/v1/users").andExpect { status { isOk() } }
    }
}

Troubleshooting

Issue	Resolution
@RequestBody null	Add jackson-module-kotlin
Coroutines not working	Enable WebFlux or use blocking

Usage

Skill("kotlin-spring")