Spring Boot REST API with Kotlin

Project Setup

start.spring.io with Kotlin, Web, JPA, Validation, H2. Kotlin plugin plus kotlin("plugin.spring") (opens classes for proxies) and kotlin("plugin.jpa") (no-arg constructors) are preconfigured.

Entity and Repository

@Entity
class Book(
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id: Long = 0,
    @field:NotBlank var title: String = "",
    var price: Int = 0
)
interface BookRepository : JpaRepository<Book, Long> {
    fun findByTitleContainingIgnoreCase(q: String): List<Book>
}

Controller

@RestController
@RequestMapping("/api/books")
class BookController(private val repo: BookRepository) {
    @GetMapping fun all() = repo.findAll()
    @PostMapping @ResponseStatus(HttpStatus.CREATED)
    fun create(@Valid @RequestBody b: Book) = repo.save(b)
}

Kotlin-Specific Gotchas

• Constructor injection with val is the default style.
• Validation annotations target fields: @field:NotBlank.
• Data classes as JPA entities need the no-arg plugin (already added by Initializr).
• Null safety plus validation equals APIs that reject bad input twice.

Key Points

• Initializr handles the Kotlin plugin wiring.
• Controllers read like pseudocode with constructor injection.
• Same testing stack: MockMvc plus Mockk or Mockito.