Part 7 · 1 chapters · ~8 min

Kotlin on Spring

Spring Boot with Kotlin: the kotlin-spring and kotlin-jpa compiler plugins, constructor injection, data classes and JPA entities (and why entities should not be data classes), coroutines in WebFlux controllers, Spring's Kotlin DSLs, configuration properties, testing with SpringBootTest and MockK, and when Spring beats Ktor.

A Spring Boot service in Kotlin

code

// build.gradle.kts
plugins { kotlin("plugin.spring") version "2.4.20"; kotlin("plugin.jpa") version "2.4.20" }

@RestController
@RequestMapping("/v1/transfers")
class TransferController(private val transfers: TransferService) {          // constructor injection
    @PostMapping
    suspend fun create(@RequestHeader("Idempotency-Key") key: String, @RequestBody req: CreateTransferRequest): ResponseEntity<Any> =
        when (val r = transfers.create(req, key)) {
            is TransferResult.Ok -> ResponseEntity.status(201).body(r.transfer.toDto())
            is TransferResult.InsufficientFunds -> ResponseEntity.unprocessableEntity().body(mapOf("error" to "insufficient_funds"))
            TransferResult.LimitExceeded -> ResponseEntity.unprocessableEntity().body(mapOf("error" to "limit_exceeded"))
        }
}

// JPA entities: regular classes, not data classes (equals/hashCode on mutable, lazily loaded state breaks Hibernate)
@Entity class TransferEntity(@Id val id: String, var state: String, val amountKobo: Long)

KOTLIN ON SPRING BOOT

the Java ecosystem with Kotlin ergonomics

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official support

Spring supports Kotlin officially: null-safety annotations in Spring's API, coroutine support in WebFlux, and Kotlin DSLs for routing and beans.

a first-class pairingcoroutines in WebFlux