Spring Boot backend for KMP apps

Built a Spring Boot backend to support a KMP mobile app. The integration is smoother than I expected.

Since both use Kotlin, sharing data models is possible with kotlinx.serialization. You can define models in your KMP common module and use them in Spring Boot:

// KMP commonMain
@Serializable
data class User(
    val id: String,
    val name: String,
    val email: String
)

// Spring Boot - same model
@Serializable
data class User(
    val id: String,
    val name: String,
    val email: String
)

But here's what got me - Spring Boot uses Jackson by default, not kotlinx.serialization. You need to configure it:

@Configuration
class SerializationConfig {
    @Bean
    fun kotlinSerializationJson(): KotlinSerializationJson {
        return KotlinSerializationJson(Json {
            ignoreUnknownKeys = true
        })
    }
}

Also, if you're using Ktor client in KMP, it works great with Spring Boot REST endpoints. The serialization format matches, so no conversion needed.

One thing I learned - Spring Boot's coroutine support works well with KMP's coroutines. You can use suspend functions in controllers:

@RestController
class UserController {
    @GetMapping("/users/{id}")
    suspend fun getUser(@PathVariable id: String): User {
        return userService.getUser(id)
    }
}

The main benefit is type safety across the stack. If you change a model in KMP, you'll catch issues at compile time in Spring Boot too, assuming you're sharing the code somehow.

Thanks for reading! Check out more posts on the blog if you'd like.