Dependency Injection

Don’t waste time and energy manually wiring your classes together. Let Koin create and wire everything instead!

Koin is a lightweight, Kotlin-native dependency injection library integrated into the Kairo ecosystem.

Why Koin?

• Kotlin-first with compile-time generation:

  • No proxies, simple Kotlin DSL.
  • Dynamic modules (if you use them) are generated at compile time. No reflection!

• Kairo-integrated:

  • Uses Kairo lifecycle hooks.
  • Your custom Features can easily provide bindings.

• Easy testing: Koin + JUnit parameter resolvers let you add parameters to your integration test functions.

Installation

Install kairo-dependency-injection-feature. You don’t need to install Koin separately — it’s included by default.

If you want to use Koin annotations (recommended), also add the relevant KSP lines from the snippet below.

build.gradle.kts

plugins {
  id("com.google.devtools.ksp")
}

dependencies {
  ksp("io.insert-koin:koin-ksp-compiler")
  implementation("software.airborne.kairo:kairo-dependency-injection-feature")
}

Usage

First, add the Feature to your Server.

val koinApplication = koinApplication()

val features = listOf(
  DependencyInjectionFeature(koinApplication),
)

Refer to Koin’s documentation for advanced usage.

Using annotations (recommended)

Bindings are created by annotating classes with @Single or @Factory.

@Single
class UserStore(
  private val database: R2dbcDatabase, // Injected automatically.
) {
  suspend fun get(id: UserId): UserModel? =
    suspendTransaction(db = database) {
      UserTable
        .selectAll()
        .where { UserTable.id eq id }
        .map(UserModel::fromRow)
        .singleNullOrThrow()
    }
}

@Single
class UserService(
  private val userStore: UserStore, // Injected automatically.
) {
  suspend fun get(id: UserId): UserModel? =
    userStore.get(id)
}

Constructor parameters are automatically injected.

Koin’s KSP-based annotation processor will pick up on these bindings, you just need to register them with your Feature.

@org.koin.core.annotation.Module
@org.koin.core.annotation.ComponentScan
class UserFeature : Feature(), HasKoinModules {
  override val name: String = "User"

  // Generated by KSP.
  override val koinModules: List<Module> = listOf(module)
}

Wiring it manually instead

Instead of using annotations and KSP codegen, you can wire it up manually if you prefer.

class UserFeature(
  private val koin: Koin,
) : Feature(), HasKoinModules {
  override val name: String = "User"

  override val koinModules: List<Module> =
    listOf(
      module {
        single<UserStore> { UserStore(...) }
        single<UserService> { UserService(...) }
      }
    )
}