Kotlin & Annotations

This tutorial lets you write a Kotlin application and use Koin dependency injection with annotations to retrieve your components. You need around 10 min to do the tutorial.

note

update - 2024-11-12

Get the code

info

The source code is available at on Github

Setup

First, check that the Koin annotations dependencies are added like below:

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

dependencies {
    // Koin for Kotlin apps
    implementation("io.insert-koin:koin-core:$koin_version")

    // Koin Annotations
    implementation("io.insert-koin:koin-annotations:$koin_annotations_version")
    ksp("io.insert-koin:koin-ksp-compiler:$koin_annotations_version")
}

Application Overview

The idea of the application is to manage a list of users, and display it in our UserApplication class:

Users -> UserRepository -> UserService -> UserApplication

The "User" Data

We will manage a collection of Users. Here is the data class:

data class User(val name: String, val email: String)

We create a "Repository" component to manage the list of users (add users or find one by name). Here below, the UserRepository interface and its implementation:

interface UserRepository {
    fun findUserOrNull(name: String): User?
    fun addUsers(users: List<User>)
}

@Singleton
class UserRepositoryImpl : UserRepository {

    private val _users = arrayListOf<User>()

    override fun findUserOrNull(name: String): User? {
        return _users.firstOrNull { it.name == name }
    }

    override fun addUsers(users: List<User>) {
        _users.addAll(users)
    }
}

note

This project uses Jakarta annotations (@Singleton from jakarta.inject) instead of Koin's @Single. Both work with Koin, but Jakarta annotations provide better interoperability with other frameworks.

The Koin module

Use the @Module annotation to declare a Koin module:

@Module
@ComponentScan("org.koin.sample")
@Configuration
class AppModule

• @Module - Declares this as a Koin module
• @ComponentScan("org.koin.sample") - Scans and registers annotated classes from the package
• @Configuration - Enables automatic module discovery with @KoinApplication

Let's declare our components by adding the @Singleton annotation:

@Singleton
class UserRepositoryImpl : UserRepository {
    // ...
}

The UserService Component

Let's write the UserService component to manage user operations:

interface UserService {
    fun getUserOrNull(name: String): User?
    fun loadUsers()
    fun prepareHelloMessage(user: User?): String
}

@Singleton
class UserServiceImpl(
    private val userRepository: UserRepository
) : UserService {

    override fun getUserOrNull(name: String): User? = userRepository.findUserOrNull(name)

    override fun loadUsers() {
        userRepository.addUsers(listOf(
            User("Alice", "alice@example.com"),
            User("Bob", "bob@example.com"),
            User("Charlie", "charlie@example.com")
        ))
    }

    override fun prepareHelloMessage(user: User?): String {
        return user?.let { "Hello '${user.name}' (${user.email})! 👋" } ?: "❌ User not found"
    }
}

UserRepository is referenced in UserServiceImpl's constructor

We declare UserService with the @Singleton annotation.

The UserApplication

The UserApplication class uses constructor injection to receive the UserService:

@Singleton
class UserApplication(
    private val userService: UserService
) {

    init {
        userService.loadUsers()
    }

    fun sayHello(name: String) {
        val user = userService.getUserOrNull(name)
        val message = userService.prepareHelloMessage(user)
        println(message)
    }
}

info

Constructor injection is the preferred way to inject dependencies. Koin will automatically resolve and inject the UserService when creating UserApplication.

Koin Application Object

Create a @KoinApplication object to mark the entry point for Koin's annotation-based configuration:

@KoinApplication
object KoinUserApplication

The @KoinApplication annotation works with the KSP processor to generate a startKoin() extension function for this object.

Start Koin

We need to start Koin with our application. Just call the generated startKoin() function in the application's main entry point:

fun main() {
    KoinUserApplication.startKoin()

    val userApplication = KoinPlatform.getKoin().get<UserApplication>()
    userApplication.sayHello("Alice")
}

Key Points:

• KoinUserApplication.startKoin() - Generated function that automatically discovers and loads all modules
• No need to manually call modules() - all annotated dependencies are discovered at compile time!
• We retrieve the UserApplication instance from Koin using KoinPlatform.getKoin().get<UserApplication>()

info

The @KoinApplication annotation with @Configuration on the module automatically discovers and loads all annotated dependencies at compile time via KSP.

Koin module: classic or autowire DSL?

Here is how our annotations-based configuration compares to the classic DSL:

With Annotations:

@Module
@ComponentScan("org.koin.sample")
@Configuration
class AppModule

@Singleton
class UserApplication(private val userService: UserService)

@Singleton
class UserRepositoryImpl : UserRepository

@Singleton
class UserServiceImpl(private val userRepository: UserRepository) : UserService

Classic DSL (from kotlin.md):

val appModule = module {
    singleOf(::UserApplication)
    singleOf(::UserRepositoryImpl) { bind<UserRepository>() }
    singleOf(::UserServiceImpl) { bind<UserService>() }
}

Both approaches achieve the same result, but annotations provide compile-time verification and automatic module discovery.