Kotlin

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

:::note update - 2024-10-21 :::

:::tip Looking for the annotations version of this tutorial? Check out Kotlin & Annotations which uses Koin Annotations for compile-time verification and automatic module discovery. :::

Get the code

:::info The source code is available at on Github :::

Setup

First, check that the koin-core dependency is added like below:

dependencies {
    
    // Koin for Kotlin apps
    implementation "io.insert-koin:koin-core:$koin_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>)
}

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)
    }
}

The Koin module

Use the module function to declare a Koin module. A Koin module is the place where we define all our components to be injected.

val appModule = module {

}

Let's declare our first component. We want a singleton of UserRepository, by creating an instance of UserRepositoryImpl

val appModule = module {
    single<UserRepositoryImpl>() bind UserRepository::class
}

:::info This tutorial uses the Koin Compiler Plugin DSL (single<T>()) which provides auto-wiring at compile time. See Compiler Plugin Setup for configuration. :::

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
}

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

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

    override fun loadUsers() {
        userRepository.addUsers(listOf(
            User("Alice", "[email protected]"),
            User("Bob", "[email protected]"),
            User("Charlie", "[email protected]")
        ))
    }

    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 in our Koin module. We declare it as a single definition:

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

Injecting Dependencies in UserApplication

The UserApplication class will help bootstrap instances out of Koin. It will resolve the UserService through constructor injection:

class UserApplication(
    private val userService: UserService
) {

    init {
        userService.loadUsers()
    }

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

That's it, your app is ready.

:::info Constructor injection is the preferred way to inject dependencies in Kotlin applications. Koin will automatically resolve and inject the UserService when creating UserApplication. :::

Start Koin

We need to start Koin with our application and add UserApplication to our module. Just call the startKoin() function in the application's main entry point, our main function:

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

fun main() {
    startKoin {
        modules(appModule)
    }

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

:::info The modules() function in startKoin loads the given list of modules. We retrieve the UserApplication instance from Koin using KoinPlatform.getKoin().get<UserApplication>(). :::

Koin module: DSL comparison

Here is the Koin module declaration using Classic DSL (manual wiring):

val appModule = module {
    single { UserApplication(get()) }
    single<UserRepository> { UserRepositoryImpl() }
    single<UserService> { UserServiceImpl(get()) }
}

With Compiler Plugin DSL (auto-wiring at compile time):

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

:::tip The Compiler Plugin DSL requires the Koin Compiler Plugin. It provides compile-time dependency resolution and cleaner syntax. :::