Ktor - Koin — ContextQMD

Ktor is a framework for building asynchronous servers and clients in connected systems using the powerful Kotlin programming language. We will use Ktor here, to build a simple web application.

Let's go 🚀

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

:::tip Looking for the annotations version of this tutorial? Check out Ktor & Annotations which uses Koin Annotations with Jakarta @Singleton for compile-time verification. :::

Get the code

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

Gradle Setup

First, add the Koin dependency like below:

kotlin

dependencies {
    // Koin for Kotlin apps
    implementation("io.insert-koin:koin-ktor:$koin_version")
    implementation("io.insert-koin:koin-logger-slf4j:$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:

kotlin

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:

kotlin

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.

kotlin

val appModule = module {

}

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

kotlin

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:

kotlin

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:

kotlin

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

HTTP Controller

Finally, we need an HTTP Controller to create the HTTP Route. In Ktor it will be expressed through a Ktor extension function:

kotlin

fun Application.main() {

    // Lazy inject UserService
    val service by inject<UserService>()
    service.loadUsers()

    // Routing section
    routing {
        get("/hello") {
            val userName = call.queryParameters["name"] ?: "Alice"
            val user = service.getUserOrNull(userName)
            val message = service.prepareHelloMessage(user)
            call.respondText(message)
        }
    }
}

The /hello endpoint accepts an optional name query parameter. If not provided, it defaults to "Alice".

Example requests:

http://localhost:8080/hello - Greets Alice (default)
http://localhost:8080/hello?name=Bob - Greets Bob

Declare your dependencies

Let's assemble our components with a Koin module:

kotlin

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

Start and Inject

Finally, let's start Koin from Ktor:

kotlin

fun Application.main() {
    // Install Koin
    install(Koin) {
        modules(appModule)
    }

    // Lazy inject UserService
    val service by inject<UserService>()
    service.loadUsers()

    // Routing section
    routing {
        get("/hello") {
            val userName = call.queryParameters["name"] ?: "Alice"
            val user = service.getUserOrNull(userName)
            val message = service.prepareHelloMessage(user)
            call.respondText(message)
        }
    }
}

Let's start Ktor:

kotlin

fun main(args: Array<String>) {
    embeddedServer(Netty, port = 8080) {
        main()
    }.start(wait = true)
}

That's it! You're ready to go. Check these URLs:

http://localhost:8080/hello - Greets Alice (default user)
http://localhost:8080/hello?name=Bob - Greets Bob