Kotlin - Annotations

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.

Get the code

info

The source code is available at on Github

Setup

Let's configure the KSP plugin like below in Gradle:

plugins {
    // Apply the org.jetbrains.kotlin.jvm Plugin to add support for Kotlin.
    id 'org.jetbrains.kotlin.jvm' version "$kotlin_version"
    // Apply the application plugin to add support for building a CLI application in Java.
    id "com.google.devtools.ksp" version "$ksp_version"
    id 'application'
}

// KSP - To use generated sources
sourceSets.main {
    java.srcDirs("build/generated/ksp/main/kotlin")
}

Let's setup the dependencies like below:

dependencies {

    // Koin
    implementation "io.insert-koin:koin-core:$koin_version"
    implementation "io.insert-koin:koin-annotations:$koin_ksp_version"
    ksp "io.insert-koin:koin-ksp-compiler:$koin_ksp_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)

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 findUser(name : String): User?
    fun addUsers(users : List<User>)
}

class UserRepositoryImpl : UserRepository {

    private val _users = arrayListOf<User>()

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

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

The Koin module

Let's declare a AppModule module class like below.

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

• We use the @Module to declare our class as Koin module
• The @ComponentScan("org.koin.sample") allow to scann any Koin definition in "org.koin.sample"package

Let's simply add @Single on UserRepositoryImpl class to declare it as singleton:

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

The UserService Component

Let's write the UserService component to request the default user:

class UserService(private val userRepository: UserRepository) {

    fun getDefaultUser() : User = userRepository.findUser(DefaultData.DEFAULT_USER.name) ?: error("Can't find default user")
}

UserRepository is referenced in UserPresenter`s constructor

Let's simply add @Single on UserService class to declare it as singleton:

@Single
class UserService(private val userRepository: UserRepository) {
    // ...
}

Injecting Dependencies in UserApplication

The UserApplication class will help bootstrap instances out of Koin. It will resolve the UserService, thanks to KoinComponent interface. This allows to inject it with the by inject() delegate function:

class UserApplication : KoinComponent {

    private val userService : UserService by inject()

    // display our data
    fun sayHello(){
        val user = userService.getDefaultUser()
        val message = "Hello '$user'!"
        println(message)
    }
}

That's it, your app is ready.

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The by inject() function allows us to retrieve Koin instances, in any class that extends KoinComponent

Start Koin

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

// generated
import org.koin.ksp.generated.*

fun main() {
    startKoin {
        modules(AppModule().module)
    }

    UserApplication().sayHello()
}

The Koin module is generated from AppModule with the .module extension: Just use the AppModule().module expression to get the Koin module from the annotations.

info

The import org.koin.ksp.generated.* import is required to allow to use generated Koin module content