Kotlin in Open Source: Top 10 GitHub Repositories for Pros

Kotlin is not just Java without getters

When JetBrains released Kotlin in 2016, many dismissed it as just another "Java killer." Eight years later, it is now the primary language for Android development, an active player in backend (thanks to Ktor and Spring Boot), and even in Data Science. But Kotlin's main strength is its community. Open Source projects in this language are not just a set of libraries, but real textbooks on modern architecture.

In this article, we have gathered the best open source GitHub repositories in Kotlin that every developer should study. You will see real code, understand how popular libraries are structured, and perhaps find inspiration for your own project. Let's go!

Why study Open Source in Kotlin?

Before diving into the list, let's answer the question: why look at someone else's code at all? Firstly, it is the best way to understand idiomatic Kotlin — not just the syntax, but the philosophy of the language. Secondly, open source repositories often contain examples of complex architectural solutions: Coroutines for asynchrony, Flow for reactive streams, Sealed classes for state modeling.

We have selected projects of various levels — from small utilities to full-fledged frameworks. Each of them can be forked, studied, and used as a template for your own tasks.

Best Open Source Repositories in Kotlin

1. Ktor — Asynchronous HTTP framework

Repository: github.com/ktorio/ktor

If you are writing a backend in Kotlin, Ktor is a must-have. Unlike Spring, it is lightweight, modular, and fully utilizes coroutines. See how simple a server looks:

import io.ktor.server.engine.*import io.ktor.server.netty.*import io.ktor.server.routing.*import io.ktor.server.response.*

fun main() {    embeddedServer(Netty, port = 8080) {        routing {            get("/") {                call.respondText("Hello, Open Source!")            }        }    }.start(wait = true)}

This code is a minimal working server. In the repository, you will find examples with WebSockets, serialization via kotlinx.serialization, and database integration. Study the module structure — it shows how to properly organize Kotlin projects.

2. Kotlin Coroutines — The foundation of asynchrony

Repository: github.com/Kotlin/kotlinx.coroutines

The official repository from JetBrains. Here you will see how launch, async, Flow, and channels work. Pay attention to the tests — they are an excellent example of how to test asynchronous code:

import kotlinx.coroutines.*import kotlinx.coroutines.test.*import org.junit.Test

class CoroutineTest {    @Test    fun testAsyncOperation() = runTest {        val result = async {            delay(1000)            "Success"        }        assertEquals("Success", result.await())    }}

By studying this code, you will understand how runTest works and why you shouldn't be afraid to test coroutines. This is the foundation for any modern Kotlin application.

3. Koin — Dependency Injection without magic

Repository: github.com/InsertKoinIO/koin

Koin is a lightweight DI framework that does not use reflection. Everything is built on Kotlin DSL constructs. Example module:

val myModule = module {    single { Database() }    factory { Repository(get()) }}

class MyApp : Application() {    override fun onCreate() {        super.onCre