How to Write a REST API in Kotlin in 30 Minutes

Introduction

Kotlin is a modern programming language that is rapidly gaining popularity in backend development. Thanks to its concise syntax, full compatibility with Java, and powerful tools such as the Ktor framework, writing a REST API in Kotlin takes literally 30 minutes. In this article, we will create a full-featured REST API for managing a task list (Todo API). You will learn how to set up a project, create endpoints, connect a database, and test the API. No unnecessary theory — just practice.

Step 1: Project setup and dependencies

For a quick start, we use IntelliJ IDEA (Community Edition) or any other editor. Let's create a new Gradle project with Kotlin DSL.

Creating the project structure

Add the following dependencies to the build.gradle.kts file:

plugins {    kotlin("jvm") version "1.9.22"    application}

repositories {    mavenCentral()}

dependencies {    implementation("io.ktor:ktor-server-core:2.3.7")    implementation("io.ktor:ktor-server-netty:2.3.7")    implementation("io.ktor:ktor-server-content-negotiation:2.3.7")    implementation("io.ktor:ktor-serialization-kotlinx-json:2.3.7")    implementation("org.jetbrains.exposed:exposed-core:0.44.1")    implementation("org.jetbrains.exposed:exposed-dao:0.44.1")    implementation("org.jetbrains.exposed:exposed-jdbc:0.44.1")    implementation("com.h2database:h2:2.2.224")    implementation("ch.qos.logback:logback-classic:1.4.14")}

application {    mainClass.set("ApplicationKt")}

We are using:

• Ktor — a lightweight asynchronous framework for HTTP servers.
• Exposed — an ORM for working with databases.
• H2 — an embedded database for development.
• Kotlinx.serialization — for working with JSON.

Step 2: Creating the data model and connecting the database

Let's define the structure of the tasks table. Create the file Database.kt:

import org.jetbrains.exposed.sql.Tableimport org.jetbrains.exposed.sql.insertimport org.jetbrains.exposed.sql.selectAllimport org.jetbrains.exposed.sql.transactions.transaction

object Tasks : Table() {    val id = integer("id").autoIncrement()    val title = varchar("title", 255)    val completed = bool("completed").default(false)

    override val primaryKey = PrimaryKey(id)}

data class Task(val id: Int, val title: String, val completed: Boolean)

fun initDatabase() {    Database.connect("jdbc:h2:mem:test", driver = "org.h2.Driver")    transaction {        SchemaUtils.create(Tasks)    }}

This code creates the Tasks table with three fields: id, title, and completed. The initDatabase() function initializes the in-memory database.

Step 3: Writing REST API controllers

Now let's create the main application file Application.kt. Here we will define all the endpoints.

Basic server setup

import io.ktor.server.application.*import io.ktor.server.engine.*import io.ktor.server.netty.*import io.ktor.server.routing.*import io.ktor.server.response.*import io.ktor.server.request.*import io.ktor.serialization.kotlinx.json.*import io.ktor.server.plugins.contentnegotiation.*

fun main() {    embeddedServer(Netty, port = 8080) {        install(ContentNegotiation) {            json()        }        initDatabase()        configureRouting()    }.start(wait = true)}

Implementing CRUD endpoints

fun Application.configureRouting() {    routing {        // Get all tasks        get("/tasks") {            val tasks = transaction {                Tasks.selectAll().map { row ->                    Task(                        id = row[Tasks.id],