How to Write a REST API in Kotlin in 30 Minutes: For Beginners

Introduction

Kotlin is a modern statically typed programming language that runs on the Java Virtual Machine (JVM) and is fully compatible with the Java ecosystem. Thanks to its conciseness, safety, and powerful functional capabilities, Kotlin has become one of the most popular languages for server-side application development, especially after Google chose it as the primary language for Android.

In this article, we will create a full-fledged REST API in Kotlin using the lightweight framework Ktor and the database library Exposed. We will implement a simple task management service (To-Do List) with support for all CRUD operations (create, read, update, delete). The entire process will take you no more than 30 minutes if you have already installed IntelliJ IDEA and JDK.

1. Project Setup

First, let's create a new Kotlin project with Gradle support. Open IntelliJ IDEA and select File → New → Project. In the project creation wizard, choose Kotlin → JVM.

Step 1.1: build.gradle.kts Structure

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

plugins {    kotlin("jvm") version "1.9.0"    id("io.ktor.plugin") version "2.3.0"    kotlin("plugin.serialization") version "1.9.0"}

dependencies {    implementation("io.ktor:ktor-server-core:2.3.0")    implementation("io.ktor:ktor-server-netty:2.3.0")    implementation("io.ktor:ktor-server-content-negotiation:2.3.0")    implementation("io.ktor:ktor-serialization-kotlinx-json:2.3.0")    implementation("org.jetbrains.exposed:exposed-core:0.41.1")    implementation("org.jetbrains.exposed:exposed-dao:0.41.1")    implementation("org.jetbrains.exposed:exposed-jdbc:0.41.1")    implementation("com.h2database:h2:2.1.214")    implementation("ch.qos.logback:logback-classic:1.4.7")}

These dependencies include: the Ktor framework itself, JSON serialization support, the Exposed library for working with the database, and the embedded H2 database for testing.

Step 1.2: Entry Point

Create a file Application.kt in the package com.example:

import io.ktor.server.engine.*import io.ktor.server.netty.*import com.example.plugins.*

fun main() {    embeddedServer(Netty, port = 8080) {        configureSerialization()        configureRouting()    }.start(wait = true)}

2. Configuring Serialization and Routing

Step 2.1: Application Plugins

Create a plugins directory and a file Serialization.kt:

package com.example.plugins

import io.ktor.serialization.kotlinx.json.*import io.ktor.server.application.*import io.ktor.server.plugins.contentnegotiation.*

fun Application.configureSerialization() {    install(ContentNegotiation) {        json()    }}

Step 2.2: Defining the Data Model

Create a file Models.kt:

package com.example.models

import kotlinx.serialization.Serializable

@Serializabledata class Task(    val id: Int = 0,    val title: String,    val description: String = "",    val completed: Boolean = false)

3. Working with the Database

Step 3.1: Configuring the Connection

Create a file DatabaseFactory.kt:

package com.example.database

import org.jetbrains.exposed.sql.Databaseimport org.jetbrains.exposed.sql.SchemaUtilsimport org.jetbrains.exposed.sql.transactions.transaction

object DatabaseFactory {    fun init() {        Database.connect("jdbc:h2:mem:test;DB_CLOSE_DELAY=-1", driver = "org.h2.Driver")        transaction {            SchemaUtils.create(TasksTable)        }    }}

Step 3.2: Defining the Table

In the same package, create TasksTable.kt:

package co