Kotlin for Web Development: Building a RESTful API with Spring Boot

Are you tired of the limitations of traditional web development languages? Do you want to learn a more efficient and modern language that can simplify your development efforts? Look no further than Kotlin! With its concise syntax, strong type checking, and interoperability with Java, Kotlin is quickly becoming the go-to language for web development. In this article, we will explore Kotlin's capabilities for building a RESTful API using Spring Boot.

What is Kotlin?

Kotlin is a statically typed programming language that runs on the Java Virtual Machine (JVM). It was developed by JetBrains, the same company that created the popular IntelliJ IDEA Integrated Development Environment (IDE). Kotlin is designed to improve upon the shortcomings of Java by providing a more concise and efficient syntax, better null safety, and more advanced language features such as coroutines.

Why use Kotlin for web development?

Kotlin offers several advantages over traditional web development languages such as Java and JavaScript. First, Kotlin's concise syntax makes it easier to read and write code, which can save time and reduce errors during development. Second, Kotlin is designed to work seamlessly with Java, allowing developers to leverage the vast library of Java frameworks and tools while still taking advantage of Kotlin's modern language features. Finally, Kotlin's approach to null safety can prevent common runtime errors that can occur in other languages, saving developers time and frustration during the debugging process.

Building a RESTful API with Spring Boot

Spring Boot is a popular framework for building web applications in Java. It provides a streamlined approach to boilerplate code and dependency management, allowing developers to focus on writing business logic rather than infrastructure. Kotlin's interoperability with Java makes it a natural fit for Spring Boot development. In this section, we will walk through the steps required to build a RESTful API using Kotlin and Spring Boot.

Setting up a Kotlin Spring Boot project

To get started, we need to set up a Kotlin Spring Boot project. We will be using Gradle as our build system, but Maven can be used as well. Open your favorite IDE and create a new Gradle project. Add the following dependencies to your build.gradle file:

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk8'
    implementation 'org.jetbrains.kotlin:kotlin-reflect'

The first dependency is the Spring Boot starter for web applications. The second and third dependencies are required for Kotlin support. Next, create a new Kotlin class named "Application" and annotate it with the "@SpringBootApplication" annotation:

class Application

fun main(args: Array<String>) {

This class serves as the entry point for our application. The "runApplication" function is provided by the Spring Boot framework and is used to start our application.

Defining a data model

Next, we need to define a data model that will represent the resources in our RESTful API. For this example, we will be building a simple todo list application. Create a new Kotlin class named "Todo" with the following properties:

data class Todo(val id: Long, val name: String, val completed: Boolean)

This class represents a todo item with an ID, name, and completed status. We will be using this class as the basis for our RESTful API.

Creating a RESTful controller

Now it's time to create a RESTful controller that will expose our data model over HTTP. Create a new Kotlin class named "TodoController" with the following code:

class TodoController {

    private val todos = mutableListOf(
        Todo(1, "Go grocery shopping", false),
        Todo(2, "Clean the house", false),
        Todo(3, "Do laundry", true)

    fun getTodos(): List<Todo> {
        return todos

    fun addTodo(@RequestBody todo: Todo): Todo {
        return todo

    fun updateTodo(@PathVariable id: Long, @RequestBody todo: Todo): Todo {
        val existingTodo = todos.find { it.id == id }
        existingTodo?.let {
            it.name = todo.name
            it.completed = todo.completed
        return existingTodo ?: throw ResponseStatusException(HttpStatus.NOT_FOUND)

    fun deleteTodo(@PathVariable id: Long) {
        todos.removeIf { it.id == id }

This controller defines the routes for our RESTful API. The "@RestController" annotation tells Spring Boot that this class should be treated as a RESTful controller. The "@RequestMapping" annotation tells Spring Boot that this route should be prefixed with "/api/todos". The controller has four routes, each mapped to a different HTTP method:

Testing the RESTful API

Now that we have our RESTful API defined, it's time to test it out. Start the Spring Boot application and navigate to "http://localhost:8080/api/todos" in your web browser. You should see a JSON array containing three todo items. Click on the "Download" button in your browser to save the JSON as a file. You can use this file to test the POST, PUT, and DELETE routes using a tool such as Postman.


Kotlin is a versatile and efficient programming language that is well-suited for web development using Spring Boot. By leveraging Kotlin's modern language features and Spring Boot's streamlined approach to web development, developers can rapidly create RESTful APIs with robust functionality. Whether you are building a simple todo list application or a complex enterprise solution, Kotlin and Spring Boot are an excellent choice for modern web development.

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