Spring Boot: Hello World, Kotlin

moelholm profile image Nicky Mølholm ・6 min read

In this post I show how you can create a Spring Boot 1.5 application using Kotlin 1.1 (as opposed to typically Java 8 in these times).

From moelholm.com: Spring Boot: Hello World, Kotlin

The example I have created is a typical "Hello World" example. I have chosen to implement a Spring MVC controller - and an awesome Spring Boot integration test. The Gradle build script uses Kotlin as well (that's pretty cool). You can find the example project in its entirety and real context on GitHub.

In this post I also explain some of the Kotlin specifics worth noticing to a typical (Spring) Java developer. If you are a Kotlin savvy developer and find that my explanations are wrong or misleading - then please leave a comment, so that I can fix them. I am by no means a Kotlin specialist (yet!).

Just want to see the code?

1 of 4: The Spring Boot application

import org.springframework.boot.SpringApplication
import org.springframework.boot.autoconfigure.SpringBootApplication

class Application

fun main(args: Array<String>) {
    SpringApplication.run(Application::class.java, *args)

Notice that we activate Spring Boot using the normal @SpringBootApplication annotation. The fun part here, if you will, is that in Kotlin classes doesn't even need to have a body. I guess many of them will have one - but here it is simply not necessary.

The main method in Kotlin is a package level function (it is not embedded inside a class).

We pass Application::class.java to the run method. That is not a typo - and it is not a Java source file reference :). It is still the Class object that represents class Application. If you just pass Application::class - then you would pass an object of type KClass - that is Kotlins own representation of what you know as the Class type from Java!

The weird *args is not a pointer - but rather Kotlins spread operator (the run method declares a vararg parameter).

( Also: Semicolons are not used - no biggie for me though )

2 of 4: The controller

import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RestController

class GreetingController {

 fun get(@PathVariable name: String) = "Hello, $name"


This is a typical Spring MVC controller. I bet you can recognise @RestController, @GetMapping and @PathVariable (if you are an experienced Spring Java developer).

Notice that the function is prefixed with fun here. It looks like a variable assignment, but it isn't. What you see there is an implicit return statement of the string Hello, $name. The function could have had a body and a return statement instead - but it isn't necessary in this case.

Notice that the the function doesn't have an explicit return type. You could have put : String just before the equals sign. But Kotlin can infer it - so not necessary in this case either.

The $name argument is replaced with the contents of the name parameter - a demonstration of Kotlins support for string interpolation. This, almost insignificant feature, would have a huge effect on many of the Java projects I have seen!

3 of 4: The integration test

import org.assertj.core.api.Assertions.assertThat
import org.junit.Test
import org.junit.runner.RunWith
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.context.SpringBootTest
import org.springframework.boot.test.web.client.TestRestTemplate
import org.springframework.test.context.junit4.SpringRunner

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class GreetingControllerIntegrationTests {

    lateinit var restTemplate: TestRestTemplate

    fun `GET when given Duke then returns "Hello, Duke"`() {

        // Given
        val name = "Duke"

        // When
        val body = restTemplate.getForObject("/hello/{name}", String::class.java, name)

        // Then
        assertThat(body).isEqualTo("Hello, $name")



Worth noticing here is the weird lateinit modifier [lateinit]. So this is the deal: Kotlin normally assume that the member is non-null and therefore it must be assigned explicitly in the constructor to a non-null value. But since the Spring TestContext Framework takes care of the injection after the constructor has run, then we need to allow it explicitly, using the lateinit modifier.

Another interesting element is the @Test function name: I actually started with the name get_whenInvokedWithDuke_thenReturnsHelloDuke. But I updated the example after a tip from one of the readers: Using backticks around function names allow us to provide sentence-like function names. Super nice for naming tests if you ask me.

In the @Test itself: notice the use of val. It's like Java's final modifier (in context of variables). Kotlin's type inference means that we aren't forced to write down the type (String).

So.... I think this seems super nice. The code is not overly verbose with type information. And at least for this Hello World case I think it is perfectly fine. I'm also pretty happy with the string interpolation again :) ... (in Java land I tend to use String.format("stuff...%s", varhere) - rather annoying).

4 of 4: The Gradle script

I have been using Gradle as a drop-in replacement for Maven for the last 1.5 year. For that period I've always used Groovy as the programming language in my Gradle scripts. And I kind of like that - but also, I must admit that the IDE assistance isn't super optimal (even in IntelliJ which I use now).

Then as I was browsing Kotlin features etc, I found out that the Gradle guys are working on supporting Kotlin as another programming language in scripts [gradlekotlin]. And not only that: they are working on making Kotlin the language of choice for developing Gradle plugins [gradlescriptkotlin] !

Okay - the code:

buildscript {

    val springBootVersion = "1.5.2.RELEASE"
    var kotlinVersion: String by extra
    kotlinVersion = "1.1.0"

    repositories {

    dependencies {


val kotlinVersion: String by extra

apply {

repositories {

dependencies {

Looks like the ordinary Groovy based script in my opinion. That's nice - so no big re-adjustment.

I am quite certain that you should use an up-to-date version of IntelliJ to get a decent IDE experience with Kotlin based build scripts. For me it seems okay (IntelliJ IDEA 2016.3.5) - but the content assist is extremely slow. I guess that will be nailed properly at some point. Also I still think I get a bunch of weird suggestions in my content assist in the different blocks.

Motivation: why Kotlin?

I love Java for it's simplicity - and I know it by heart.

But, to be honest I feel there are a bunch of nice features in most of the other popular languages today that Java simply doesn't have. Kotlin is one of these new JVM languages - and it has a truly remarkable feature list. Head over to the reference manual and browse through it [kotlinreference]; I promise that you will be clapping your hands as you read through the individual features.

Lately I've noticed how Pivotal embraces Kotlin - through examples, public demo sessions (fx by our favorite rockstar, Starbuxman [starbuxman]) and nonetheless by making the core Spring Framework seamless to use from Kotlin [springkotlin]. Being an avid Spring Boot fan that got my attention.

I haven't tried Kotlin on a real world project yet. But I definitely hope that I will get the chance very soon.


[kotlinreference] :Â Kotlin Reference

[starbuxman] :Â Spring Tips: The Kotlin Programming language

[springkotlin] :Â Introducing Kotlin support in Spring Framework 5.0

[lateinit]: The Kotlin lateinit modifier:

[gradlekotlin]: Kotlin Meets Gradle

[gradlescriptkotlin]: Gradle Script Kotlin - FAQ


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