Spring Boot: A bit more cool with Kotlin

In the context of my favorite framework, Spring Boot, I have recently started to explore the effect of using the Kotlin 1.1 programming language (as an alternative to Java 8). This post describes a few language features that may be interesting to a typical Java 8 developer. Also, I hope you will see that:

Spring Boot is a bit more cool with Kotlin


I have created an example application on GitHub. That application contains all the code that you see here. It is based on Spring Boot 2.0 and Kotlin 1.1.

The example contains: a JPA entity, a JPA repository, a REST controller, a REST client and 2 x integration tests. The database is H2 and the schema is created using a single Flyway migration.

It is going to get messy now: I’ll just pick and choose Kotlin features, in the context of that example, that I find interesting. Hang tight 🙂

JPA – Kotlin style

Here’s the JPA entity:

import javax.persistence.Entity
import javax.persistence.Id
import javax.validation.constraints.NotNull

class Message(@Id val id: String, @NotNull val text: String)

A one liner. It has two properties: id and text. It uses standard JPA annotations. So that’s one of Kotlin’s nice features: properties – see [kotlin-reference]. Another, is the possibility to develop extremely compact class definitions.

A JPA repository that can be used to perform CRUD operations on the entity:

import org.springframework.data.jpa.repository.JpaRepository
import org.springframework.stereotype.Repository

interface MessageRepository : JpaRepository<Message, String>

This is a Spring Data powered JPA repository in action. To be honest it’s quite similar to the same if it was written Java. Just wanted to show it for completeness.

REST controller – Kotlin style

import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController
import javax.annotation.PostConstruct

class MessageController(val messageRepository: MessageRepository) {

    fun postConstruct() = messageRepository.save(
            listOf(Message("1", "Hello World"), Message("2", "Hej Verden")))

    fun list() = messageRepository.findAll()

    fun get(@PathVariable id: String) = messageRepository.findOne(id)


This is a normal Spring MVC REST controller. It uses constructor injection (see the class header), defines two HTTP GET methods and a lifecycle method.

The methods are one liners: In Kotlin, functions doesn’t have to supply a body.

Notice that the methods doesn’t explicitly declare any return types. That’s an example of Kotlin’s advanced type inference. You are definitely allowed to declare the types also. Being a die hard Java developer, I may argue that it could add clarity to the reader – but mostly for more advanced examples (fx functions with bodies having x amount of code).

Together these features reduce the typical Java ceremonies a lot.

REST client – Kotlin style

Here’s a rather funny REST client (it’s just a client to the controller you saw above):

import kotlinx.coroutines.experimental.CommonPool
import kotlinx.coroutines.experimental.async
import kotlinx.coroutines.experimental.runBlocking
import org.springframework.beans.factory.annotation.Value
import org.springframework.boot.web.client.RestTemplateBuilder
import org.springframework.stereotype.Component
import java.util.concurrent.TimeUnit

class MessageClient(templateBuilder: RestTemplateBuilder, @Value("\${server.port}") val port: Int) {

    val restTemplate = templateBuilder.rootUri("http://localhost:$port").build()

    fun getMessages(): List<Message> {

        fun asyncGetForObject(id: Int) = async(CommonPool) {
            restTemplate.getForObject("/messages/$id/", Message::class.java)

        val messages = listOf(asyncGetForObject(1), asyncGetForObject(2))

        return runBlocking { messages.map { it.await() } }



This class has two properties: port and restTemplate. The latter is initialized in the class body. Take a look at that initialization code: it uses Kotlin’s string template support for string interpolation [kotlin-stringtemplates]. I know it’s small stuff – but thats really a neat little feature. Also – you know what? Kotlin even supports multiline strings 🙂

Take a close look at the getMessages function. It has a nested function called asyncGetForObject. At first this may seem nasty to a Java developer. But to be honest, after having given it a bit of thought, I think it can be okay. In this case I think it is, because: the function is only supposed to be used from within the getMessages function and it is rather small.

Also, getMessages uses the Kotlin 1.1 experimental “coroutine” functionality [kotlin-coroutines]. The coroutine is the async(CommonPool){} block. Invoking that, as we do twice in the val messages = … line is non-blocking (hence the async hint :)). At that very moment 2 x REST requests run in parallel. The last block runBlocking{} is where we await the results and return them when they are ready.

There is a lot more to the coroutine story. And remember it is experimental in Kotlin 1.1. But still: That’s damn interesting in my opinion :).

Did you notice the collection map functionality?? The messages.map { it.await() } code. (We pass a lambda to the List.map() method). In Kotlin single-argument lambdas, we can just reference the it variable. Also, no collect() call there. That’s a really nice lambda functionality in Kotlin, right? 🙂

Tests – Kotlin style

Here’s a Spring Boot integration test of the REST client:

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.context.SpringBootTest.WebEnvironment.DEFINED_PORT
import org.springframework.test.context.junit4.SpringRunner
import kotlin.system.measureTimeMillis

@SpringBootTest(webEnvironment = DEFINED_PORT, value = "server.port=8090")
class MessageClientIntegrationTests {

    lateinit var messageClient: MessageClient

    fun `getMessages() should fetch messages in parallel`() {

        val durationInMilliseconds = measureTimeMillis {

            val messages = messageClient.getMessages()

            assertThat(messages).extracting { it.id }.contains("1", "2")
            assertThat(messages).extracting { it.text }.contains("Hello World", "Hej Verden")





The test function name is more pleasant to read than a typical Java based test.

Notice Kotlin’s measureTimeMillis function. For Java developers that may look like a build in language construct. Like the Java synchronized( this ) {} blocks fx. It is a function actually: In Kotlin, when a lambda is the last argument, then it may be supplied after the function call (measureTimeMillis() {} or measureTimeMillis {}).

The measureTimeMillis function itself – that’s not bad at all either I think. Here it is super handy and nevertheless easy to use.

There are many more cool features in Kotlin. That’s subject for another post though.

I told you it was going to get messy 🙂


[kotlin-reference] : Kotlin Reference

[kotlin-coroutines] : Kotlin Reference : Coroutines

[kotlin-stringtemplates] : Kotlin Reference : String templates

Tagged with: ,
Posted in Kotlin, Spring
2 comments on “Spring Boot: A bit more cool with Kotlin
  1. gorolwo says:

    It looks cool in a simple example, but things are getting complicated when you have real entities with ton of relationships….

    • moelholm says:

      I agree 🙂

      When I’m writing “intro” posts like this I deliberately make them very simple. It’s a trade-off.

      I tend to make them super simple, as that is the way I prefer to learn new technologies myself. It keeps me focused on the actual technology – and not the complexity of, say, a “real” domain model.

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