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Building web applications with Spring Boot and Kotlin

This tutorial shows you how to build efficiently a sample blog application by combining the power of Spring Boot and Kotlin.

If you are starting with Kotlin, you can learn the language by reading the reference documentation, following the online Kotlin Koans tutorial or just using Spring Framework reference documentation which now provides code samples in Kotlin.

Spring Kotlin support is documented in the Spring Framework and Spring Boot reference documentation. If you need help, search or ask questions with the spring and kotlin tags on StackOverflow or come discuss in the #spring channel of Kotlin Slack.

Creating a New Project

First we need to create a Spring Boot application, which can be done in a number of ways.

Using the Initializr Website

Visit https://start.spring.io and choose the Kotlin language. Gradle is the most commonly used build tool in Kotlin, and it provides a Kotlin DSL which is used by default when generating a Kotlin project, so this is the recommended choice. But you can also use Maven if you are more comfortable with it. Notice that you can use https://start.spring.io/#!language=kotlin&type=gradle-project-kotlin to have Kotlin and Gradle selected by default.

  1. Select "Gradle - Kotlin" or "Maven" depending on which build tool you want to use

  2. Enter the following artifact coordinates: blog

  3. Add the following dependencies:

    • Spring Web

    • Mustache

    • Spring Data JPA

    • H2 Database

    • Spring Boot DevTools

  4. Click "Generate Project".

The .zip file contains a standard project in the root directory, so you might want to create an empty directory before you unpack it.

Using command line

You can use the Initializr HTTP API from the command line with, for example, curl on a UN*X like system:

$ mkdir blog && cd blog
$ curl https://start.spring.io/starter.zip -d language=kotlin -d type=gradle-project-kotlin -d dependencies=web,mustache,jpa,h2,devtools -d packageName=com.example.blog -d name=Blog -o blog.zip

Add -d type=gradle-project if you want to use Gradle.

Using IntelliJ IDEA

Spring Initializr is also integrated in IntelliJ IDEA Ultimate edition and allows you to create and import a new project without having to leave the IDE for the command-line or the web UI.

To access the wizard, go to File | New | Project, and select Spring Initializr.

Follow the steps of the wizard to use the following parameters:

  • Artifact: "blog"

  • Type: "Gradle - Kotlin" or "Maven"

  • Language: Kotlin

  • Name: "Blog"

  • Dependencies: "Spring Web Starter", "Mustache", "Spring Data JPA", "H2 Database" and "Spring Boot DevTools"

Understanding the Gradle Build

If you’re using a Maven Build, you can skip to the dedicated section.

Plugins

In addition to the obvious Kotlin Gradle plugin, the default configuration declares the kotlin-spring plugin which automatically opens classes and methods (unlike in Java, the default qualifier is final in Kotlin) annotated or meta-annotated with Spring annotations. This is useful to be able to create @Configuration or @Transactional beans without having to add the open qualifier required by CGLIB proxies for example.

In order to be able to use Kotlin non-nullable properties with JPA, Kotlin JPA plugin is also enabled. It generates no-arg constructors for any class annotated with @Entity, @MappedSuperclass or @Embeddable.

build.gradle.kts

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

plugins {
  id("org.springframework.boot") version "3.2.2"
  id("io.spring.dependency-management") version "1.1.4"
  kotlin("jvm") version "1.9.22"
  kotlin("plugin.spring") version "1.9.22"
  kotlin("plugin.jpa") version "1.9.22"
}

Compiler options

One of Kotlin’s key features is null-safety - which cleanly deals with null values at compile time rather than bumping into the famous NullPointerException at runtime. This makes applications safer through nullability declarations and expressing "value or no value" semantics without paying the cost of wrappers like Optional. Note that Kotlin allows using functional constructs with nullable values; check out this comprehensive guide to Kotlin null-safety.

Although Java does not allow one to express null-safety in its type-system, Spring Framework provides null-safety of the whole Spring Framework API via tooling-friendly annotations declared in the org.springframework.lang package. By default, types from Java APIs used in Kotlin are recognized as platform types for which null-checks are relaxed. Kotlin support for JSR 305 annotations + Spring nullability annotations provide null-safety for the whole Spring Framework API to Kotlin developers, with the advantage of dealing with null related issues at compile time.

This feature can be enabled by adding the -Xjsr305 compiler flag with the strict options.

build.gradle.kts

tasks.withType<KotlinCompile> {
  kotlinOptions {
    freeCompilerArgs += "-Xjsr305=strict"
  }
}

Dependencies

2 Kotlin specific libraries are required (the standard library is added automatically with Gradle) for such Spring Boot web application and configured by default:

  • kotlin-reflect is Kotlin reflection library

  • jackson-module-kotlin adds support for serialization/deserialization of Kotlin classes and data classes (single constructor classes can be used automatically, and those with secondary constructors or static factories are also supported)

build.gradle.kts

dependencies {
  implementation("org.springframework.boot:spring-boot-starter-data-jpa")
  implementation("org.springframework.boot:spring-boot-starter-mustache")
  implementation("org.springframework.boot:spring-boot-starter-web")
  implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
  implementation("org.jetbrains.kotlin:kotlin-reflect")
  runtimeOnly("com.h2database:h2")
  runtimeOnly("org.springframework.boot:spring-boot-devtools")
  testImplementation("org.springframework.boot:spring-boot-starter-test")
}

Recent versions of H2 require special configuration to properly escape reserved keywords like user.

src/main/resources/application.properties

spring.jpa.properties.hibernate.globally_quoted_identifiers=true
spring.jpa.properties.hibernate.globally_quoted_identifiers_skip_column_definitions=true

Spring Boot Gradle plugin automatically uses the Kotlin version declared via the Kotlin Gradle plugin.

You can now take a deeper look at the generated application.

Understanding the Maven Build

Plugins

In addition to the obvious Kotlin Maven plugin, the default configuration declares the kotlin-spring plugin which automatically opens classes and methods (unlike in Java, the default qualifier is final in Kotlin) annotated or meta-annotated with Spring annotations. This is useful to be able to create @Configuration or @Transactional beans without having to add the open qualifier required by CGLIB proxies for example.

In order to be able to use Kotlin non-nullable properties with JPA, Kotlin JPA plugin is also enabled. It generates no-arg constructors for any class annotated with @Entity, @MappedSuperclass or @Embeddable.

pom.xml

<build>
    <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
    <testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
      <plugin>
        <groupId>org.jetbrains.kotlin</groupId>
        <artifactId>kotlin-maven-plugin</artifactId>
        <configuration>
          <compilerPlugins>
            <plugin>jpa</plugin>
            <plugin>spring</plugin>
          </compilerPlugins>
          <args>
            <arg>-Xjsr305=strict</arg>
          </args>
        </configuration>
        <dependencies>
          <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-noarg</artifactId>
            <version>${kotlin.version}</version>
          </dependency>
          <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-allopen</artifactId>
            <version>${kotlin.version}</version>
          </dependency>
        </dependencies>
      </plugin>
    </plugins>
  </build>

One of Kotlin’s key features is null-safety - which cleanly deals with null values at compile time rather than bumping into the famous NullPointerException at runtime. This makes applications safer through nullability declarations and expressing "value or no value" semantics without paying the cost of wrappers like Optional. Note that Kotlin allows using functional constructs with nullable values; check out this comprehensive guide to Kotlin null-safety.

Although Java does not allow one to express null-safety in its type-system, Spring Framework provides null-safety of the whole Spring Framework API via tooling-friendly annotations declared in the org.springframework.lang package. By default, types from Java APIs used in Kotlin are recognized as platform types for which null-checks are relaxed. Kotlin support for JSR 305 annotations + Spring nullability annotations provide null-safety for the whole Spring Framework API to Kotlin developers, with the advantage of dealing with null related issues at compile time.

This feature can be enabled by adding the -Xjsr305 compiler flag with the strict options.

Notice also that Kotlin compiler is configured to generate Java 8 bytecode (Java 6 by default).

Dependencies

3 Kotlin specific libraries are required for such Spring Boot web application and configured by default:

  • kotlin-stdlib is the Kotlin standard library

  • kotlin-reflect is Kotlin reflection library

  • jackson-module-kotlin adds support for serialization/deserialization of Kotlin classes and data classes (single constructor classes can be used automatically, and those with secondary constructors or static factories are also supported)

pom.xml

<dependencies>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-mustache</artifactId>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
  </dependency>
  <dependency>
    <groupId>com.fasterxml.jackson.module</groupId>
    <artifactId>jackson-module-kotlin</artifactId>
  </dependency>
  <dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-reflect</artifactId>
  </dependency>
  <dependency>
    <groupId>org.jetbrains.kotlin</groupId>
    <artifactId>kotlin-stdlib</artifactId>
  </dependency>

  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
  </dependency>
  <dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
  </dependency>
</dependencies>

Understanding the generated Application

src/main/kotlin/com/example/blog/BlogApplication.kt

package com.example.blog

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class BlogApplication

fun main(args: Array<String>) {
  runApplication<BlogApplication>(*args)
}

Compared to Java, you can notice the lack of semicolons, the lack of brackets on empty class (you can add some if you need to declare beans via @Bean annotation) and the use of runApplication top level function. runApplication<BlogApplication>(*args) is Kotlin idiomatic alternative to SpringApplication.run(BlogApplication::class.java, *args) and can be used to customize the application with following syntax.

src/main/kotlin/com/example/blog/BlogApplication.kt

fun main(args: Array<String>) {
  runApplication<BlogApplication>(*args) {
    setBannerMode(Banner.Mode.OFF)
  }
}

Writing your first Kotlin controller

Let’s create a simple controller to display a simple web page.

src/main/kotlin/com/example/blog/HtmlController.kt

package com.example.blog

import org.springframework.stereotype.Controller
import org.springframework.ui.Model
import org.springframework.ui.set
import org.springframework.web.bind.annotation.GetMapping

@Controller
class HtmlController {

  @GetMapping("/")
  fun blog(model: Model): String {
    model["title"] = "Blog"
    return "blog"
  }

}

Notice that we are using here a Kotlin extension that allows to add Kotlin functions or operators to existing Spring types. Here we import the org.springframework.ui.set extension function in order to be able to write model["title"] = "Blog" instead of model.addAttribute("title", "Blog"). The Spring Framework KDoc API lists all the Kotlin extensions provided to enrich the Java API.

We also need to create the associated Mustache templates.

src/main/resources/templates/header.mustache

<html>
<head>
  <title>{{title}}</title>
</head>
<body>

src/main/resources/templates/footer.mustache

</body>
</html>

src/main/resources/templates/blog.mustache

{{> header}}

<h1>{{title}}</h1>

{{> footer}}

Start the web application by running the main function of BlogApplication.kt, and go to http://localhost:8080/, you should see a sober web page with a "Blog" headline.

Testing with JUnit 5

JUnit 5 now used by default in Spring Boot provides various features very handy with Kotlin, including autowiring of constructor/method parameters which allows to use non-nullable val properties and the possibility to use @BeforeAll/@AfterAll on regular non-static methods.

Writing JUnit 5 tests in Kotlin

For the sake of this example, let’s create an integration test in order to demonstrate various features:

  • We use real sentences between backticks instead of camel-case to provide expressive test function names

  • JUnit 5 allows to inject constructor and method parameters, which is a good fit with Kotlin read-only and non-nullable properties

  • This code leverages getForObject and getForEntity Kotlin extensions (you need to import them)

src/test/kotlin/com/example/blog/IntegrationTests.kt

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class IntegrationTests(@Autowired val restTemplate: TestRestTemplate) {

  @Test
  fun `Assert blog page title, content and status code`() {
    val entity = restTemplate.getForEntity<String>("/")
    assertThat(entity.statusCode).isEqualTo(HttpStatus.OK)
    assertThat(entity.body).contains("<h1>Blog</h1>")
  }

}

Test instance lifecycle

Sometimes you need to execute a method before or after all tests of a given class. Like Junit 4, JUnit 5 requires by default these methods to be static (which translates to companion object in Kotlin, which is quite verbose and not straightforward) because test classes are instantiated one time per test.

But Junit 5 allows you to change this default behavior and instantiate test classes one time per class. This can be done in various ways, here we will use a property file to change the default behavior for the whole project:

src/test/resources/junit-platform.properties

junit.jupiter.testinstance.lifecycle.default = per_class

With this configuration, we can now use @BeforeAll and @AfterAll annotations on regular methods like shown in updated version of IntegrationTests above.

src/test/kotlin/com/example/blog/IntegrationTests.kt

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class IntegrationTests(@Autowired val restTemplate: TestRestTemplate) {

  @BeforeAll
  fun setup() {
    println(">> Setup")
  }

  @Test
  fun `Assert blog page title, content and status code`() {
    println(">> Assert blog page title, content and status code")
    val entity = restTemplate.getForEntity<String>("/")
    assertThat(entity.statusCode).isEqualTo(HttpStatus.OK)
    assertThat(entity.body).contains("<h1>Blog</h1>")
  }

  @Test
  fun `Assert article page title, content and status code`() {
    println(">> TODO")
  }

  @AfterAll
  fun teardown() {
    println(">> Tear down")
  }

}

Creating your own extensions

Instead of using util classes with abstract methods like in Java, it is usual in Kotlin to provide such functionalities via Kotlin extensions. Here we are going to add a format() function to the existing LocalDateTime type in order to generate text with the English date format.

src/main/kotlin/com/example/blog/Extensions.kt

fun LocalDateTime.format(): String = this.format(englishDateFormatter)

private val daysLookup = (1..31).associate { it.toLong() to getOrdinal(it) }

private val englishDateFormatter = DateTimeFormatterBuilder()
    .appendPattern("yyyy-MM-dd")
    .appendLiteral(" ")
    .appendText(ChronoField.DAY_OF_MONTH, daysLookup)
    .appendLiteral(" ")
    .appendPattern("yyyy")
    .toFormatter(Locale.ENGLISH)

private fun getOrdinal(n: Int) = when {
  n in 11..13 -> "${n}th"
  n % 10 == 1 -> "${n}st"
  n % 10 == 2 -> "${n}nd"
  n % 10 == 3 -> "${n}rd"
  else -> "${n}th"
}

fun String.toSlug() = lowercase(Locale.getDefault())
    .replace("\n", " ")
    .replace("[^a-z\\d\\s]".toRegex(), " ")
    .split(" ")
    .joinToString("-")
    .replace("-+".toRegex(), "-")

We will leverage these extensions in the next section.

Persistence with JPA

In order to make lazy fetching working as expected, entities should be open as described in KT-28525. We are going to use the Kotlin allopen plugin for that purpose.

With Gradle:

build.gradle.kts

plugins {
  ...
  kotlin("plugin.allopen") version "1.9.22"
}

allOpen {
  annotation("jakarta.persistence.Entity")
  annotation("jakarta.persistence.Embeddable")
  annotation("jakarta.persistence.MappedSuperclass")
}

Or with Maven:

pom.xml

<plugin>
  <artifactId>kotlin-maven-plugin</artifactId>
  <groupId>org.jetbrains.kotlin</groupId>
  <configuration>
    ...
    <compilerPlugins>
      ...
      <plugin>all-open</plugin>
    </compilerPlugins>
    <pluginOptions>
      <option>all-open:annotation=jakarta.persistence.Entity</option>
      <option>all-open:annotation=jakarta.persistence.Embeddable</option>
      <option>all-open:annotation=jakarta.persistence.MappedSuperclass</option>
    </pluginOptions>
  </configuration>
</plugin>

Then we create our model by using Kotlin primary constructor concise syntax which allows to declare at the same time the properties and the constructor parameters.

src/main/kotlin/com/example/blog/Entities.kt

@Entity
class Article(
    var title: String,
    var headline: String,
    var content: String,
    @ManyToOne var author: User,
    var slug: String = title.toSlug(),
    var addedAt: LocalDateTime = LocalDateTime.now(),
    @Id @GeneratedValue var id: Long? = null)

@Entity
class User(
    var login: String,
    var firstname: String,
    var lastname: String,
    var description: String? = null,
    @Id @GeneratedValue var id: Long? = null)

Notice that we are using here our String.toSlug() extension to provide a default argument to the slug parameter of Article constructor. Optional parameters with default values are defined at the last position in order to make it possible to omit them when using positional arguments (Kotlin also supports named arguments). Notice that in Kotlin it is not unusual to group concise class declarations in the same file.

Here we don’t use data classes with val properties because JPA is not designed to work with immutable classes or the methods generated automatically by data classes. If you are using other Spring Data flavor, most of them are designed to support such constructs so you should use classes like data class User(val login: String, …​) when using Spring Data MongoDB, Spring Data JDBC, etc.
While Spring Data JPA makes it possible to use natural IDs (it could have been the login property in User class) via Persistable, it is not a good fit with Kotlin due to KT-6653, that’s why it is recommended to always use entities with generated IDs in Kotlin.

We also declare our Spring Data JPA repositories as following.

src/main/kotlin/com/example/blog/Repositories.kt

interface ArticleRepository : CrudRepository<Article, Long> {
  fun findBySlug(slug: String): Article?
  fun findAllByOrderByAddedAtDesc(): Iterable<Article>
}

interface UserRepository : CrudRepository<User, Long> {
  fun findByLogin(login: String): User?
}

And we write JPA tests to check whether basic use cases work as expected.

src/test/kotlin/com/example/blog/RepositoriesTests.kt

@DataJpaTest
class RepositoriesTests @Autowired constructor(
    val entityManager: TestEntityManager,
    val userRepository: UserRepository,
    val articleRepository: ArticleRepository) {

  @Test
  fun `When findByIdOrNull then return Article`() {
    val johnDoe = User("johnDoe", "John", "Doe")
    entityManager.persist(johnDoe)
    val article = Article("Lorem", "Lorem", "dolor sit amet", johnDoe)
    entityManager.persist(article)
    entityManager.flush()
    val found = articleRepository.findByIdOrNull(article.id!!)
    assertThat(found).isEqualTo(article)
  }

  @Test
  fun `When findByLogin then return User`() {
    val johnDoe = User("johnDoe", "John", "Doe")
    entityManager.persist(johnDoe)
    entityManager.flush()
    val user = userRepository.findByLogin(johnDoe.login)
    assertThat(user).isEqualTo(johnDoe)
  }
}
We use here the CrudRepository.findByIdOrNull Kotlin extension provided by default with Spring Data, which is a nullable variant of the Optional based CrudRepository.findById. Read the great Null is your friend, not a mistake blog post for more details.

Implementing the blog engine

We update the "blog" Mustache templates.

src/main/resources/templates/blog.mustache

{{> header}}

<h1>{{title}}</h1>

<div class="articles">

  {{#articles}}
    <section>
      <header class="article-header">
        <h2 class="article-title"><a href="/article/{{slug}}">{{title}}</a></h2>
        <div class="article-meta">By  <strong>{{author.firstname}}</strong>, on <strong>{{addedAt}}</strong></div>
      </header>
      <div class="article-description">
        {{headline}}
      </div>
    </section>
  {{/articles}}
</div>

{{> footer}}

And we create an "article" new one.

src/main/resources/templates/article.mustache

{{> header}}

<section class="article">
  <header class="article-header">
    <h1 class="article-title">{{article.title}}</h1>
    <p class="article-meta">By  <strong>{{article.author.firstname}}</strong>, on <strong>{{article.addedAt}}</strong></p>
  </header>

  <div class="article-description">
    {{article.headline}}

    {{article.content}}
  </div>
</section>

{{> footer}}

We update the HtmlController in order to render blog and article pages with the formatted date. ArticleRepository constructor parameter will be automatically autowired since HtmlController has a single constructor (implicit @Autowired).

src/main/kotlin/com/example/blog/HtmlController.kt

@Controller
class HtmlController(private val repository: ArticleRepository) {

  @GetMapping("/")
  fun blog(model: Model): String {
    model["title"] = "Blog"
    model["articles"] = repository.findAllByOrderByAddedAtDesc().map { it.render() }
    return "blog"
  }

  @GetMapping("/article/{slug}")
  fun article(@PathVariable slug: String, model: Model): String {
    val article = repository
        .findBySlug(slug)
        ?.render()
        ?: throw ResponseStatusException(HttpStatus.NOT_FOUND, "This article does not exist")
    model["title"] = article.title
    model["article"] = article
    return "article"
  }

  fun Article.render() = RenderedArticle(
      slug,
      title,
      headline,
      content,
      author,
      addedAt.format()
  )

  data class RenderedArticle(
      val slug: String,
      val title: String,
      val headline: String,
      val content: String,
      val author: User,
      val addedAt: String)

}

Then, we add data initialization to a new BlogConfiguration class.

src/main/kotlin/com/example/blog/BlogConfiguration.kt

@Configuration
class BlogConfiguration {

  @Bean
  fun databaseInitializer(userRepository: UserRepository,
              articleRepository: ArticleRepository) = ApplicationRunner {

    val johnDoe = userRepository.save(User("johnDoe", "John", "Doe"))
    articleRepository.save(Article(
        title = "Lorem",
        headline = "Lorem",
        content = "dolor sit amet",
        author = johnDoe
    ))
    articleRepository.save(Article(
        title = "Ipsum",
        headline = "Ipsum",
        content = "dolor sit amet",
        author = johnDoe
    ))
  }
}
Notice the usage of named parameters to make the code more readable.

And we also update the integration tests accordingly.

src/test/kotlin/com/example/blog/IntegrationTests.kt

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class IntegrationTests(@Autowired val restTemplate: TestRestTemplate) {

  @BeforeAll
  fun setup() {
    println(">> Setup")
  }

  @Test
  fun `Assert blog page title, content and status code`() {
    println(">> Assert blog page title, content and status code")
    val entity = restTemplate.getForEntity<String>("/")
    assertThat(entity.statusCode).isEqualTo(HttpStatus.OK)
    assertThat(entity.body).contains("<h1>Blog</h1>", "Lorem")
  }

  @Test
  fun `Assert article page title, content and status code`() {
    println(">> Assert article page title, content and status code")
    val title = "Lorem"
    val entity = restTemplate.getForEntity<String>("/article/${title.toSlug()}")
    assertThat(entity.statusCode).isEqualTo(HttpStatus.OK)
    assertThat(entity.body).contains(title, "Lorem", "dolor sit amet")
  }

  @AfterAll
  fun teardown() {
    println(">> Tear down")
  }

}

Start (or restart) the web application, and go to http://localhost:8080/, you should see the list of articles with clickable links to see a specific article.

Exposing HTTP API

We are now going to implement the HTTP API via @RestController annotated controllers.

src/main/kotlin/com/example/blog/HttpControllers.kt

@RestController
@RequestMapping("/api/article")
class ArticleController(private val repository: ArticleRepository) {

  @GetMapping("/")
  fun findAll() = repository.findAllByOrderByAddedAtDesc()

  @GetMapping("/{slug}")
  fun findOne(@PathVariable slug: String) =
      repository.findBySlug(slug) ?: throw ResponseStatusException(HttpStatus.NOT_FOUND, "This article does not exist")

}

@RestController
@RequestMapping("/api/user")
class UserController(private val repository: UserRepository) {

  @GetMapping("/")
  fun findAll() = repository.findAll()

  @GetMapping("/{login}")
  fun findOne(@PathVariable login: String) =
      repository.findByLogin(login) ?: throw ResponseStatusException(HttpStatus.NOT_FOUND, "This user does not exist")
}

For tests, instead of integration tests, we are going to leverage @WebMvcTest and Mockk which is similar to Mockito but better suited for Kotlin.

Since @MockBean and @SpyBean annotations are specific to Mockito, we are going to leverage SpringMockK which provides similar @MockkBean and @SpykBean annotations for Mockk.

With Gradle:

build.gradle.kts

testImplementation("org.springframework.boot:spring-boot-starter-test") {
  exclude(module = "mockito-core")
}
testImplementation("org.junit.jupiter:junit-jupiter-api")
testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
testImplementation("com.ninja-squad:springmockk:4.0.2")

Or with Maven:

pom.xml

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.junit.jupiter</groupId>
  <artifactId>junit-jupiter-engine</artifactId>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>com.ninja-squad</groupId>
  <artifactId>springmockk</artifactId>
  <version>4.0.2</version>
  <scope>test</scope>
</dependency>

src/test/kotlin/com/example/blog/HttpControllersTests.kt

@WebMvcTest
class HttpControllersTests(@Autowired val mockMvc: MockMvc) {

  @MockkBean
  lateinit var userRepository: UserRepository

  @MockkBean
  lateinit var articleRepository: ArticleRepository

  @Test
  fun `List articles`() {
    val johnDoe = User("johnDoe", "John", "Doe")
    val lorem5Article = Article("Lorem", "Lorem", "dolor sit amet", johnDoe)
    val ipsumArticle = Article("Ipsum", "Ipsum", "dolor sit amet", johnDoe)
    every { articleRepository.findAllByOrderByAddedAtDesc() } returns listOf(lorem5Article, ipsumArticle)
    mockMvc.perform(get("/api/article/").accept(MediaType.APPLICATION_JSON))
        .andExpect(status().isOk)
        .andExpect(content().contentType(MediaType.APPLICATION_JSON))
        .andExpect(jsonPath("\$.[0].author.login").value(johnDoe.login))
        .andExpect(jsonPath("\$.[0].slug").value(lorem5Article.slug))
        .andExpect(jsonPath("\$.[1].author.login").value(johnDoe.login))
        .andExpect(jsonPath("\$.[1].slug").value(ipsumArticle.slug))
  }

  @Test
  fun `List users`() {
    val johnDoe = User("johnDoe", "John", "Doe")
    val janeDoe = User("janeDoe", "Jane", "Doe")
    every { userRepository.findAll() } returns listOf(johnDoe, janeDoe)
    mockMvc.perform(get("/api/user/").accept(MediaType.APPLICATION_JSON))
        .andExpect(status().isOk)
        .andExpect(content().contentType(MediaType.APPLICATION_JSON))
        .andExpect(jsonPath("\$.[0].login").value(johnDoe.login))
        .andExpect(jsonPath("\$.[1].login").value(janeDoe.login))
  }
}
$ needs to be escaped in strings as it is used for string interpolation.

Configuration properties

In Kotlin, the recommended way to manage your application properties is to use read-only properties.

src/main/kotlin/com/example/blog/BlogProperties.kt

@ConfigurationProperties("blog")
data class BlogProperties(var title: String, val banner: Banner) {
  data class Banner(val title: String? = null, val content: String)
}

Then we enable it at BlogApplication level.

src/main/kotlin/com/example/blog/BlogApplication.kt

@SpringBootApplication
@EnableConfigurationProperties(BlogProperties::class)
class BlogApplication {
  // ...
}

To generate your own metadata in order to get these custom properties recognized by your IDE, kapt should be configured with the spring-boot-configuration-processor dependency as following.

build.gradle.kts

plugins {
  ...
  kotlin("kapt") version "1.9.22"
}

dependencies {
  ...
  kapt("org.springframework.boot:spring-boot-configuration-processor")
}
Note that some features (such as detecting the default value or deprecated items) are not working due to limitations in the model kapt provides. Also annotation processing is not yet supported with Maven due to KT-18022, see initializr#438 for more details.

In IntelliJ IDEA:

  • Make sure Spring Boot plugin in enabled in menu File | Settings | Plugins | Spring Boot

  • Enable annotation processing via menu File | Settings | Build, Execution, Deployment | Compiler | Annotation Processors | Enable annotation processing

  • Since Kapt is not yet integrated in IDEA, you need to run manually the command ./gradlew kaptKotlin to generate the metadata

Your custom properties should now be recognized when editing application.properties (autocomplete, validation, etc.).

src/main/resources/application.properties

blog.title=Blog
blog.banner.title=Warning
blog.banner.content=The blog will be down tomorrow.

Edit the template and the controller accordingly.

src/main/resources/templates/blog.mustache

{{> header}}

<div class="articles">

  {{#banner.title}}
  <section>
    <header class="banner">
      <h2 class="banner-title">{{banner.title}}</h2>
    </header>
    <div class="banner-content">
      {{banner.content}}
    </div>
  </section>
  {{/banner.title}}

  ...

</div>

{{> footer}}

src/main/kotlin/com/example/blog/HtmlController.kt

@Controller
class HtmlController(private val repository: ArticleRepository,
           private val properties: BlogProperties) {

  @GetMapping("/")
  fun blog(model: Model): String {
    model["title"] = properties.title
    model["banner"] = properties.banner
    model["articles"] = repository.findAllByOrderByAddedAtDesc().map { it.render() }
    return "blog"
  }

  // ...

Restart the web application, refresh http://localhost:8080/, you should see the banner on the blog homepage.

Conclusion

We have now finished to build this sample Kotlin blog application. The source code is available on Github. You can also have a look to Spring Framework and Spring Boot reference documentation if you need more details on specific features.

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