Kotlin is a very popular programming language that targets the JVM (amongst other environments). Kotlin has experienced a surge in popularity the last few years making it the most popular JVM language, except for Java of course.

Quarkus provides first class support for using Kotlin as will be explained in this guide.

Prerequisites

To complete this guide, you need:

  • Roughly 15 minutes

  • An IDE

  • JDK 17+ installed with JAVA_HOME configured appropriately

  • Apache Maven 3.9.6

  • Optionally the Quarkus CLI if you want to use it

  • Optionally Mandrel or GraalVM installed and configured appropriately if you want to build a native executable (or Docker if you use a native container build)

NB: For Gradle project setup please see below, and for further reference consult the guide in the Gradle setup page.

Creating the Maven project

First, we need a new Kotlin project. This can be done using the following command:

CLI
quarkus create app org.acme:rest-kotlin-quickstart \
    --extension='kotlin,resteasy-reactive-jackson'
cd rest-kotlin-quickstart

To create a Gradle project, add the --gradle or --gradle-kotlin-dsl option.

For more information about how to install and use the Quarkus CLI, see the Quarkus CLI guide.

Maven
mvn io.quarkus.platform:quarkus-maven-plugin:3.7.4:create \
    -DprojectGroupId=org.acme \
    -DprojectArtifactId=rest-kotlin-quickstart \
    -Dextensions='kotlin,resteasy-reactive-jackson'
cd rest-kotlin-quickstart

To create a Gradle project, add the -DbuildTool=gradle or -DbuildTool=gradle-kotlin-dsl option.

For Windows users:

  • If using cmd, (don’t use backward slash \ and put everything on the same line)

  • If using Powershell, wrap -D parameters in double quotes e.g. "-DprojectArtifactId=rest-kotlin-quickstart"

When adding kotlin to the extensions list, the Maven plugin will generate a project that is properly configured to work with Kotlin. Furthermore, the org.acme.ReactiveGreetingResource class is implemented as Kotlin source code (as is the case with the generated tests). The addition of resteasy-reactive-jackson in the extension list results in importing the RESTEasy Reactive and Jackson extensions.

ReactiveGreetingResource looks like this:

ReactiveGreetingResource.kt
package org.acme

import jakarta.ws.rs.GET
import jakarta.ws.rs.Path
import jakarta.ws.rs.Produces
import jakarta.ws.rs.core.MediaType

@Path("/hello")
class ReactiveGreetingResource {

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    fun hello() = "Hello from RESTEasy Reactive"
}

Update code

In order to show a more practical example of Kotlin usage we will add a simple data class called Greeting like so:

Greeting.kt
package org.acme.rest

data class Greeting(val message: String = "")

We also update the ReactiveGreetingResource class like so:

import jakarta.ws.rs.GET
import jakarta.ws.rs.Path
import jakarta.ws.rs.core.MediaType

@Path("/hello")
class ReactiveGreetingResource {

    @GET
    fun hello() = Greeting("hello")
}

With these changes in place the /hello endpoint will reply with a JSON object instead of a simple String.

To make the test pass, we also need to update ReactiveGreetingResourceTest like so:

import org.hamcrest.Matchers.equalTo

@QuarkusTest
class ReactiveGreetingResourceTest {

    @Test
    fun testHelloEndpoint() {
        given()
          .`when`().get("/hello")
          .then()
             .statusCode(200)
             .body("message", equalTo("hello"))
    }

}

Kotlin version

The Quarkus Kotlin extension already declares a dependency on some base Kotlin libraries like kotlin-stdlib-jdk8 and kotlin-reflect. The Kotlin version of these dependencies is declared in the Quarkus BOM and is currently at 1.9.22. It is therefore recommended to use the same Kotlin version for other Kotlin libraries. When adding a dependency to another base Kotlin library (e.g. kotlin-test-junit5) you don’t need to specify the version, since the Quarkus BOM includes the Kotlin BOM.

This being said, you still need to specify the version of the Kotlin compiler to use. Again, it is recommended to use the same version which Quarkus uses for the Kotlin libraries.

Using a different Kotlin version in a Quarkus application is typically not recommended. But in order to do so, you must import the Kotlin BOM before the Quarkus BOM.

Important Maven configuration points

The generated pom.xml contains the following modifications compared to its counterpart when Kotlin is not selected:

  • The quarkus-kotlin artifact is added to the dependencies. This artifact provides support for Kotlin in the live reload mode (more about this later on)

  • The kotlin-stdlib-jdk8 is also added as a dependency.

  • Maven’s sourceDirectory and testSourceDirectory build properties are configured to point to Kotlin sources (src/main/kotlin and src/test/kotlin respectively)

  • The kotlin-maven-plugin is configured as follows:

pom.xml
<plugin>
    <artifactId>kotlin-maven-plugin</artifactId>
    <groupId>org.jetbrains.kotlin</groupId>
    <version>${kotlin.version}</version>
    <executions>
        <execution>
            <id>compile</id>
            <goals>
                <goal>compile</goal>
            </goals>
        </execution>
        <execution>
            <id>test-compile</id>
            <goals>
                <goal>test-compile</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <compilerPlugins>
            <plugin>all-open</plugin> (1)
        </compilerPlugins>

        <pluginOptions>
            <!-- Each annotation is placed on its own line -->
            <option>all-open:annotation=jakarta.ws.rs.Path</option>
        </pluginOptions>
    </configuration>

    <dependencies>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-allopen</artifactId>
            <version>${kotlin.version}</version>
        </dependency>
    </dependencies>
</plugin>
1 Enables the all-open annotation plugin (see discussion below)

The important thing to note is the use of the all-open Kotlin compiler plugin. In order to understand why this plugin is needed, first we need to note that by default all the classes generated from the Kotlin compiler are marked as final.

Having final classes however does not work well with various frameworks that need to create Dynamic Proxies.

Thus, the all-open Kotlin compiler plugin allows us to configure the compiler to not mark as final classes that have certain annotations. In the snippet above, we have specified that classes annotated with jakarta.ws.rs.Path should not be final.

If your application contains Kotlin classes annotated with jakarta.enterprise.context.ApplicationScoped for example, then <option>all-open:annotation=jakarta.enterprise.context.ApplicationScoped</option> needs to be added as well. Same goes for any class that needs to have a dynamic proxy created at runtime, like a JPA Entity class.

Future versions of Quarkus will configure the Kotlin compiler plugin in a way that will make it unnecessary to alter this configuration.

Important Gradle configuration points

Similar to the Maven configuration, when using Gradle, the following modifications are required when Kotlin is selected:

  • The quarkus-kotlin artifact is added to the dependencies. This artifact provides support for Kotlin in the live reload mode (more about this later on)

  • The kotlin-stdlib-jdk8 is also added as a dependency.

  • The Kotlin plugin is activated, which implicitly adds sourceDirectory and testSourceDirectory build properties to point to Kotlin sources (src/main/kotlin and src/test/kotlin respectively)

  • The all-open Kotlin plugin tells the compiler not to mark as final, those classes with the annotations highlighted (customize as required)

  • When using native-image, the use of http (or https) protocol(s) must be declared

  • An example configuration follows:

plugins {
    id 'java'
    id 'io.quarkus'

    id "org.jetbrains.kotlin.jvm" version "1.9.22" (1)
    id "org.jetbrains.kotlin.plugin.allopen" version "1.9.22" (1)
}

repositories {
    mavenLocal()
    mavenCentral()
}

dependencies {
    implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.9.22'

   implementation enforcedPlatform("${quarkusPlatformGroupId}:${quarkusPlatformArtifactId}:${quarkusPlatformVersion}")

    implementation 'io.quarkus:quarkus-resteasy-reactive'
    implementation 'io.quarkus:quarkus-resteasy-reactive-jackson'
    implementation 'io.quarkus:quarkus-kotlin'

    testImplementation 'io.quarkus:quarkus-junit5'
    testImplementation 'io.rest-assured:rest-assured'
}

group = '...' // set your group
version = '1.0.0-SNAPSHOT'

java {
    sourceCompatibility = JavaVersion.VERSION_11
    targetCompatibility = JavaVersion.VERSION_11
}

allOpen { (2)
    annotation("jakarta.ws.rs.Path")
    annotation("jakarta.enterprise.context.ApplicationScoped")
    annotation("jakarta.persistence.Entity")
    annotation("io.quarkus.test.junit.QuarkusTest")
}

compileKotlin {
    kotlinOptions.jvmTarget = JavaVersion.VERSION_11
    kotlinOptions.javaParameters = true
}

compileTestKotlin {
    kotlinOptions.jvmTarget = JavaVersion.VERSION_11
}
1 The Kotlin plugin version needs to be specified.
2 The all-open configuration required, as per Maven guide above

or, if you use the Gradle Kotlin DSL:

plugins {
    kotlin("jvm") version "1.9.22" (1)
    kotlin("plugin.allopen") version "1.9.22"
    id("io.quarkus")
}

repositories {
    mavenLocal()
    mavenCentral()
}

val quarkusPlatformGroupId: String by project
val quarkusPlatformArtifactId: String by project
val quarkusPlatformVersion: String by project

group = "..."
version = "1.0.0-SNAPSHOT"


repositories {
    mavenLocal()
    mavenCentral()
}

dependencies {
    implementation(kotlin("stdlib-jdk8"))

    implementation(enforcedPlatform("${quarkusPlatformGroupId}:${quarkusPlatformArtifactId}:${quarkusPlatformVersion}"))

    implementation("io.quarkus:quarkus-kotlin")
    implementation("io.quarkus:quarkus-resteasy-reactive")
    implementation("io.quarkus:quarkus-resteasy-reactive-jackson")

    testImplementation("io.quarkus:quarkus-junit5")
    testImplementation("io.rest-assured:rest-assured")
}

group = '...' // set your group
version = "1.0.0-SNAPSHOT"

java {
    sourceCompatibility = JavaVersion.VERSION_11
    targetCompatibility = JavaVersion.VERSION_11
}

allOpen { (2)
    annotation("jakarta.ws.rs.Path")
    annotation("jakarta.enterprise.context.ApplicationScoped")
    annotation("jakarta.persistence.Entity")
    annotation("io.quarkus.test.junit.QuarkusTest")
}

tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
    kotlinOptions.jvmTarget = JavaVersion.VERSION_11.toString()
    kotlinOptions.javaParameters = true
}
1 The Kotlin plugin version needs to be specified.
2 The all-open configuration required, as per Maven guide above

Overriding the Quarkus BOM Kotlin version (Gradle)

If you want to use a different version than the one specified by Quarkus' BOM in your application (for example, to try pre-release features or for compatibility reasons), you can do so by using the strictly {} version modifier in your Gradle dependencies. For instance:

plugins {
    id("io.quarkus")
    kotlin("jvm") version "1.7.0-Beta"
    kotlin("plugin.allopen") version "1.7.0-Beta"
}

configurations.all {
    resolutionStrategy {
        force "org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.7.0-Beta"
        force "org.jetbrains.kotlin:kotlin-reflect:1.7.0-Beta"
    }
}

Live reload

Quarkus provides support for live reloading changes made to source code. This support is also available to Kotlin, meaning that developers can update their Kotlin source code and immediately see their changes reflected.

To see this feature in action, first execute:

CLI
quarkus dev
Maven
./mvnw quarkus:dev
Gradle
./gradlew --console=plain quarkusDev

When executing an HTTP GET request against http://localhost:8080/hello, you see a JSON message with the value hello as its message field.

Now using your favorite editor or IDE, update ReactiveGreetingResource.kt and change the hello method to the following:

fun hello() = Greeting("hi")

When you now execute an HTTP GET request against http://localhost:8080/hello, you should see a JSON message with the value hi as its message field.

One thing to note is that the live reload feature is not available when making changes to both Java and Kotlin source that have dependencies on each other. We hope to alleviate this limitation in the future.

Configuring live reload compiler

If you need to customize the compiler flags used by kotlinc in development mode, you can configure them in the quarkus plugin:

Maven
<plugin>
  <groupId>${quarkus.platform.group-id}</groupId>
  <artifactId>quarkus-maven-plugin</artifactId>
  <version>${quarkus.platform.version}</version>

  <configuration>
    <source>${maven.compiler.source}</source>
    <target>${maven.compiler.target}</target>
    <compilerOptions>
      <compiler>
        <name>kotlin</name>
        <args>
          <arg>-Werror</arg>
        </args>
      </compiler>
    </compilerOptions>
  </configuration>

  ...
</plugin>
Gradle (Groovy DSL)
quarkusDev {
    compilerOptions {
        compiler("kotlin").args(['-Werror'])
    }
}
Gradle (Kotlin DSL)
tasks.quarkusDev {
     compilerOptions {
        compiler("kotlin").args(["-Werror"])
    }
}

Packaging the application

As usual, the application can be packaged using:

CLI
quarkus build
Maven
./mvnw install
Gradle
./gradlew build

and executed with java -jar target/quarkus-app/quarkus-run.jar.

You can also build the native executable using:

CLI
quarkus build --native
Maven
./mvnw install -Dnative
Gradle
./gradlew build -Dquarkus.package.type=native

Kotlin and Jackson

If the com.fasterxml.jackson.module:jackson-module-kotlin dependency and the quarkus-jackson extension (or one of the quarkus-resteasy-jackson or quarkus-resteasy-reactive-jackson extensions) have been added to the project, then Quarkus automatically registers the KotlinModule to the ObjectMapper bean (see this guide for more details).

When using Kotlin data classes with native-image you may experience serialization errors that do not occur with the JVM version, despite the Kotlin Jackson Module being registered. This is especially so if you have a more complex JSON hierarchy, where an issue on a lower node causes a serialization failure. The error message displayed is a catch-all and typically displays an issue with the root object, which may not necessarily be the case.

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `Address` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)

To ensure full-compatibility with native-image, it is recommended to apply the Jackson @field:JsonProperty("fieldName") annotation, and set a nullable default, as illustrated below. You can automate the generation of Kotlin data classes for your sample JSON using IntelliJ IDEA plugins (such as JSON to Kotlin Class), and easily enable the Jackson annotation and select nullable parameters as part of the auto-code generation.

import com.fasterxml.jackson.annotation.JsonProperty

data class Response(
        @field:JsonProperty("chart")
        val chart: ChartData? = null
)

data class ChartData(
        @field:JsonProperty("result")
        val result: List<ResultItem?>? = null,

        @field:JsonProperty("error")
        val error: Any? = null
)

data class ResultItem(
        @field:JsonProperty("meta")
        val meta: Meta? = null,

        @field:JsonProperty("indicators")
        val indicators: IndicatorItems? = null,

        @field:JsonProperty("timestamp")
        val timestamp: List<Int?>? = null
)

...

Kotlin and the Kubernetes Client

When working with the quarkus-kubernetes extension and have Kotlin classes bound to CustomResource definitions (like you do for building operators), you need to be aware that the underlying Fabric8 Kubernetes Client uses its own static Jackson ObjectMapper s, which can be configured as follows with the KotlinModule:

import io.fabric8.kubernetes.client.utils.Serialization
import com.fasterxml.jackson.module.kotlin.KotlinModule

...
val kotlinModule = KotlinModule.Builder().build()
Serialization.jsonMapper().registerModule(kotlinModule)
Serialization.yamlMapper().registerModule(kotlinModule)

Please test this carefully on compilation to native images and fallback to Java-compatible Jackson bindings if you experience problems.

Coroutines support

Extensions

The following extensions provide support for Kotlin Coroutines by allowing the use of Kotlin’s suspend keyword on method signatures.

Extension Comments

quarkus-resteasy-reactive

Support is provided for Jakarta REST Resource Methods

quarkus-rest-client-reactive

Support is provided for REST Client interface methods

quarkus-smallrye-reactive-messaging

Support is provided for Reactive messaging methods

quarkus-scheduler

Support is provided for scheduler methods

quarkus-smallrye-fault-tolerance

Support is provided for the declarative annotation-based API

Kotlin coroutines and Mutiny

Kotlin coroutines provide an imperative programming model that actually gets executed in an asynchronous, reactive manner. To simplify the interoperability between Mutiny and Kotlin there is the module io.smallrye.reactive:mutiny-kotlin, described here.

CDI @Inject with Kotlin

Kotlin reflection annotation processing differs from Java. You may experience an error when using CDI @Inject such as: "kotlin.UninitializedPropertyAccessException: lateinit property xxx has not been initialized"

In the example below, this can be easily solved by adapting the annotation, adding @field: Default, to handle the lack of a @Target on the Kotlin reflection annotation definition.

import jakarta.inject.Inject
import jakarta.enterprise.inject.Default
import jakarta.enterprise.context.ApplicationScoped

import jakarta.ws.rs.GET
import jakarta.ws.rs.Path
import jakarta.ws.rs.Produces
import jakarta.ws.rs.core.MediaType



@ApplicationScoped
class GreetingService {

    fun greeting(name: String): String {
        return "hello $name"
    }

}

@Path("/")
class ReactiveGreetingResource {

    @Inject
    @field: Default (1)
    lateinit var service: GreetingService

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    @Path("/hello/{name}")
    fun greeting(name: String): String {
        return service.greeting(name)
    }

}
1 Kotlin requires a @field: xxx qualifier as it has no @Target on the annotation definition. Add @field: xxx in this example. @Default is used as the qualifier, explicitly specifying the use of the default bean.

Alternatively, prefer the use of constructor injection which works without modification of the Java examples, increases testability and complies best to a Kotlin programming style.

import jakarta.enterprise.context.ApplicationScoped

import jakarta.ws.rs.GET
import jakarta.ws.rs.Path
import jakarta.ws.rs.Produces
import jakarta.ws.rs.core.MediaType

@ApplicationScoped
class GreetingService {
    fun greeting(name: String): String {
        return "hello $name"
    }
}

@Path("/")
class ReactiveGreetingResource(
    private val service: GreetingService
) {

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    @Path("/hello/{name}")
    fun greeting(name: String): String {
        return service.greeting(name)
    }

}