Quarkus - Using Kotlin

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:

  • less than 10 minutes

  • an IDE

  • JDK 1.8+ installed with JAVA_HOME configured appropriately

  • Apache Maven 3.5.3+

Creating the Maven project

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

mvn io.quarkus:quarkus-maven-plugin:0.22.0:create \
    -DprojectGroupId=org.acme \
    -DprojectArtifactId=rest-kotlin \
    -DclassName="org.acme.rest.GreetingResource" \
    -Dpath="/greeting" \
    -Dextensions="kotlin,resteasy-jsonb"

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.rest.GreetingResource class is implemented as Kotlin source code (as is the case with the generated tests). The addition of resteasy-jsonb in the extension list results in importing the RESTEasy/JAX-RS and JSON-B extensions.

GreetingResource.kt looks like this:

package org.acme.rest

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

@Path("/greeting")
class GreetingResource {

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    fun hello() = "hello"
}

Update code

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

package org.acme.rest

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

We also update the GreetingResource.kt like so:

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

@Path("/greeting")
class GreetingResource {

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    fun hello() = Greeting("hello")
}

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

To make the test pass, we also need to update GreetingResourceTest.kt like so:

@QuarkusTest
open class GreetingResourceTest {

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

}

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:

<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>
        </compilerPlugins>

        <pluginOptions>
            <!-- Each annotation is placed on its own line -->
            <option>all-open:annotation=javax.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>

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 javax.ws.rs.Path should not be final.

If your application contains classes annotated with javax.enterprise.context.ApplicationScoped for example, then <option>all-open:annotation=javax.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.

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

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: ./mvnw compile quarkus:dev

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

Now using your favorite editor or IDE, update GreetingResource.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/greeting, 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.

Packaging the application

As usual, the application can be packaged using ./mvnw clean package and executed using the -runner.jar file. You can also build the native executable using ./mvnw package -Pnative.