Hello World - Kotlin

A simple web app written in Kotlin using Ktor that you can use for testing. It reads in an env variable TARGET and prints “Hello \${TARGET}“. If TARGET is not specified, it will use “World” as the TARGET.

Follow the steps below to create the sample code and then deploy the app to your cluster. You can also download a working copy of the sample, by running the following commands:

  git clone -b "release-0.6" https://github.com/knative/docs knative-docs
  cd knative-docs/serving/samples/hello-world/helloworld-kotlin

Before you begin

  • A Kubernetes cluster with Knative installed. Follow the installation instructions if you need to create one.
  • Docker installed and running on your local machine, and a Docker Hub account configured (we’ll use it for a container registry).

Recreating the sample code

  1. Create a new directory and cd into it:

    mkdir hello
    cd hello
  2. Create a file named Main.kt at src/main/kotlin/com/example/hello and copy the code block below into it:

    mkdir -p src/main/kotlin/com/example/hello
    package com.example.hello
    import io.ktor.application.*
    import io.ktor.http.*
    import io.ktor.response.*
    import io.ktor.routing.*
    import io.ktor.server.engine.*
    import io.ktor.server.netty.*
    fun main(args: Array<String>) {
       val target = System.getenv("TARGET") ?: "World"
       val port = System.getenv("PORT") ?: "8080"
       embeddedServer(Netty, port.toInt()) {
           routing {
               get("/") {
                   call.respondText("Hello $target!", ContentType.Text.Html)
       }.start(wait = true)
  3. Switch back to hello directory

  4. Create a new file, build.gradle and copy the following setting

    buildscript {
        ext.kotlin_version = '1.2.61'
        ext.ktor_version = '0.9.4'
        repositories {
        dependencies {
            classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    apply plugin: 'java'
    apply plugin: 'kotlin'
    apply plugin: 'application' 
    sourceCompatibility = 1.8
    compileKotlin {
        kotlinOptions.jvmTarget = "1.8"
    compileTestKotlin {
        kotlinOptions.jvmTarget = "1.8"
    repositories {
        maven { url "https://dl.bintray.com/kotlin/ktor" }
    mainClassName = 'com.example.hello.MainKt'
    jar {
        manifest {
            attributes 'Main-Class': mainClassName
       from { configurations.compile.collect { it.isDirectory() ? it : zipTree(it) } }
    dependencies {
        compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
        compile "io.ktor:ktor-server-netty:$ktor_version"
        testCompile group: 'junit', name: 'junit', version: '4.12'
  5. Create a file named Dockerfile and copy the code block below into it.

    # Use the official gradle image to create a build artifact.
    # https://hub.docker.com/_/gradle
    FROM gradle:4.10 as builder
    # Copy local code to the container image.
    COPY build.gradle .
    COPY src ./src
    # Build a release artifact.
    RUN gradle clean build --no-daemon
    # Use AdoptOpenJDK for base image.
    # It's important to use OpenJDK 8u191 or above that has container support enabled.
    # https://hub.docker.com/r/adoptopenjdk/openjdk8
    # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
    FROM adoptopenjdk/openjdk8:jdk8u202-b08-alpine-slim
    # Copy the jar to the production image from the builder stage.
    COPY --from=builder /home/gradle/build/libs/gradle.jar /helloworld.jar
    # Run the web service on container startup.
    CMD [ "java", "-jar", "-Djava.security.egd=file:/dev/./urandom", "/helloworld.jar" ]
  6. Create a new file, service.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username.

    apiVersion: serving.knative.dev/v1alpha1
    kind: Service
      name: helloworld-kotlin
      namespace: default
          - image: docker.io/{username}/helloworld-kotlin
              - name: TARGET
                value: "Kotlin Sample v1"

Build and deploy this sample

Once you have recreated the sample code files (or used the files in the sample folder) you’re ready to build and deploy the sample app.

  1. Use Docker to build the sample code into a container. To build and push with Docker Hub, run these commands replacing {username} with your Docker Hub username:

    # Build the container on your local machine
    docker build -t {username}/helloworld-kotlin .
    # Push the container to docker registry
    docker push {username}/helloworld-kotlin
  2. After the build has completed and the container is pushed to docker hub, you can deploy the app into your cluster. Ensure that the container image value in service.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

    kubectl apply --filename service.yaml
  3. Now that your service is created, Knative will perform the following steps:

    • Create a new immutable revision for this version of the app.
    • Network programming to create a route, ingress, service, and load balance for your app.
    • Automatically scale your pods up and down (including to zero active pods).
  4. To find the IP address for your service, use kubectl get service knative-ingressgateway --namespace istio-system to get the ingress IP for your cluster. If your cluster is new, it may take sometime for the service to get assigned an external IP address.

    kubectl get service knative-ingressgateway --namespace istio-system
    NAME                     TYPE           CLUSTER-IP     EXTERNAL-IP      PORT(S)                                      AGE
    knative-ingressgateway   LoadBalancer   80:32380/TCP,443:32390/TCP,32400:32400/TCP   2d
    # Now you can assign the external IP address to the env variable.
    export IP_ADDRESS=<EXTERNAL-IP column from the command above>
    # Or just execute:
    export IP_ADDRESS=$(kubectl get svc $INGRESSGATEWAY \
      --namespace istio-system \
      --output jsonpath="{.status.loadBalancer.ingress[*].ip}")
  5. To find the URL for your service, use

    kubectl get ksvc helloworld-kotlin  --output=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain
    NAME                DOMAIN
    helloworld-kotlin   helloworld-kotlin.default.example.com
  6. Now you can make a request to your app to see the result. Presuming, the IP address you got in the step above is in the ${IP_ADDRESS} env variable:

    curl -H "Host: helloworld-kotlin.default.example.com" http://${IP_ADDRESS}
    Hello Kotlin Sample v1!

Remove the sample app deployment

To remove the sample app from your cluster, delete the service record:

kubectl delete --filename service.yaml