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.


  • 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).

Steps to recreate the sample code

While you can clone all of the code from this directory, hello world apps are generally more useful if you build them step-by-step. The following instructions recreate the source files from this folder.

  1. Create a new directory and cd into it:
   mkdir hello
   cd hello
  1. 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)
  1. Switch back to hello directory

  2. 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'
  1. 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 the Official OpenJDK image for a lean production stage of our multi-stage build.
   # https://hub.docker.com/_/openjdk
   # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
   FROM openjdk:8-jre-alpine

   # 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" ]
  1. 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
  1. 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
  1. 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).
  2. 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}")
  1. 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
  1. 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