Hello World - Java (Micronaut)

Learn how to deploy a simple web app that is written in Java and uses Micronaut.

This samples uses Docker to build locally. The app reads in a TARGET env variable and then prints “Hello World: ${TARGET}!”. If a value for TARGET is not specified, the “NOT SPECIFIED” default value is used.

Use this sample to walk you through the steps of creating and modifying the sample app, building and pushing your container image to a registry, and then deploying your app to your Knative cluster.

Before you begin

You must meet the following requirements to complete this sample:

Tip: You can clone the Knatve/docs repo and then modify the source files. Alternatively, learn more by manually creating the files yourself.

Creating and configuring the sample code

To create and configure the source files in the root of your working directory:

  1. Create the pom.xml file:

                         <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                         <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>
  2. Create the HelloWorldController.java file in the src/main/java/com/example/helloworld directory. The [ROOT]/src/main/java/com/example/helloworld/HelloWorldController.java file handles requests to the root URI /.

     package com.example.helloworld;
     import io.micronaut.http.MediaType;
     import io.micronaut.http.annotation.Controller;
     import io.micronaut.http.annotation.Get;
     public class HelloWorldController {
         @Get(value = "/", produces = MediaType.TEXT_PLAIN)
         public String index() {
             String target = System.getenv("TARGET");
             if (target == null) {
                 target = "NOT SPECIFIED";
             return "Hello World: " + target;
  3. The Micronaut application is configured via src/main/resources/application.yml:

        name: helloworld-java-micronaut
        port: ${PORT:8080}
  4. Create the Dockerfile file:

    # Use the official maven/Java 8 image to create a build artifact.
    # https://hub.docker.com/_/maven
    FROM maven:3.5-jdk-8-alpine as builder
    # Copy local code to the container image.
    WORKDIR /app
    COPY pom.xml .
    COPY src ./src
    # Build a release artifact.
    RUN mvn package -DskipTests
    # 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 /app/target/helloworld-1.0.0-SNAPSHOT.jar /helloworld.jar
    # Run the web service on container startup.
    CMD ["java","-jar","/helloworld.jar"]
  5. Create the service.yaml file. You must specify your Docker Hub username in {username}. You can also configure the TARGET, for example you can modify the Micronaut Sample v1 value.

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

Building and deploying the sample

To build a container image, push your image to the registry, and then deploy your sample app to your cluster:

  1. Use Docker to build your container image and then push that image to your Docker Hub registry. You must replace the {username} variables in the following commands with your Docker Hub username.

    # Build the container on your local machine
    docker build -t {username}/helloworld-java-micronaut .
    # Push the container to docker registry
    docker push {username}/helloworld-java-micronaut
  2. Now that your container image is in the registry, you can deploy it to your Knative cluster by running the kubectl apply command:

    kubectl apply --filename service.yaml

    Result: A service name helloworld-java-micronaut is created in your cluster along with the following resources:

    • A new immutable revision for the version of the app that you just deployed.
    • The following networking resources are created for your app:
      • route
      • ingress
      • service
      • load balancer
    • Auto scaling is enable to allow your pods to scale up to meet traffic, and also back down to zero when there is no traffic.

Testing the sample app

To verify that your sample app has been successfully deployed:

  1. Retrieve the URL for your service, by running the following kubectl get command:

    kubectl get ksvc helloworld-java-micronaut  --output=custom-columns=NAME:.metadata.name,URL:.status.url

    Example result:

    NAME                          URL
    helloworld-java-micronaut     http://helloworld-java-micronaut.default.
  2. Now you can make a request to your app and see the result. Replace the URL below with the URL returned in the previous command.

    curl http://helloworld-java-micronaut.default.

    Example result:

     Hello World: Micronaut Sample v1

Congratulations on deploying your sample Java app to Knative!

Removing the sample app deployment

To remove the sample app from your cluster, run the following kubectl delete command:

kubectl delete --filename service.yaml