Hello World - Spring Boot Java

A simple web app written in Java using Spring Boot 2.0 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.

Prerequisites

  • 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).
  • You have installed Java SE 8 or later JDK.

Recreating 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. From the console, create a new empty web project using the curl and unzip commands:
   curl https://start.spring.io/starter.zip \
       -d dependencies=web \
       -d name=helloworld \
       -d artifactId=helloworld \
       -o helloworld.zip
   unzip helloworld.zip

If you don’t have curl installed, you can accomplish the same by visiting the Spring Initializr page. Specify Artifact as helloworld and add the Web dependency. Then click Generate Project, download and unzip the sample archive.

  1. Update the SpringBootApplication class in src/main/java/com/example/helloworld/HelloworldApplication.java by adding a @RestController to handle the “/” mapping and also add a @Value field to provide the TARGET environment variable:
   package com.example.helloworld;

   import org.springframework.beans.factory.annotation.Value;
   import org.springframework.boot.SpringApplication;
   import org.springframework.boot.autoconfigure.SpringBootApplication;
   import org.springframework.web.bind.annotation.GetMapping;
   import org.springframework.web.bind.annotation.RestController;

   @SpringBootApplication
   public class HelloworldApplication {

       @Value("${TARGET:World}")
       String target;

       @RestController
       class HelloworldController {
           @GetMapping("/")
           String hello() {
               return "Hello " + target + "!";
           }
       }

       public static void main(String[] args) {
           SpringApplication.run(HelloworldApplication.class, args);
       }
   }
  1. Run the application locally:
   ./mvnw package && java -jar target/helloworld-0.0.1-SNAPSHOT.jar

Go to http://localhost:8080/ to see your Hello World! message.

  1. In your project directory, create a file named Dockerfile and copy the code block below into it. For detailed instructions on dockerizing a Spring Boot app, see Spring Boot with Docker. For additional information on multi-stage docker builds for Java see Creating Smaller Java Image using Docker Multi-stage Build.
   # 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 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 /app/target/helloworld-*.jar /helloworld.jar

   # Run the web service on container startup.
   CMD ["java","-Djava.security.egd=file:/dev/./urandom","-Dserver.port=${PORT}","-jar","/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
   metadata:
     name: helloworld-java
     namespace: default
   spec:
     runLatest:
       configuration:
         revisionTemplate:
           spec:
             container:
               image: docker.io/{username}/helloworld-java
               env:
                 - name: TARGET
                   value: "Spring Boot Sample v1"

Building and deploying the 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-java .

   # Push the container to docker registry
   docker push {username}/helloworld-java
  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 balancer 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. If your cluster is new, it may take sometime for the service to get asssigned an external IP address.

   # In Knative 0.2.x and prior versions, the `knative-ingressgateway` service was used instead of `istio-ingressgateway`.
   INGRESSGATEWAY=knative-ingressgateway

   # The use of `knative-ingressgateway` is deprecated in Knative v0.3.x.
   # Use `istio-ingressgateway` instead, since `knative-ingressgateway`
   # will be removed in Knative v0.4.
   if kubectl get configmap config-istio -n knative-serving &> /dev/null; then
       INGRESSGATEWAY=istio-ingressgateway
   fi

   kubectl get svc $INGRESSGATEWAY --namespace istio-system

   NAME                     TYPE           CLUSTER-IP     EXTERNAL-IP      PORT(S)                                      AGE
   xxxxxxx-ingressgateway   LoadBalancer   10.23.247.74   35.203.155.229   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-java \
       --output=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain

   NAME                DOMAIN
   helloworld-java     helloworld-java.default.example.com

   # Or simply:
   export DOMAIN_NAME=$(kubectl get ksvc helloworld-java \
     --output jsonpath={.status.domain}
  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: ${DOMAIN_NAME}" http://${IP_ADDRESS}

   Hello Spring Boot Sample v1!

Removing the sample app deployment

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

kubectl delete --filename service.yaml