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Hello World - Spring Boot Java sample

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 World: ${TARGET}!". If TARGET is not specified, it will use "NOT SPECIFIED" 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.

  2. 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:NOT SPECIFIED}")
    String target;

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

    public static void main(String[] args) {
        SpringApplication.run(HelloworldApplication.class, args);
    }
}

  3. 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.

    FROM maven:3.5-jdk-8-alpine as build
ADD pom.xml ./pom.xml
ADD src ./src
RUN mvn package -DskipTests

FROM openjdk:8-jre-alpine
COPY --from=build /target/helloworld-*.jar /helloworld.jar
VOLUME /tmp
ENTRYPOINT ["java","-Djava.security.egd=file:/dev/./urandom","-jar","/helloworld.jar"]

  4. 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

  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 balancer 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 svc knative-ingressgateway -n istio-system to get the ingress IP for your cluster. If your cluster is new, it may take sometime for the service to get asssigned an external IP address.

    kubectl get svc knative-ingressgateway --namespace istio-system

NAME                     TYPE           CLUSTER-IP     EXTERNAL-IP      PORT(S)                                      AGE
knative-ingressgateway   LoadBalancer   10.23.247.74   35.203.155.229   80:32380/TCP,443:32390/TCP,32400:32400/TCP   2d

  5. 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

    Note: ksvc is an alias for services.serving.knative.dev. If you have an older version (version 0.1.0) of Knative installed, you'll need to use the long name until you upgrade to version 0.1.1 or higher. See Checking Knative Installation Version to learn how to see what version you have installed.

  6. Now you can make a request to your app to see the result. Replace {IP_ADDRESS} with the address you see returned in the previous step.

    curl -H "Host: helloworld-java.default.example.com" http://{IP_ADDRESS}
Hello World: 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