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 -f 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 -n 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 services.serving.knative.dev helloworld-java  -o=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain
   NAME                DOMAIN
   helloworld-java     helloworld-java.default.example.com
   

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 -f service.yaml