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.

Follow the steps below to create the sample code and then deploy the app to your cluster. You can also download a working copy of the sample, by running the following commands:

```shell

git clone -b "release-0.6" https://github.com/knative/docs knative-docs cd knative-docs/serving/samples/hello-world/helloworld-java-spring


 ## Before you begin

- A Kubernetes cluster with Knative installed. Follow the
  [installation instructions](../../../../install/index.html) if you need to
  create one.
- [Docker](https://www.docker.com) 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](http://www.oracle.com/technetwork/java/javase/downloads/index.html).

## Recreating the sample code

1. From the console, create a new empty web project using the curl and unzip
   commands:

    ```shell
    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](https://start.spring.io/) 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:

    ```java
    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:

    ```shell
    ./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](https://spring.io/guides/gs/spring-boot-docker/).
   For additional information on multi-stage docker builds for Java see
   [Creating Smaller Java Image using Docker Multi-stage Build](http://blog.arungupta.me/smaller-java-image-docker-multi-stage-build/).

    ```docker
    # 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-*.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.

    ```yaml
    apiVersion: serving.knative.dev/v1alpha1
    kind: Service
    metadata:
      name: helloworld-java-spring
      namespace: default
    spec:
      template:
        spec:
          containers:
            - image: docker.io/{username}/helloworld-java-spring
              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:

    ```shell
    # Build the container on your local machine
    docker build -t {username}/helloworld-java-spring .

    # Push the container to docker registry
    docker push {username}/helloworld-java-spring
    ```

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`:

    ```shell
    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).

1. 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.

    ```shell
    # 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

    ```shell
    kubectl get ksvc helloworld-java-spring \
       --output=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain

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

    # Or simply:
    export DOMAIN_NAME=$(kubectl get ksvc helloworld-java-spring \
      --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:

    ```shell
    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:

```shell
kubectl delete --filename service.yaml