docs/serving/samples/helloworld-java/README.md

# 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](https://github.com/knative/install/) 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

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:

    ```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: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);
        }
    }
    ```

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
    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"]
    ```

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
      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:

    ```shell
    # 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`:

    ```shell
    kubectl apply -f 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 balance 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
   `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.

    ```shell
    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

    ```

1. 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
    ```

1. 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.

    ```shell
    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:

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