docs/code-samples/serving/hello-world/helloworld-java-spark

Hello World - Spark Java Framework

A simple web app written in Java using Spark Java Framework that you can use for testing. This guide describes the steps required to to create the helloworld-java sample app and deploy it to your cluster.

Prerequisites

You will need:

• A Kubernetes cluster with Knative installed and DNS configured. See Install Knative Serving.
• Docker installed and running on your local machine, and a Docker Hub account configured.
• Java SE 8 or later JDK.

Develop

The sample app reads a TARGET environment variable, and prints Hello ${TARGET}!. If TARGET is not specified, World is used as the default value. You can also download a working copy of the sample, by running the following commands:

git clone https://github.com/knative/docs.git knative-docs
cd knative-docs/code-samples/serving/hello-world/helloworld-java

1. Run the application locally:

   ./mvnw package && java -jar target/helloworld-0.0.1-SNAPSHOT-jar-with-dependencies.jar
   

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

2. In your project directory, create a file named Dockerfile and copy the following code block into it. For detailed instructions on dockerizing a Spark Java app, see Spark with Docker. For additional information on multi-stage docker builds for Java see Creating Smaller Java Image using Docker Multi-stage Build. Navigate to your project directory and copy the following code into a new file named Dockerfile:

    FROM maven:3.5-jdk-8-alpine as builder
   
    # Copy local code to the container image.
    WORKDIR /app
    COPY pom.xml .
    COPY src ./src
   
    RUN mvn package -DskipTests
   
    FROM openjdk:8-jre-alpine
   
    # Copy the jar to the production image from the builder stage.
    COPY --from=builder /app/target/helloworld-0.0.1-SNAPSHOT-jar-with-dependencies.jar helloworld.jar
   
    ENV PORT 8080
    EXPOSE 8080
    # Run the web service on container startup.
    CMD ["java","-jar","helloworld.jar"]
   

3. To build the sample code into a container, and push using Docker Hub, enter the following commands and replace {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
   

Deploy

1. After the build has completed and the container is pushed to Docker Hub, you can deploy the app into your cluster. Choose one of the following methods:

=== "kn"

   Use `kn` to deploy the service, make sure to replace `{username}` with your Docker Hub username:

   ```bash
   kn service create helloworld-java --image=docker.io/{username}/helloworld-java --env TARGET="SparkJava Sample v1"
   ```

   This will wait until your service is deployed and ready, and ultimately it will print the URL through which you can access the service.

=== "kubectl"

   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/v1
      kind: Service
      metadata:
        name: helloworld-java
        namespace: default
      spec:
        template:
          spec:
            containers:
              - image: docker.io/{username}/helloworld-java
                env:
                  - name: TARGET
                    value: "SparkJava Sample v1"
      ```

   1. Ensure that the container image value in `service.yaml` matches the container you built in the previous step. Apply the configuration using `kubectl`:

   ```bash
   kubectl apply --filename service.yaml
   ```

After 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).

Verify

1. Run one of the followings commands to find the domain URL for your service.

=== "kn"

   ```bash
   kn service describe helloworld-java -o url
   ```

   Example:

   ```bash
   http://helloworld-java.default.1.2.3.4.xip.io
   ```

=== "kubectl" bash kubectl get ksvc helloworld-java --output=custom-columns=NAME:.metadata.name,URL:.status.url

   Example:

   ```bash
   NAME                      URL
   helloworld-java    http://helloworld-java.default.1.2.3.4.xip.io
   ```

1. Now you can make a request to your app and see the result. Replace the following URL with the URL returned in the previous command.

   Example:

   curl http://helloworld-java.default.1.2.3.4.sslip.io
   Hello SparkJava Sample v1!
   # Even easier with kn:
   curl $(kn service describe helloworld-java -o url)
   

   Note: Add -v option to get more detail if the curl command failed.

Delete

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

=== "kn" bash kn service delete helloworld-java

=== "kubectl" bash kubectl delete --filename service.yaml