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Hello World - Spring Boot Java

This guide describes the steps required to create the helloworld-java-spring sample app and deploy it to your cluster.

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-spring

Prerequisites

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.
Optional. The Knative CLI client kn can be used to simplify the deployment. Alternatively, you can use kubectl to apply YAML resource files.

Building the sample app

From the console, create a new, empty web project by 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.

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:World}")
 String target;

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

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

Run the application locally:

mvn wrapper:wrapper
./mvnw package && java -jar target/helloworld-0.0.1-SNAPSHOT.jar

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

In your project directory, create a file named Dockerfile and copy the following code block into it:

# 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 the Official OpenJDK image for a lean production stage of our multi-stage build.
# https://hub.docker.com/_/openjdk
# https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
FROM openjdk:8-jre-alpine

# 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", "-jar", "/helloworld.jar"]

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.

Use Docker to build the sample code into a container, then push the container to the Docker registry:

# Build and push the container on your local machine.
docker buildx build --platform linux/arm64,linux/amd64 -t "{username}/helloworld-java-spring" --push .

Where {username} is your Docker Hub username.

Deploying the app

After the build has completed and the container is pushed to Docker Hub, you can deploy the app into your cluster.

During the creation of a Service, Knative performs 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 scaling down to zero active pods.

Choose one of the following methods to deploy the app:

yaml

Create a new file named service.yaml and copy the following service definition into the file:

apiVersion: serving.knative.dev/v1
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"

Where {username} is your Docker Hub username.

Note: Ensure that the container image value in service.yaml matches the container you built in the previous step.

Apply the YAML file by running the command:
```
kubectl apply -f service.yaml
```

kn

With kn you can deploy the service with
```
kn service create helloworld-java-spring --image=docker.io/{username}/helloworld-java-spring --env TARGET="Spring Boot 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.

Verification

Find the domain URL for your service:

For kubectl, run:

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

Example:

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

For kn, run:

kn service describe helloworld-java-spring -o url