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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 ${TARGET}!
. If TARGET
is not specified, World
is used as the default value.
You can also download a working copy of the sample, by running the following commands:
git clone -b "master" https://github.com/knative/docs knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-java-spring
Prerequisites
- A Kubernetes cluster with Knative installed and DNS configured. Follow the installation instructions.
- Docker installed and running on your local machine, and a Docker Hub account configured.
- (optional) The Knative CLI client kn can be used to simplify the deployment. Alternatively, you can use
kubectl
, and apply resource files directly.
Building the sample app
-
From the console, create a new, empty web project by using the curl and unzip commands:
curl https://start.spring.io/starter.zip \ -d dependencies=web \ -d name=helloworld \ -d artifactId=helloworld \ -o helloworld.zip unzip helloworld.zip
If you don’t have
curl
installed, you can accomplish the same by visiting the Spring Initializr page. Specify Artifact ashelloworld
and add theWeb
dependency. Then clickGenerate Project
, download and unzip the sample archive. -
Update the
SpringBootApplication
class insrc/main/java/com/example/helloworld/HelloworldApplication.java
by adding a@RestController
to handle the “/” mapping and also add a@Value
field to provide theTARGET
environment variable:package com.example.helloworld; import org.springframework.beans.factory.annotation.Value; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.web.bind.annotation.GetMapping; import org.springframework.web.bind.annotation.RestController; @SpringBootApplication public class HelloworldApplication { @Value("${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:
./mvnw package && java -jar target/helloworld-0.0.1-SNAPSHOT.jar
Go to
http://localhost:8080/
to see yourHello World!
message. -
In your project directory, create a file named
Dockerfile
and copy the code block below 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 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", "-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.
NOTE: 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.
-
Use Docker to build the sample code into a container, then push the container to the Docker registry:
# 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
Deploying the app
After the build has completed and the container is pushed to Docker Hub, you can deploy the app into your cluster.
-
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/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: "Java Spring Sample v1"
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 --filename service.yaml
With kn
you can deploy the service with
kn service create helloworld-java-spring --image=docker.io/{username}/helloworld-java-spring --env TARGET="Java Spring 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.
During the creation of your 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 balance for your app.
- Automatically scale your pods up and down, including scaling down to zero active pods.
Verification
-
Find the domain URL for your service:
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
kn service describe helloworld-java-spring -o url
Example:
http://helloworld-java-spring.default.1.2.3.4.xip.io
-
Make a request to your app and observe the result. Replace the URL below with the URL returned in the previous command.
Example:
curl http://helloworld-java-spring.default.1.2.3.4.xip.io Hello Java Spring Sample v1! # Even easier with kn: curl $(kn service describe helloworld-java-spring -o url)
Note: Add
-v
option to get more detail if thecurl
command failed.
Removing
To remove the sample app from your cluster, delete the service record.
kubectl delete --filename service.yaml
kn service delete helloworld-java-spring
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