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Hello World - Java (Micronaut)
Learn how to deploy a simple web app that is written in Java and uses Micronaut.
This samples uses Docker to build locally. The app reads in a TARGET
env
variable and then prints “Hello World: ${TARGET}!". If a value for TARGET
is
not specified, the “NOT SPECIFIED” default value is used.
Use this sample to walk you through the steps of creating and modifying the sample app, building and pushing your container image to a registry, and then deploying your app to your Knative cluster.
Before you begin
You must meet the following requirements to complete this sample:
- A version of the Knative Serving component installed and DNS configured. Follow the Knative installation instructions if you need to create a Knative cluster.
- The following software downloaded and install on your loacal machine:
- Java SE 8 or later JDK.
- Micronaut 1.1.
- Docker for building and pushing your container image.
- curl to test the sample app after deployment.
- A Docker Hub account where you can push your container image.
Tip: You can clone the Knatve/docs repo and then modify the source files. Alternatively, learn more by manually creating the files yourself.
Creating and configuring the sample code
To create and configure the source files in the root of your working directory:
-
Create the
pom.xml
file:<modelVersion>4.0.0</modelVersion> <groupId>com.example.micronaut</groupId> <artifactId>helloworld</artifactId> <version>1.0.0-SNAPSHOT</version> <properties> <micronaut.version>1.1.0</micronaut.version> <jdk.version>1.8</jdk.version> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> <exec.mainClass>com.example.helloworld.Application</exec.mainClass> </properties> <dependencyManagement> <dependencies> <dependency> <groupId>io.micronaut</groupId> <artifactId>micronaut-bom</artifactId> <version>${micronaut.version}</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies> </dependencyManagement> <dependencies> <dependency> <groupId>io.micronaut</groupId> <artifactId>micronaut-inject</artifactId> <scope>compile</scope> </dependency> <dependency> <groupId>io.micronaut</groupId> <artifactId>micronaut-validation</artifactId> <scope>compile</scope> </dependency> <dependency> <groupId>io.micronaut</groupId> <artifactId>micronaut-runtime</artifactId> <scope>compile</scope> </dependency> <dependency> <groupId>io.micronaut</groupId> <artifactId>micronaut-http-client</artifactId> <scope>compile</scope> </dependency> <dependency> <groupId>io.micronaut</groupId> <artifactId>micronaut-http-server-netty</artifactId> <scope>compile</scope> </dependency> <dependency> <groupId>ch.qos.logback</groupId> <artifactId>logback-classic</artifactId> <version>1.2.3</version> <scope>runtime</scope> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-shade-plugin</artifactId> <version>3.1.0</version> <executions> <execution> <phase>package</phase> <goals> <goal>shade</goal> </goals> <configuration> <transformers> <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer"> <mainClass>${exec.mainClass}</mainClass> </transformer> <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/> </transformers> </configuration> </execution> </executions> </plugin> </plugins> </build> </project>
-
Create the
HelloWorldController.java
file in thesrc/main/java/com/example/helloworld
directory. The[ROOT]/src/main/java/com/example/helloworld/HelloWorldController.java
file handles requests to the root URI/
.package com.example.helloworld; import io.micronaut.http.MediaType; import io.micronaut.http.annotation.Controller; import io.micronaut.http.annotation.Get; @Controller("/") public class HelloWorldController { @Get(value = "/", produces = MediaType.TEXT_PLAIN) public String index() { String target = System.getenv("TARGET"); if (target == null) { target = "NOT SPECIFIED"; } return "Hello World: " + target; } }
-
The Micronaut application is configured via
src/main/resources/application.yml
:micronaut: application: name: helloworld-java-micronaut server: port: ${PORT:8080}
-
Create the
Dockerfile
file:# 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-1.0.0-SNAPSHOT.jar /helloworld.jar # Run the web service on container startup. CMD ["java","-jar","/helloworld.jar"]
-
Create the
service.yaml
file. You must specify your Docker Hub username in{username}
. You can also configure theTARGET
, for example you can modify theMicronaut Sample v1
value.apiVersion: serving.knative.dev/v1 kind: Service metadata: name: helloworld-java-micronaut namespace: default spec: template: spec: containers: - image: docker.io/{username}/helloworld-java-micronaut env: - name: TARGET value: "Micronaut Sample v1"
Building and deploying the sample
To build a container image, push your image to the registry, and then deploy your sample app to your cluster:
-
Use Docker to build your container image and then push that image to your Docker Hub registry. You must replace the
{username}
variables in the following commands with your Docker Hub username.# Build the container on your local machine docker build -t {username}/helloworld-java-micronaut . # Push the container to docker registry docker push {username}/helloworld-java-micronaut
-
Now that your container image is in the registry, you can deploy it to your Knative cluster by running the
kubectl apply
command:kubectl apply --filename service.yaml
Result: A service name
helloworld-java-micronaut
is created in your cluster along with the following resources:- A new immutable revision for the version of the app that you just deployed.
- The following networking resources are created for your app:
- route
- ingress
- service
- load balancer
- Auto scaling is enable to allow your pods to scale up to meet traffic, and also back down to zero when there is no traffic.
Testing the sample app
To verify that your sample app has been successfully deployed:
-
Retrieve the URL for your service, by running the following
kubectl get
command:kubectl get ksvc helloworld-java-micronaut --output=custom-columns=NAME:.metadata.name,URL:.status.url
Example result:
NAME URL helloworld-java-micronaut http://helloworld-java-micronaut.default.1.2.3.4.xip.io
-
Now you can make a request to your app and see the result. Replace the URL below with the URL returned in the previous command.
curl http://helloworld-java-micronaut.default.1.2.3.4.xip.io
Example result:
Hello World: Micronaut Sample v1
Congratulations on deploying your sample Java app to Knative!
Removing the sample app deployment
To remove the sample app from your cluster, run the following kubectl delete
command:
kubectl delete --filename service.yaml
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