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Andrea Peruffo

Typed Java shell for Kubernetes extensions at fingertips

·3 mins

Working with Java vibrant tooling ecosystem on Kubernetes nowadays is a blast and you should know about it!


Wait, wait, what’s going on???

Let’s go through the BOM (Bill of Material) of the used tools first:

🌐 Kubernetes extensions #

On Kubernetes, the Operator pattern has quickly become a popular way to provide “Kubernetes native” experience for various services and applications.

The contract for “talking” to Operators is based on Custom Resource Definitions (CRD) and Custom Resources (CR) which are usually shipped as plain and agnostic yaml files.

I work on the Keycloak project and we will use its CRD for this post.

Try out your own!

🔥 Fabric8 kubernetes-client #

In the Java world, there is a top-notch kubernetes-client that enables seamless interaction with a Kubernetes cluster.

In particular, the fabric8 kubernetes-client provides a fully typed API for resources that gives Java developers a huge kick for safety and productivity.

Gimme more!

Starting from the upcoming version there is a new module called java-generator that takes CRDs as input spits output plain Java files that are directly usable from the previously mentioned resources typed API.

❗ Jbang #

jbang is an awesome tool for running Java programs with ease. Not just that, it already evolved into a pretty comprehensive tool for managing/packaging/using Java programs from the command line with minimal to no setup.

🔁 JShell #

jshell is a Read-Evaluate-Print Loop (REPL) for Java that can be used for learning and prototyping Java code. Here we are (ab)using it to be our interactive shell.

🍡 All together #

Glue all of this with a little bit of bash and you have a little shell script that can be executed as a one-liner. The script takes one argument being the URL of the (yaml or json) CRD we want to interact with, e.g.:

bash <(curl -sL \

The script will, in a temp directory:

  • download the CRD
  • generate the Java code for the CRD using the latest version (-SNAPSHOT) of the java-generator invoked using jbang
  • generate a little .java file containing a basic dependencies/sources configuration for the following steps
  • produce a single jar containing everything needed using jbang export
  • start a jshell session using jbang interactive mode

✈️ now enjoy the fun of experimenting with Java code on the fly! Quickly sort out the imports:

import io.fabric8.kubernetes.client.*;
import org.keycloak.k8s.v2alpha1.*;

Create a client for Kubernetes:

var client = new DefaultKubernetesClient();

You can already do pretty nifty things on your cluster with (typed!) commands like:

client.pods().list().getItems().stream().map(p -> p.getMetadata().getName()).collect(Collectors.joining(", "));

Create a client for the Keycloak CRD:

var kcClient = client.resources(Keycloak.class);

Play with your keycloak CRs in the cluster using, again, a fully typed API!

var kc = kcClient.list().getItems().get(0);

✌️ Conclusions #

Java ecosystem and tooling are growing, maturing, and improving at an incredible pace making it a first-tier choice for developing on top of Kubernetes as of today. In this post, we explored how to lively interact with a Kubernetes cluster and custom CRDs through a fully typed, generated on-the-fly, Java native API.

🏃‍♂️ Go and try it at home! 🏃‍♂️

Bear in mind that we are using some bleeding edge tech here but you can already play and take advantage of it as a developer. 😇