Kubernetes with Java - Running in the Cluster

Using Kubernetes API through a Java app from inside the Kubernetes cluster with RBAC

What are we going to do?

This post builds on top of Kubernetes with Java - Introduction and Kubernetes with Java - Asynchronous APIs posts which showed us how to interact with the Kubernetes API to list deployment resources and provide an API of our own to list teams and apps belonging to those teams running in the cluster. In this post, we will

  • Configure a spring-boot app to be able to access the Kubernetes API when running inside a Kubernetes cluster
  • Learn how to package up a spring-boot app into an OCI image (aka Docker image)
  • Configure RBAC with Role, RoleBinding, and ServiceAccount so our spring-boot app has the ability to run with only the Kubernetes APIs that we think it should have access to and nothing more
  • Create Deployment and Service resources to deploy our spring-boot app and expose its API within the cluster


  • Access to source code for the project - https://github.com/fnjoin/blog-team-apps
  • Access to a running Kubernetes cluster - we will assume minikube
  • A cluster admin role so we can use RBAC based role and service-account within Kubernetes. If you started a minikube instance, you already have this role

Connecting to Kubernetes API inside Kubernetes cluster

Earlier, we created an ApiClientConfig class in Kubernetes with Java - Introduction and added the mechanism to create an api-client which connects to the Kubernetes API server when the app is running outside of the cluster. We will now introduce a new bean which creates an api-client which knows how to communicate with the Kubernetes API server while running inside the cluster. Since other parts of the application expect only one api-client bean in the whole system, we have to configure that api-client appropriately depending on where the app is running. We will use the spring-boot profiles mechanism to differentiate between which environment we are running in when creating this bean.

We will introduce a cloud profile. When cloud profile is active, we will assume that the app is running inside the Kubernetes cluster, otherwise it will assume to be running outside. The code will look like the following:

public ApiClient internalApiClient() throws IOException {
    log.info("Using cluster client");
    return ClientBuilder

public ApiClient externalApiClient() throws IOException {
    KubeConfig kubeConfig = KubeConfig.loadKubeConfig(new FileReader(getConfigFile()));
    log.info("Current Context: Name={}", kubeConfig.getCurrentContext());
    return ClientBuilder

private File getConfigFile() {
    String kubeConfigVar = System.getenv("KUBECONFIG");
    if (StringUtils.isNotBlank(kubeConfigVar)) {
        log.info("Using KUBECONFIG variable: Value='{}'", kubeConfigVar);
        return new File(kubeConfigVar);
    } else {
        File configDir = new File(System.getProperty("user.home"), ".kube");
        File configFile = new File(configDir, "config");
        log.info("Using home file: Path='{}'", configFile.getPath());
        return configFile;

Notice, the usage of @Profile annotations. Also, see how simple the internalApiClient() method is. This is because, connecting to Kubernetes API while running inside the cluster itslef is super easy and we will be relying on Kubernetes Role Based Access Control (RBAC) security mechanisms to define which Kubernetes API our app will have access to. Also, the API endpoint, its hostname, port, and the way communication is secured is standard across all distributions of Kubernetes and that is why we didn’t have to specify any of that when creating this bean.

These are all the changes needed in our spring-boot application to be able to connect to the Kubernetes API while running inside the cluster itself. Now, we just need to make sure to run with the cloud profile when running inside the Kubernetes cluster.

Packaging spring-boot app in an OCI image

Now that our code is ready to be run inside Kubernetes, we need to package it as an OCI image. We also need to settle on the image name and which image registry the image will be hosted in. For this to happen, we need access to a running docker daemon, be able to to connect to it, and have acsess to an image regitry where our OCI image will be stored.

Linux has the capability to run a docker deamon natively. Also, docker-desktop is a good choice for Windows or Mac. Another interesting choice is exposing the docker daemon running inside minikube to the host machine - we will go with that last option.

Since we are using spring-boot version 2.5.4, it is very easy to package up our application as an OCI image. The capability to create OCI images is actually a spring-boot maven/gradle plugin capablity. It has been around since spring-boot version 2.3.0. For earlier versions of spring-boot, the Jib plugins for maven/gradle would have been a good option. In this day and age, rolling your own image with a Dockerfile is not worth it for spring-boot applications because of these other options.

We can kill two birds with one stone (only figuratively, we love birds!) by exposing the docker daemon running inside our minikube instance. This docker daemon will not only build our app image but will act as the image registry when it is time for Kubernetes to download/run the image - there will be nothing to download as the image will already be there. On Linux and Mac, the following will do the trick:

eval $(minikube -p minikube docker-env)

This command will expose the docker daemon in the current shell only. Next we will pick an image name. Lets say the image name will be sample/team-apps:1.0, we will issue the following command to create the image:

./gradlew bootBuildImage --imageName=sample/team-apps:1.0

This will build the app into an executable spring-boot jar, and then bundle it up as a OCI image using the Paketo Buildpacks. First time run takes a couple of minutes but subsequent image builds only take a few seconds. The output will look something like this for a first time build:

> Task :bootBuildImage
Building image 'docker.io/sample/team-apps:1.0'

 > Pulling builder image 'docker.io/paketobuildpacks/builder:base' ..................................................
 > Pulled builder image 'paketobuildpacks/builder@sha256:deeb603fef3faa8ef43b04f6d7d284a78a136a0872fcf7d6091f8d5a0a8fecc1'
 > Pulling run image 'docker.io/paketobuildpacks/run:base-cnb' ..................................................
 > Pulled run image 'paketobuildpacks/run@sha256:c6e071ac324b948037be8f950199e765a6b9eccce4e0b74f83f38755d528f534'
 > Executing lifecycle version v0.11.4
 > Using build cache volume 'pack-cache-21ce65b290df.build'

 > Running creator
    [creator]     ===> DETECTING
    [creator]     5 of 18 buildpacks participating
    [creator]     paketo-buildpacks/ca-certificates   2.4.1
    [creator]     paketo-buildpacks/bellsoft-liberica 8.5.0
    [creator]     paketo-buildpacks/executable-jar    5.2.1
    [creator]     paketo-buildpacks/dist-zip          4.2.1
    [creator]     paketo-buildpacks/spring-boot       4.6.0
    [creator]     ===> ANALYZING
    [creator]     Previous image with name "docker.io/sample/team-apps:1.0" not found
    [creator]     ===> RESTORING
    [creator]     ===> BUILDING
    [creator]     Paketo CA Certificates Buildpack 2.4.1
    [creator]       https://github.com/paketo-buildpacks/ca-certificates
    [creator]       Launch Helper: Contributing to layer
    [creator]         Creating /layers/paketo-buildpacks_ca-certificates/helper/exec.d/ca-certificates-helper
    [creator]     Paketo BellSoft Liberica Buildpack 8.5.0
    [creator]       https://github.com/paketo-buildpacks/bellsoft-liberica
    [creator]       Build Configuration:
    [creator]         $BP_JVM_TYPE                 JRE             the JVM type - JDK or JRE
    [creator]         $BP_JVM_VERSION              11.*            the Java version
    [creator]       Launch Configuration:
    [creator]         $BPL_HEAP_DUMP_PATH                          write heap dumps on error to this path
    [creator]         $BPL_JVM_HEAD_ROOM           0               the headroom in memory calculation
    [creator]         $BPL_JVM_LOADED_CLASS_COUNT  35% of classes  the number of loaded classes in memory calculation
    [creator]         $BPL_JVM_THREAD_COUNT        250             the number of threads in memory calculation
    [creator]         $JAVA_TOOL_OPTIONS                           the JVM launch flags
    [creator]       BellSoft Liberica JRE 11.0.12: Contributing to layer
    [creator]         Downloading from https://github.com/bell-sw/Liberica/releases/download/11.0.12+7/bellsoft-jre11.0.12+7-linux-amd64.tar.gz
    [creator]         Verifying checksum
    [creator]         Expanding to /layers/paketo-buildpacks_bellsoft-liberica/jre
    [creator]         Adding 129 container CA certificates to JVM truststore
    [creator]         Writing env.launch/BPI_APPLICATION_PATH.default
    [creator]         Writing env.launch/BPI_JVM_CACERTS.default
    [creator]         Writing env.launch/BPI_JVM_CLASS_COUNT.default
    [creator]         Writing env.launch/BPI_JVM_SECURITY_PROVIDERS.default
    [creator]         Writing env.launch/JAVA_HOME.default
    [creator]         Writing env.launch/JAVA_TOOL_OPTIONS.append
    [creator]         Writing env.launch/JAVA_TOOL_OPTIONS.delim
    [creator]         Writing env.launch/MALLOC_ARENA_MAX.default
    [creator]       Launch Helper: Contributing to layer
    [creator]         Creating /layers/paketo-buildpacks_bellsoft-liberica/helper/exec.d/active-processor-count
    [creator]         Creating /layers/paketo-buildpacks_bellsoft-liberica/helper/exec.d/java-opts
    [creator]         Creating /layers/paketo-buildpacks_bellsoft-liberica/helper/exec.d/jvm-heap
    [creator]         Creating /layers/paketo-buildpacks_bellsoft-liberica/helper/exec.d/link-local-dns
    [creator]         Creating /layers/paketo-buildpacks_bellsoft-liberica/helper/exec.d/memory-calculator
    [creator]         Creating /layers/paketo-buildpacks_bellsoft-liberica/helper/exec.d/openssl-certificate-loader
    [creator]         Creating /layers/paketo-buildpacks_bellsoft-liberica/helper/exec.d/security-providers-configurer
    [creator]         Creating /layers/paketo-buildpacks_bellsoft-liberica/helper/exec.d/security-providers-classpath-9
    [creator]       Java Security Properties: Contributing to layer
    [creator]         Writing env.launch/JAVA_SECURITY_PROPERTIES.default
    [creator]         Writing env.launch/JAVA_TOOL_OPTIONS.append
    [creator]         Writing env.launch/JAVA_TOOL_OPTIONS.delim
    [creator]     Paketo Executable JAR Buildpack 5.2.1
    [creator]       https://github.com/paketo-buildpacks/executable-jar
    [creator]       Class Path: Contributing to layer
    [creator]         Writing env/CLASSPATH.delim
    [creator]         Writing env/CLASSPATH.prepend
    [creator]       Process types:
    [creator]         executable-jar: java org.springframework.boot.loader.JarLauncher (direct)
    [creator]         task:           java org.springframework.boot.loader.JarLauncher (direct)
    [creator]         web:            java org.springframework.boot.loader.JarLauncher (direct)
    [creator]     Paketo Spring Boot Buildpack 4.6.0
    [creator]       https://github.com/paketo-buildpacks/spring-boot
    [creator]       Creating slices from layers index
    [creator]         dependencies
    [creator]         spring-boot-loader
    [creator]         snapshot-dependencies
    [creator]         application
    [creator]       Launch Helper: Contributing to layer
    [creator]         Creating /layers/paketo-buildpacks_spring-boot/helper/exec.d/spring-cloud-bindings
    [creator]       Spring Cloud Bindings 1.8.0: Contributing to layer
    [creator]         Downloading from https://repo.spring.io/release/org/springframework/cloud/spring-cloud-bindings/1.8.0/spring-cloud-bindings-1.8.0.jar
    [creator]         Verifying checksum
    [creator]         Copying to /layers/paketo-buildpacks_spring-boot/spring-cloud-bindings
    [creator]       Web Application Type: Contributing to layer
    [creator]         Reactive web application detected
    [creator]         Writing env.launch/BPL_JVM_THREAD_COUNT.default
    [creator]       4 application slices
    [creator]       Image labels:
    [creator]         org.springframework.boot.version
    [creator]     ===> EXPORTING
    [creator]     Adding layer 'paketo-buildpacks/ca-certificates:helper'
    [creator]     Adding layer 'paketo-buildpacks/bellsoft-liberica:helper'
    [creator]     Adding layer 'paketo-buildpacks/bellsoft-liberica:java-security-properties'
    [creator]     Adding layer 'paketo-buildpacks/bellsoft-liberica:jre'
    [creator]     Adding layer 'paketo-buildpacks/executable-jar:classpath'
    [creator]     Adding layer 'paketo-buildpacks/spring-boot:helper'
    [creator]     Adding layer 'paketo-buildpacks/spring-boot:spring-cloud-bindings'
    [creator]     Adding layer 'paketo-buildpacks/spring-boot:web-application-type'
    [creator]     Adding 5/5 app layer(s)
    [creator]     Adding layer 'launcher'
    [creator]     Adding layer 'config'
    [creator]     Adding layer 'process-types'
    [creator]     Adding label 'io.buildpacks.lifecycle.metadata'
    [creator]     Adding label 'io.buildpacks.build.metadata'
    [creator]     Adding label 'io.buildpacks.project.metadata'
    [creator]     Adding label 'org.springframework.boot.version'
    [creator]     Setting default process type 'web'
    [creator]     Saving docker.io/sample/team-apps:1.0...
    [creator]     *** Images (47750fba3362):
    [creator]           docker.io/sample/team-apps:1.0

Successfully built image 'docker.io/sample/team-apps:1.0'


There is a lot that went on with that image build - all the heavy lifting was done by the buildpacks. We will leave the details of using buikdpacks for to build app images for a future post.

Setup RBAC in Kubernetes

To keep things simple, we will run our app in the same namespace (dev) that our sample apps from Kubernetes with Java - Introduction post were running in. The details of the Kubernetes RBAC mechanism is described very well in the official Kubernetes documentation but we will just go over the highlights here.

We need to create a Role and a ServiceAccount first. Then create a RoleBinding to link the Role and ServiceAccount together. A ServiceAccount can be associated with many Role objects, the reverse is true as well. All three of these resources are configured using YAML definitions with kubectl. They are included in the source code in the k8s directory as rbac.yaml. Apply them to the cluster using the following command (assuming that we are running from the root of the project):

kubectl apply -f k8s/rbac.yaml

The rbac.yaml contents look like:

apiVersion: v1
kind: ServiceAccount
  name: team-apps
  namespace: dev
apiVersion: rbac.authorization.k8s.io/v1
kind: Role
  name: team-apps-role
  namespace: dev
  - apiGroups:
      - apps
      - deployments
      - watch
      - get
      - list
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
  name: team-apps-role-binding
  namespace: dev
  - kind: ServiceAccount
    name: team-apps
    namespace: dev
  kind: Role
  name: team-apps-role
  apiGroup: rbac.authorization.k8s.io

The service-account definition is just a name at this point. The role is where the Kubernetes API permissions are defined. In this case, we are only allowing watch, get, and list operations for the deployment resources. The deployment resources are part of the apps API group - this is the same reason when we dealt with the V1Deployment objects in Java, we got hold of them using the AppsV1Api.listNamespacedDeployment*() method. Finally, the role-binding binds the role and service-account together.

Deploy the app in Kubernetes

The source code includes the app.yaml file in k8s directory. This file includes the Deployment and Service resource definitions that we need to apply using kubectl. The command will look like the following:

kubectl apply -f k8s/app.yaml

The contents of app.yaml look like:

apiVersion: apps/v1
kind: Deployment
  name: team-apps
  namespace: dev
  replicas: 1
      app: team-apps
        app: team-apps
      serviceAccountName: team-apps
      restartPolicy: Always
        - name: app
          image: example/team-apps:1.0
            - name: SPRING_PROFILES_ACTIVE
              value: cloud,async
            - name: web
              containerPort: 8080
apiVersion: v1
kind: Service
  name: team-apps-service
  namespace: dev
    app: team-apps
    - port: 8181
      targetPort: web

Notice the usage of serviceAccountName in the deployment manifest. Without speciying that, our app will not have permissions to use the Kubernetes API when running inside the cluster. Also, notice that we supplied the SPRING_PROFILES_ACTIVE environment variable be set to cloud,async so those Spring profiles are active when the app starts up.

Accessing the team-apps API

If you want to access the API inside the cluster and you are trying to do it from a pod running in the dev namesapce then the URL would simply be - http://team-apps-service:8181

If you want to access the API inside the cluster but you are trying to do it from a pod running in a namespace other than dev then the URL would be - http://team-apps-service.dev.svc.cluster.local:8181

If you want to access the API from outside the cluster, there are a lot more options and complexities to be considered. But the easiest way to verify that the service is properly working is simply setting up a port-forward - which can be done with the following command:

kubectl port-forward service/team-apps-service -n dev 8282:8181

This will make port 8282 on localhost forward to port 8181 on the team-apps-service service, which will then forward the request to port 8080 on the app container.

The decision to use different ports (8080, 8181, 8282) is just for demonstration purposes. It is to show the levels of port-forwarding or proxying that is involved in this way of accessing our app

See also