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Building your first Kubernetes controller in Java starts with a simple loop: observe what the cluster has, compare it with what a user asked for, and make changes until the two agree. For a Java implementation, the Java Operator SDK (JOSDK) adds controller and reconciliation machinery on top of the Fabric8 Kubernetes client; neither is required by Kubernetes itself.
What a Kubernetes controller does
A controller is an API client that continuously observes Kubernetes objects and works to make actual cluster state match desired state. It is not a one-time script: the controller may be called again after an event, a retry, or a later observation, so its work must remain safe when repeated.
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An operator is a common way to package this pattern around an application-specific API. Kubernetes describes an operator as an API client acting as a controller for a custom resource. For example, a custom resource could declare the desired version and replica count of an application; the controller would create or update ordinary Kubernetes resources to move the cluster toward that specification.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11A typical operator combines a CustomResourceDefinition (CRD), controller code, and a container image. The CRD makes a new resource type available to the Kubernetes API; the controller watches that resource and manages related state. Controllers commonly run outside the control plane and can run in the cluster as a Deployment. Kubernetes does not prescribe Java, JOSDK, or a particular client library. Kubernetes: Operator pattern
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Choose the smallest useful first project
Start with behavior that has an observable desired state and a manageable set of resources. A custom resource is useful when users need to declare that state through a Kubernetes API object. If that is unnecessary, a controller for an existing built-in resource can still be a valid learning project; JOSDK supports standard-resource controllers as well as custom resources.
Before adding code, write down what the controller should observe, what it may change, and what success looks like. A narrow example might be: “When this application resource exists, ensure one Deployment has the requested image and replica count.” Avoid beginning with a broad application lifecycle or many dependent resource types.
Choose an implementation level
JOSDK and Fabric8 are not competing client ecosystems: JOSDK uses Fabric8 as its Kubernetes client foundation. The practical choice is how much operator runtime machinery you want to adopt.
| Approach | What it provides | When it fits |
|---|---|---|
| JOSDK with Fabric8 | Controller runtime and operator-oriented features such as event handling, dependent resources, retries, scheduling, error handling, and testing support. | You want a higher-level structure for reconciliation and common operator lifecycle concerns. |
| Fabric8 directly | A Java Kubernetes client for API interactions, with configuration options and a mock server for testing API responses. | You want direct control over client interactions and are prepared to build more of the controller lifecycle and conventions yourself. |
| Official Kubernetes Java client | A Java client documented by Kubernetes for accessing the API. | You want to use the official client and have selected its conventions and runtime approach deliberately. |
JOSDK is a framework option, not a Kubernetes-mandated standard. Fabric8 and the official Kubernetes Java client should be evaluated against the APIs you need, the Kubernetes versions supported by the specific client release, project conventions, and how much operator runtime support you want. The Kubernetes Java client documentation points to release information for compatibility; no single current compatibility matrix or artifact combination is established here. Check current project release documentation and keep dependencies from a compatible release set rather than combining versions copied from unrelated examples. Java Operator SDK repository Fabric8 Kubernetes Client repository Kubernetes: Accessing the API
Shape the custom resource API
If the project needs a custom resource, decide its API shape before writing reconciliation logic. Put user-controlled desired state in the specification, define validation deliberately, and decide what useful outcome or observed state belongs in status. Treat this as a public API: changing field meaning later can affect manifests, users, and deployed controllers.
JOSDK documentation describes generating CRD manifests from annotated Java custom-resource classes with Fabric8’s crd-generator-apt. Generated files are placed under target/classes/META-INF/fabric8. You can instead author and review the CRD manifest directly. Whichever route you choose, include the resulting CRD in the release and deployment workflow so the API type exists before users create instances. Quarkus extension users do not need to add the generator dependency separately. Java Operator SDK features
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Implement reconciliation as a repeatable operation
A reconciler should read the resource and relevant dependent state, compare what exists with the requested state, and make only the changes needed. It should also report meaningful progress or errors so an operator or user can understand what happened.
Make repeated calls converge
Idempotency is essential because reconciliation can happen more than once. The Java Operator SDK Reconciler API documentation states: “The implementation of this operation is required to be idempotent.” In practical terms, processing the same desired state repeatedly should converge on the same result rather than create duplicate resources or repeat unsafe side effects.
For example, do not blindly create a Deployment on every call. Look up the corresponding object, create it if absent, and update fields that differ when it exists. Consider deletion and ownership behavior explicitly as well: a controller should only remove resources it is responsible for, and should handle a resource that has already disappeared.
Use status updates deliberately
JOSDK’s UpdateControl manages updates to the custom resource, usually its status. Keep user intent in the specification and report controller observations in status rather than rewriting the desired configuration. The API contract and available control options are documented in the JOSDK Reconciler API source.
Test decisions separately from cluster behavior
Test the desired-state decisions independently from the mechanics of calling the Kubernetes API. Then test API interactions using the chosen framework’s testing support. Fabric8 documents a mock server that can return expected API responses; it is useful for checking client behavior, but it is not a complete Kubernetes API server and cannot establish every behavior of a real cluster. JOSDK also provides framework-level testing support. Fabric8 Kubernetes Client repository Java Operator SDK repository
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Finally, run an integration check against a real cluster for behaviors that depend on API-server validation, permissions, watches, or resource interactions that mocks do not fully reproduce. The exact test setup depends on the Kubernetes distribution and resources your controller uses.
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Configure cluster access and deploy
Where the controller runs determines how it authenticates. During local development, Kubernetes Java client guidance describes kubeconfig-based access. Fabric8 also documents kubeconfig and service-account configuration. In a cluster, a controller commonly runs as a Deployment and uses a service account with permissions appropriate to its work. Kubernetes: Accessing the API Fabric8 Kubernetes Client repository
Derive RBAC from the resources the controller actually watches and changes, including the verbs it needs. A controller that reads a resource and updates a related object needs different permissions from one that also deletes resources or updates custom-resource status. Avoid granting broad access merely to make an initial deployment work. Package the controller as a containerized workload and deploy the CRD with the rest of the operator’s installation artifacts.
Quick Recap
A practical build sequence
- Pick one behavior. State the desired outcome and the Kubernetes objects involved; use a custom resource only if users need an API object to express that intent.
- Select the runtime and client. Decide whether JOSDK’s higher-level reconciliation support or a direct client approach fits, then choose compatible releases from current project documentation.
- Define the API. Specify fields, validation, and status responsibilities. Generate the CRD from annotated Java classes with Fabric8’s generator or maintain the manifest directly.
- Write the reconciler. Observe relevant state, compare it with the specification, apply only necessary changes, and make repeated calls safe.
- Test at multiple levels. Check desired-state logic, exercise API interactions with framework or client test tools, and verify cluster-dependent behavior with an integration test.
- Package and grant access. Ship the controller and CRD, deploy the workload, and scope its service-account permissions to the resources and operations it needs.
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