Using Kubernetes ConfigMaps with Java: Best Practices and Examples
Tags: kubernetes, cloudnative, java, configmap
Published: 2023-01-11 08:44:55

In Kubernetes, a `configMap` is a way to store configuration data as key-value pairs. The data stored in a `configMap` can be used to configure pods and other objects in a Kubernetes cluster.

A configMap can be created and managed using the Kubernetes API, `kubectl` command-line tool, or declaratively in a yaml file. Once a `configMap` is created, it can be referenced by pods and other objects in a Kubernetes cluster.

Some common use cases for configMap are:

- Storing configuration files, such as application properties files, that need to be shared across multiple pods or containers.
- Storing environment variables that need to be passed to a container running in a pod.
- Storing initialization scripts or command-line arguments that need to be passed to a container at runtime.
- In addition to being referenced by pods, configMap can also be consumed by other resources such as `Deployment`, `StatefulSet` using the `envFrom` field.

It is worth noting that configMap is great for storing non-sensitive data like configuration properties, environment variables, and other metadata. When dealing with sensitive data like passwords and secret keys, it's recommended to use Secret instead.

In this article will try to consume `configMap` by Java code. This solution assumes the `configMap` is present and valid in the system and our code will use the Kubernetes Java client library.

To set up the Kubernetes Java client library in your Java project, you can use a build tool such as Maven or Gradle.

Here's an example of how to set it up with Maven:

Add the following dependency to your pom.xml file:

```
<dependency>
    <groupId>io.kubernetes</groupId>
    <artifactId>client-java</artifactId>
    <version>12.2.2</version>
</dependency>
```
This will include the Kubernetes Java client library and its dependencies in your project.

Below sample Java program to read a configMap from a Kubernetes cluster:

```
public class ConfigMapReader {
    public static void main(String[] args) {
        try {
            // Set up the Kubernetes API client
            ApiClient client = Configuration.getDefaultApiClient();
            // Configure the client to use the Kubernetes service account
            client.setAuth("BearerToken", "YOUR_BEARER_TOKEN");
            // Create a CoreV1Api object
            CoreV1Api api = new CoreV1Api(client);
            // Specify the name of the configMap and namespace
            String configMapName = "example-configmap";
            String namespace = "default";
            // Read the configMap from the Kubernetes API
            V1ConfigMap configMap = api.readNamespacedConfigMap(configMapName, namespace, null, null, null);
            // Print the configMap data
            System.out.println(configMap.getData());
        } catch (ApiException e) {
            System.err.println("ApiException: " + e.getResponseBody());
        } catch (Exception e) {
            System.err.println("Exception: " + e.getMessage());
        }
    }
}
```

This example uses the Kubernetes Java client library to interact with the Kubernetes API. It requires to set up the Kubernetes API client, and configure the client to use a bearer token. It then creates a `CoreV1Api` object and uses it to read a `configMap`. The `readNamespacedConfigMap` method takes several arguments, including the name of the `configMap` and the `namespace` it belongs to, and returns a `V1ConfigMap` object, containing the data of the `configMap`.

This is a simple example that only demonstrate the basic mechanism to read a `configmap`. In a real-world application, you will probably want to handle potential exceptions, make sure that the client is configured correctly, and handle cases where the `configMap` is not found or the API call fails.

Additionally, to read the values from the `configMap`, you can use `configMap.getData().get("KEY_NAME")`

It is also good to note that you should follow the best practices for handling authentication and authorization for your use-case.

More details of Kubernetes Client library can be found:
 
- [client-libraries](https://kubernetes.io/docs/reference/using-api/client-libraries/)
- [Java-Kubernetes-client](https://github.com/kubernetes-client/java)

If you have any further questions or thoughts on this topic, please don't hesitate to reach out in the comments section.
Thank you for reading!