Project: CDK for Kubernetes (CDK8s) (Provisioning / Automation & Configuration)
File: java.md

In this guide, we'll walk you through the process of building a basic Kubernetes application using cdk8s for Java. To install the cdk8s CLI, you need the installed on your local machine. cdk8s for Java supports Java Development Kit (JDK) versions 8+. Check your Java version: ```bash java --version ``` To get setup, follow these general steps: To begin, you'll need Maven 3.6.3+ installed. If you're not familiar with this setup, use these instructions to . To initialize a cdk8s project and auto-generate Kubernetes manifests based on our code, we need the cdk8s CLI: Run the following command to install the CLI using . For more installation methods, see . ```console npm install -g cdk8s-cli ``` Next, well initialize a project to create the directory structure and install the necessary dependencies using the command. In a terminal window, run the following command in an empty directory: ```console cdk8s init java-app ``` In your preferred code editor, open the `Main.java` file. ```java package com.mycompany.app; import software.constructs.Construct; import org.cdk8s.App; import org.cdk8s.Chart; import org.cdk8s.ChartProps; public class Main extends Chart { public Main(final Construct scope, final String id) { this(scope, id, null); } public Main(final Construct scope, final String id, final ChartProps options) { super(scope, id, options); // define resources here } public static void main(String[] args) { final App app = new App(); new Main(app, "java"); app.synth(); } } ``` This sample shows the basic structure of a cdk8s application with the essential libraries: constructs and cdk8s. These libraries supply the foundational classes and methods required for working with cdk8s. It includes the following components: A custom `Main` class inherits from the base class provided by the cdk8s library, serving as a representation of the Kubernetes resources to be generated and managed. The `Main` constructor method is responsible for initializing the base class () and specifying the Kubernetes resources. This method is invoked when creating an instance of the class. An instance of the class is created in the `main` function, which acts the primary entry point of the cdk8s application and oversees the application's lifecycle and resources. An instance of the `Main` class is created by passing the app instance and a string identifier, "java", as arguments. This action generates and registers the Kubernetes resources defined in the `Main` class within the application. Note that in this example, the "java" string identifier is autogenerated based on the current directory name. The `synth` method is called on the `app` instance, which produces the required Kubernetes YAML manifest files based on the defined resources. Note that in this example, we haven't defined any resources within the `Main` constructor, so running the "cdk8s synth" command in the CLI would generate a blank Kubernetes

Now, we are ready to define Kubernetes resources for our application. In this sample, we define a basic Kubernetes for a sample application. We start by importing the `imports.k8s` sub-directory, which contains all the cdk8s Kubernetes classes and functions we'll need. Copy and paste the following code sample into the existing `Main.java` file of your project. ```java package com.mycompany.app; import software.constructs.Construct; import org.cdk8s.App; import org.cdk8s.Chart; import org.cdk8s.ChartProps; import imports.k8s.Container; import imports.k8s.ContainerPort; import imports.k8s.DeploymentSpec; import imports.k8s.KubeDeployment; import imports.k8s.KubeDeploymentProps; import imports.k8s.LabelSelector; import imports.k8s.ObjectMeta; import imports.k8s.PodSpec; import imports.k8s.PodTemplateSpec; import java.util.Collections; import java.util.List; import java.util.Map; public class Main extends Chart { public Main(final Construct scope, final String id, final String appLabel) { this(scope, id, null, appLabel); } public Main(final Construct scope, final String id, final ChartProps props, final String appLabel) { super(scope, id, props); Map<String, String> label = Collections.singletonMap("app", appLabel); new KubeDeployment(this, "my-deployment", KubeDeploymentProps.builder() .spec(DeploymentSpec.builder() .replicas(3) .selector(LabelSelector.builder() .matchLabels(label) .build()) .template(PodTemplateSpec.builder() .metadata(ObjectMeta.builder().labels(label) .build()) .spec(PodSpec.builder() .containers(List.of(Container.builder() .name("app-container-test") .image("nginx:1.19.10") .ports(List.of(ContainerPort.builder() .containerPort(80) .build())) .build())) .build()) .build()) .build()) .build()); } public static void main(String[] args) { final App app = new App(); new Main(app, "getting-started", "my-app"); app.synth(); } } ``` A few things worth noting about this sample: The constructor in the custom `Main` class leverages Java's object-oriented programming features to construct a Kubernetes Deployment. The Deployment is defined with specific parameters, including the number of replicas, label selectors, and pod specifications. This constructor employs Java's strong typing and encapsulation to dynamically set the "app" key in label selectors and metadata labels for our Kubernetes resources, creating a clear and concise way to set key configuration details. After you have defined the Kubernetes resources for your application, you are ready to generate the Kubernetes manifest that will define your Deployment resource. Open a terminal and navigate to your project directory. Run the command. This command generates a Kubernetes manifest file in the `dist` folder of your project directory. The manifest file contains all the resources you defined inside the `MyChart` class. ```console cdk8s synth ``` Open the `dist/getting-started.k8s.yaml` file. You should see a Kubernetes manifest similar to the following: ```yaml apiVersion: apps/v1 kind: Deployment metadata: name: getting-started-my-deployment-c85252a6 spec: replicas: 3 selector: matchLabels: app: my-app template: metadata: labels: app: my-app spec: containers: image: nginx:1.19.10 name: app-container ports: containerPort: 80 ``` Throughout this guide, we introduced you to the cdk8s Java library and guided you through the process of creating a cdk8s Java application. We initiated a simple project and constructed a Kubernetes Deployment using cdk8s code. This included leveraging Java-specific programming language conventions to dynamically set the "app" key in "label" selectors and "metadata" labels for Kubernetes resources using Java's versatile strong typing and encapsulation features. To run a complete code sample, we recommend diving into the Kubernetes and using the sample application.