Deploying Java Applications on Kubernetes: Step-by-Step Guide

Deploying Java Applications on Kubernetes: From Dockerfile to a Working Service

Modern Java development is unthinkable without containerization and orchestration. Kubernetes (K8s) has become the standard for managing microservices, and the ability to deploy Java applications in a cluster is a mandatory skill for a backend developer. In this article, we will walk through the complete deployment cycle in practice: from building a Docker image with a Java application to configuring load balancing via Ingress.

1. Preparing the Java Application and Dockerfile

Before getting into Kubernetes, your Java application must be packaged into a Docker image. Let's consider a classic case — a Spring Boot application built into a JAR file.

Building the Project

Suppose we have a project that is built using Maven. Run the command in the project root:

mvn clean package -DskipTests

As a result, the file my-app-1.0.0.jar will appear in the target/ folder.

Creating a Dockerfile

To minimize the image size and increase security, we use a multi-stage build.

# Stage 1: build\nFROM maven:3.8.6-openjdk-17 AS builder\nWORKDIR /app\nCOPY pom.xml .\nRUN mvn dependency:go-offline -B\nCOPY src ./src\nRUN mvn package -DskipTests\n\n# Stage 2: final image\nFROM openjdk:17-jdk-slim\nWORKDIR /app\nCOPY --from=builder /app/target/my-app-1.0.0.jar app.jar\nEXPOSE 8080\nENTRYPOINT ["java", "-jar", "app.jar"]

Building and Publishing the Image

docker build -t my-java-app:1.0.0 .\ndocker tag my-java-app:1.0.0 your-dockerhub/my-java-app:1.0.0\ndocker push your-dockerhub/my-java-app:1.0.0

2. Creating Kubernetes Manifests

Kubernetes uses a declarative approach. We will describe the desired state of the system in YAML files.

Deployment — Managing Pods

Create a file deployment.yaml:

apiVersion: apps/v1\nkind: Deployment\nmetadata:\n  name: java-app-deployment\n  labels:\n    app: java-app\nspec:\n  replicas: 3\n  selector:\n    matchLabels:\n      app: java-app\n  template:\n    metadata:\n      labels:\n        app: java-app\n    spec:\n      containers:\n      - name: java-app\n        image: your-dockerhub/my-java-app:1.0.0\n        ports:\n        - containerPort: 8080\n        resources:\n          requests:\n            memory: "256Mi"\n            cpu: "250m"\n          limits:\n            memory: "512Mi"\n            cpu: "500m"\n        livenessProbe:\n          httpGet:\n            path: /actuator/health\n            port: 8080\n          initialDelaySeconds: 30\n          periodSeconds: 10\n        readinessProbe:\n          httpGet:\n            path: /actuator/health\n            port: 8080\n          initialDelaySeconds: 20\n          periodSeconds: 5

Service — Providing Access

Create a file service.yaml:

apiVersion: v1\nkind: Service\nmetadata:\n  name: java-app-service\nspec:\n  selector:\n    app: java-app\n  ports:\n  - protocol: TCP\n    port: 80\n    targetPort: 8080\n  type: ClusterIP

ConfigMap — Externalizing Configuration

To store environment variables, create configmap.yaml:

apiVersion: v1\nkind: ConfigMap\nmetadata:\n  name: app-config\ndata:\n  SPRING_PROFILES_ACTIVE: "prod"\n  DB_URL: "jdbc:postgresql://postgres-service:5432/mydb"

3. Deploying to the Cluster and Verification

Now let's apply all manifests to the cluster. Make sure you have kubectl configured and are connected to the correct context.

Applying Manifests

kubectl apply -f configmap.yaml\nkubectl apply -f deployment.yaml\nkubectl apply -f service.yaml

Checking Status

kubectl get pods\nkubectl get deployments\nkubectl get services

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