Docker enables developers to package applications along with their dependencies into portable containers. This approach simplifies deployment across different environments. A common use case involves containerizing a Sprinng Boot or other Java application packaged as an executable JAR file.

Prerequisites

• An executable .jar file (e.g., myapp.jar)
• A working Docker installation
• A Dockerfile in the project root

Writing the Dockerfile

The Dockerfile defines the steps to build the image. Below is a minimal and efficient example:

FROM eclipse-temurin:17-jre-alpine
WORKDIR /app
COPY myapp.jar app.jar
ENTRYPOINT ["java", "-jar", "app.jar"]

This configuration:

• Uses a lightweight JDK 17 JRE base image (eclipse-temurin:17-jre-alpine)
• Sets /app as the working directory inside the container
• Copies the local JAR into the container as app.jar
• Defines the startup command using ENTRYPOINT for better signal handling

Building the Image

From the directory containing both the JAR and the Dockerfile, run:

docker build -t my-java-app .

This creates a Docker image tagged as my-java-app.

Running the Container

To start the container and expose the application port (assuming it listens on port 8080):

docker run -d --name my-running-app -p 8080:8080 my-java-app

The application will now be accessible at http://localhost:8080. The -d flag runs the container in detached mode.