In this section, we will explore how to containerize a Spring Boot application using Docker. Docker is a platform that allows you to package applications and their dependencies into a standardized unit called a container. Containers are lightweight, portable, and ensure that your application runs consistently across different environments.

• Understand the basics of Docker.
• Learn how to create a Dockerfile for a Spring Boot application.
• Build and run a Docker image.
• Use Docker Compose to manage multi-container applications.

Docker is an open-source platform designed to automate the deployment, scaling, and management of applications. It uses containerization technology to package an application and its dependencies into a container.

• Image: A lightweight, standalone, and executable software package that includes everything needed to run a piece of software, including the code, runtime, libraries, and settings.
• Container: A runtime instance of an image. Containers are isolated from each other and the host system.
• Dockerfile: A text file that contains a series of instructions on how to build a Docker image.

A Dockerfile is a script that contains a series of instructions on how to build a Docker image. Below is an example of a Dockerfile for a Spring Boot application.

1. FROM openjdk:11-jre-slim: Specifies the base image to use. In this case, it's an OpenJDK runtime.
2. WORKDIR /app: Sets the working directory inside the container to /app.
3. COPY target/my-spring-boot-app.jar /app/my-spring-boot-app.jar: Copies the Spring Boot jar file from the host machine to the container.
4. EXPOSE 8080: Informs Docker that the container listens on port 8080.
5. ENTRYPOINT ["java", "-jar", "my-spring-boot-app.jar"]: Specifies the command to run the application.

To build the Docker image, navigate to the directory containing the Dockerfile and run the following command:

To run the Docker container, use the following command:

This command maps port 8080 on the host to port 8080 in the container.

Docker Compose is a tool for defining and running multi-container Docker applications. With Docker Compose, you can use a YAML file to configure your application's services.

To start the application using Docker Compose, run the following command:

This command builds the image (if it doesn't already exist) and starts the container.

1. Create a simple Spring Boot application.
2. Write a Dockerfile to containerize the application.
3. Build the Docker image.
4. Run the Docker container.
5. Use Docker Compose to manage the container.

1. Create a Spring Boot Application: Use Spring Initializr to generate a simple Spring Boot application.
2. Write a Dockerfile: Use the example Dockerfile provided above.
3. Build the Docker Image: Run docker build -t my-spring-boot-app .
4. Run the Docker Container: Run docker run -p 8080:8080 my-spring-boot-app
5. Use Docker Compose: Create a docker-compose.yml file and run docker-compose up

• Forgetting to expose the port: Ensure you include the EXPOSE instruction in your Dockerfile.
• Incorrect file paths: Double-check the paths in the COPY instruction to ensure the jar file is correctly copied.
• Not cleaning up: Use docker system prune to clean up unused images and containers to free up space.

In this section, you learned how to containerize a Spring Boot application using Docker. You created a Dockerfile, built a Docker image, and ran a Docker container. Additionally, you used Docker Compose to manage multi-container applications. These skills are essential for deploying and managing Spring Boot applications in a consistent and portable manner.