Crafting Docker Containers for Your Spring Boot Applications

Introduction

Docker containers offer a lightweight and portable way to package and deploy your Spring Boot applications. This article guides you through the essential steps of creating a Docker container for your Spring Boot project, enabling you to streamline your development and deployment workflows.

Prerequisites

• Docker installed and running on your system. Here !.
• A Spring Boot project with a generated executable JAR file (typically located in target/ directory).
• Basic understanding of Docker concepts.

Create a Dockerfile:

In the root directory of your Spring Boot project, create a plain text file named Dockerfile. This file specifies the instructions for building your Docker image.

Define the Base Image:

The first line in your Dockerfile specifies the base image upon which your Spring Boot application will run. A common choice is openjdk:21-slim (or a similar JDK version) as it provides a lightweight Java runtime environment.

# Dockerfile
FROM openjdk:21-slim

Copy the JAR File:

Use the COPY instruction to copy your Spring Boot application's JAR file from your project directory to a suitable location within the container image.

# Dockerfile
FROM openjdk:21-slim

ARG JAR_FILE=target/*.jar

COPY ${JAR_FILE} app.jar

Define the Entrypoint:

The ENTRYPOINT instruction specifies the command to be executed when the container starts. If your Spring Boot application has a main method, you can use the following:

# Dockerfile
FROM openjdk:21-slim

ARG JAR_FILE=target/*.jar

COPY ${JAR_FILE} app.jar

ENTRYPOINT ["java", "-jar", "app.jar"]

Build Docker Image :

docker build -t <image-name> .

Check The list of images :

docker images

Run the Container:

Use the docker run command to start a container from your newly built image. Here's the basic syntax:

docker run -d -p <host-port>:<container-port> <image-name>

1. host-port: The port on your host machine that will be mapped to the container's application port (usually the default Spring Boot port, 8080).

2. container-port : The port within the container that your application listens on (defaults to 8080 for Spring Boot).

3. image-name: The name of your Docker image created.

Example:

 docker run -d -p 8081:8081 --name test-microservice test-image

 docker ps # list of running containers

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