Using Docker with Spring Boot: A Comprehensive Guide

In the ever-evolving world of development, achieving a seamless and efficient development-to-deployment flow is paramount. This has been made incredibly simple with the power couple: Spring Boot and Docker. When it comes to Java-based microservice applications, Spring Boot is a frontrunner, and Docker’s containerization technique makes the deployment process as smooth as it can get. In this post, we will delve deep into integrating Spring Boot with Docker, ensuring that you get the most out of these technologies.

Why Docker with Spring Boot?

• Consistency: Docker ensures that your application runs the same regardless of where the container is being executed. This means you avoid the common “it works on my machine” dilemma.
• Simplicity: With Docker, you can have a complete environment for your Spring Boot application including its dependencies, libraries, and even the OS, packaged into a single unit.
• Scalability: Dockerized applications can be easily scaled up or down depending on the demand, ensuring optimal use of resources.

Starting with a Simple Spring Boot Application

Let’s kick things off by developing a basic Spring Boot REST API application. If you’re familiar with Spring Boot, this should be a walk in the park.

package com.example.dockerdemo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
public class DockerDemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DockerDemoApplication.class, args);
    }
}

@RestController
class HelloController {
    @GetMapping("/")
    public String sayHello() {
        return "Hello from Spring Boot and Docker!";
    }
}

To run the Spring Boot application, use the command:

./mvnw spring-boot:run

Once it starts, you can navigate to http://localhost:8080/ in your browser and see the greeting message.

Dockerizing the Spring Boot Application

For our Spring Boot application to run inside a Docker container, we need a Dockerfile. This file defines the environment in which our application will run.

Here’s a simple Dockerfile for our Spring Boot application:

FROM openjdk:11-jre-slim

ARG JAR_FILE=target/*.jar

COPY ${JAR_FILE} app.jar

EXPOSE 8080

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

Understanding the Dockerfile:

• We’re using an official Java image openjdk:11-jre-slim, which is a slim version suitable for running Java applications.
• The JAR file generated by our Spring Boot application is copied to the Docker image as app.jar.
• We’re exposing port 8080, which our application uses.
• The ENTRYPOINT directive indicates how the application should be started.

With the Dockerfile in place, navigate to the root directory of your project in your terminal and build the Docker image:

docker build -t spring-boot-docker 

After building the image, you can run the application inside a Docker container:

docker run -p 8080:8080 spring-boot-docke

Your Spring Boot application is now running inside a Docker container!

Final Thoughts

Combining Spring Boot with Docker brings forth the prowess of modern-day application development and deployment. Spring Boot’s convention-over-configuration philosophy marries perfectly with Docker’s promise of consistent and isolated environments. For Java developers, this integration is not just a necessity for modern application lifecycles, but also a step forward in ensuring scalability, consistency, and efficiency.

Incorporating these tools into your workflow will undoubtedly usher your development process into the next era of cloud-native applications. It’s time to embrace the future of application deployment and take full advantage of the synergy between Spring Boot and Docker.