Spring Boot 4 Microservices & REST APIs · Lesson

Dockerizing Spring Boot Applications

Create Dockerfiles to package your Spring Boot applications into lightweight, portable containers.

Dockerizing Spring Boot Applications is a free Spring Boot 4 Microservices & REST APIs lesson on CoddyKit — lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Spring Boot 4 Microservices & REST APIs learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why Dockerize Spring Boot?

Welcome to containerizing Spring Boot apps! Docker helps package your application into a standardized unit called a container.

Why is this great for Spring Boot?

Portability: Run your app consistently anywhere Docker runs.
Isolation: Your app and its dependencies are bundled together, avoiding conflicts.
Consistency: The environment is the same from development to production.

Docker Fundamentals for Apps

Before we dive in, let's quickly define key Docker terms:

Image: A lightweight, standalone, executable package that includes everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and config files. Think of it as a blueprint.
Container: A runnable instance of an image. You can create, start, stop, move, or delete a container. It's the actual running application.
Dockerfile: A text file that contains all the commands, in order, to build a given image.

Our Sample Spring Boot App

Let's imagine we have a simple Spring Boot application. When we build it, it typically produces a single executable JAR file (e.g., my-app.jar).

Here's a basic Spring Boot application structure:

package com.coddykit.docker;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DockerApp {

  public static void main(String[] args) {
    SpringApplication.run(DockerApp.class, args);
    // This will print to the container's standard output
    System.out.println("Spring Boot app started inside Docker!");
  }
}

Introducing the Dockerfile

A Dockerfile is a script of instructions that Docker uses to build an image. It's like a recipe for creating your container environment.

We'll create a file named Dockerfile (no extension) in the root directory of our Spring Boot project. Each line in the Dockerfile is an instruction.

Choosing a Base Image (FROM)

Every Dockerfile starts with the FROM instruction. This specifies the base image your application will build upon. For Java applications, a common choice is an OpenJDK image.

openjdk:17-jdk-slim provides a Java 17 Development Kit (JDK) in a slim, smaller image, which is great for production.

FROM openjdk:17-jdk-slim

Copying Your Application (COPY)

Next, we need to get our compiled Spring Boot JAR file into the Docker image. The COPY instruction does exactly that.

Assuming your built JAR is in target/my-app.jar, we copy it into the image's root directory and rename it to app.jar for simplicity.

COPY target/my-app.jar app.jar

Exposing Ports (EXPOSE)

Spring Boot applications typically run on a specific port, often 8080. The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime.

While EXPOSE doesn't actually publish the port, it serves as documentation and helps with port mapping when running the container.

EXPOSE 8080

Defining the Entrypoint (ENTRYPOINT)

The ENTRYPOINT instruction configures a container that will run as an executable. This is how we tell Docker to start our Spring Boot application when the container launches.

We use the java -jar app.jar command to execute our application's JAR file.

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

Putting It All Together: Dockerfile

Here's what our complete Dockerfile looks like. You would place this file in the root of your Spring Boot project.

FROM openjdk:17-jdk-slim
VOLUME /tmp
COPY target/docker-app-0.0.1-SNAPSHOT.jar app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "/app.jar"]

Note: The JAR name might vary based on your project. VOLUME /tmp is often added to support Spring Boot's embedded Tomcat.

Building Your Docker Image

Once your Dockerfile is ready and your Spring Boot application is compiled (e.g., using mvn package), you can build the Docker image.

Navigate to your project's root directory in the terminal and run:

docker build -t my-spring-app .

This command:

docker build: Initiates the image build process.
-t my-spring-app: Tags the image with a name (my-spring-app).
.: Specifies the build context (the current directory, where the Dockerfile is located).

Dockerfile Instruction Check

Which Dockerfile instruction is used to define the command that runs when a container starts from an image?

Recap: Dockerizing Your App

You've successfully learned the basics of Dockerizing a Spring Boot application!

We understood the role of Dockerfiles, images, and containers.
We explored key Dockerfile instructions: FROM, COPY, EXPOSE, and ENTRYPOINT.
You now know how to bundle your Spring Boot JAR into a portable Docker image.

This sets the stage for running your applications consistently in any Docker environment!

Frequently asked questions

Is the “Dockerizing Spring Boot Applications” lesson free?

Yes — the full text of “Dockerizing Spring Boot Applications” is free to read here on the web, and the Spring Boot 4 Microservices & REST APIs course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Spring Boot 4 Microservices & REST APIs course, upgrade to CoddyKit PRO.

What will I learn in “Dockerizing Spring Boot Applications”?

Create Dockerfiles to package your Spring Boot applications into lightweight, portable containers. You practise Spring Boot 4 Microservices & REST APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Spring Boot 4 Microservices & REST APIs?

No prior experience is required. Spring Boot 4 Microservices & REST APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Dockerizing Spring Boot Applications” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Spring Boot 4 Microservices & REST APIs lesson?

Yes. Every Spring Boot 4 Microservices & REST APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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