Spring Boot 애플리케이션 Docker 이미지 만들기

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.

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.

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:

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.

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.

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.

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.

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.

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

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

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:

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).

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

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!