Dockerizing a Spring Boot Application: A Step-by-Step Guide

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Dockerizing your Spring Boot application can provide numerous benefits, including consistent deployment environments, ease of scaling, and simplified dependency management. This guide will walk you through the process of creating a Docker image for your Spring Boot application and running it in a Docker container.

Prerequisites

• JDK 17 or later
• Maven or Gradle
• Docker installed on your machine
• IDE (IntelliJ IDEA, Eclipse, etc.)

Step 1: Set Up a Spring Boot Project

Use Spring Initializr to create a new project with the following configuration:

• Project: Maven Project
• Language: Java
• Spring Boot: 3.2.x
• Dependencies: Spring Web

Download and unzip the project, then open it in your IDE.

Example Spring Boot Application

Create a simple Spring Boot application that includes a REST controller.

1.1 Application Class

package com.example.demo; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class DemoApplication { public static void main(String[] args) { SpringApplication.run(DemoApplication.class, args); } } 

1.2 REST Controller

Create a REST controller class named HelloController in the com.example.demo.controller package.

package com.example.demo.controller; import org.springframework.web.bind.annotation.GetMapping; import org.springframework.web.bind.annotation.RestController; @RestController public class HelloController { @GetMapping("/hello") public String hello() { return "Hello, Docker!"; } } 

1.3 Maven Configuration

Ensure that your pom.xml is configured correctly. The default configuration provided by Spring Initializr should suffice.

<dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> <!-- Additional dependencies can be added here --> </dependencies> <build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> </plugin> </plugins> </build> 

Step 2: Create a Dockerfile

A Dockerfile is a script that contains instructions to assemble a Docker image.

2.1 Create a Dockerfile

Create a file named Dockerfile in the root directory of your project.

# Use the official OpenJDK base image FROM openjdk:17-jdk-alpine # Set the working directory inside the container WORKDIR /app # Copy the built jar file into the container COPY target/demo-0.0.1-SNAPSHOT.jar app.jar # Expose port 8080 EXPOSE 8080 # Run the application ENTRYPOINT ["java", "-jar", "app.jar"] 

Explanation:

• FROM openjdk:17-jdk-alpine: Uses the official OpenJDK base image with JDK 17.
• WORKDIR /app: Sets the working directory inside the container to /app.
• COPY target/demo-0.0.1-SNAPSHOT.jar app.jar: Copies the built jar file from the target directory into the container.
• EXPOSE 8080: Exposes port 8080 to the host.
• ENTRYPOINT ["java", "-jar", "app.jar"]: Specifies the command to run the application.

Step 3: Build the Spring Boot Application

3.1 Build the Jar File

Run the following command to build the jar file of your Spring Boot application:

./mvnw clean package 

This command will generate a jar file in the target directory, for example, target/demo-0.0.1-SNAPSHOT.jar.

Step 4: Build the Docker Image

4.1 Build the Docker Image

Run the following command to build the Docker image:

docker build -t demo-app . 

Explanation:

• docker build: The Docker command to build an image.
• -t demo-app: Tags the image with the name demo-app.
• .: Specifies the current directory as the build context.

Step 5: Run the Docker Container

5.1 Run the Docker Container

Run the following command to start a Docker container from the image:

docker run -p 8080:8080 demo-app 

Explanation:

• docker run: The Docker command to run a container.
• -p 8080:8080: Maps port 8080 of the container to port 8080 on the host machine.
• demo-app: The name of the Docker image to run.

Step 6: Test the Application

6.1 Access the Application

Open a web browser or a tool like Postman and navigate to the following URL:

http://localhost:8080/hello 

You should see the message "Hello, Docker!" returned from the HelloController.

Step 7: Additional Docker Commands

7.1 List Docker Images

To list all Docker images on your system, run:

docker images 

7.2 List Running Containers

To list all running Docker containers, run:

docker ps 

7.3 Stop a Running Container

To stop a running Docker container, run:

docker stop <container_id> 

Replace <container_id> with the actual container ID obtained from the docker ps command.

7.4 Remove a Docker Container

To remove a Docker container, run:

docker rm <container_id> 

Replace <container_id> with the actual container ID.

7.5 Remove a Docker Image

To remove a Docker image, run:

docker rmi demo-app 

Conclusion

In this guide, you have learned how to dockerize a Spring Boot application. We covered:

• Setting up a Spring Boot project.
• Creating a Dockerfile.
• Building the Spring Boot application.
• Building the Docker image.
• Running the Docker container.
• Testing the application.

By following these steps, you can easily package your Spring Boot application into a Docker container, providing a consistent and portable deployment environment.