Dockerizing a Spring Boot Application
“It works on my machine” is an excuse that died with Docker. Containerizing your Java application ensures it runs the same way on your laptop, the testing server, and production.
Here is the standard 4-step process I use for all my Spring Boot projects.
1. The Application
Assume you have a standard Maven structure. First, ensure you can build it locally:
./mvnw clean package
2. The Dockerfile
Create a file named Dockerfile in the root directory. We’ll use a multi-stage build to keep the image small.
# Stage 1: Build the JAR
FROM maven:3.8.4-openjdk-17 as builder
WORKDIR /app
COPY . .
RUN mvn clean package -DskipTests
# Stage 2: Create the Runtime Image
FROM openjdk:17-jdk-alpine
VOLUME /tmp
WORKDIR /app
COPY --from=builder /app/target/*.jar app.jar
ENTRYPOINT ["java","-jar","/app/app.jar"]
Why Multi-stage? The Maven image is huge (600MB+) because it contains all the build tools. The Alpine JDK runtime is tiny (<200MB). We build in the big image, copy only the JAR to the small image, and throw the big image away.
3. Building and Running
Build the image:
docker build -t my-spring-app .
Run it:
docker run -p 8080:8080 my-spring-app
4. Environment Variables
Never hardcode database URLs. Pass them at runtime.
In application.properties:
spring.datasource.url=${DB_URL}
Run with the env var:
docker run -p 8080:8080 -e DB_URL=jdbc:postgresql://host.docker.internal:5432/mydb my-spring-app
Now your app is portable, scalable, and ready for Kubernetes!