A Docker image

Docker packages an application together with its environment — the right Java version and system settings — into an image. You start a container from the image, and it runs the same on your laptop, on a server and in the cloud. Spring Boot offers two ways to get an image: write a short Dockerfile or let the Maven plugin build it. This lesson needs Docker installed.

A Dockerfile

Create a file in the project root, next to pom.xml:

# Dockerfile
FROM eclipse-temurin:25-jre
WORKDIR /app
COPY target/blog-0.0.1-SNAPSHOT.jar app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "app.jar"]

Build the JAR first, then the image, and run a container:

./mvnw clean package
docker build -t blog .
docker run -p 8080:8080 blog

-t blog names the image, -p 8080:8080 links your computer's port to the container's port. The blog opens at http://localhost:8080, as before.

Without a Dockerfile

The Spring Boot plugin can build an image itself with Cloud Native Buildpacks:

./mvnw spring-boot:build-image
docker run -p 8080:8080 docker.io/library/blog:0.0.1-SNAPSHOT

The plugin picks Java from java.version in pom.xml and splits the JAR into layers: libraries apart from your code. Rebuilding after a code change is then faster.

Together with the database

On a server the application needs PostgreSQL. Both containers are handy to describe in one file:

# compose.yaml
services:
  db:
    image: postgres:18
    environment:
      POSTGRES_DB: blog
      POSTGRES_USER: blog
      POSTGRES_PASSWORD: changeme
  app:
    image: blog
    ports:
      - "8080:8080"
    environment:
      SPRING_PROFILES_ACTIVE: prod
      SPRING_DATASOURCE_URL: jdbc:postgresql://db:5432/blog
      SPRING_DATASOURCE_USERNAME: blog
      SPRING_DATASOURCE_PASSWORD: changeme
    depends_on:
      - db

docker compose up starts both containers. Settings are passed as environment variables — just as in the previous lesson. The real password is not written into the file: on a server it comes from secrets or from a .env file that stays out of Git.

Common mistake

Inside a container localhost is the container itself, not your computer and not the database container. With the address jdbc:postgresql://localhost:5432/blog the application does not start:

Connection to localhost:5432 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.

In compose.yaml the database is reached by its service name — db, as in the example above.