Why Multi-stage Docker builds in Spring Boot? - Purpose & Use Cases

The Big Idea

Discover how one Dockerfile can save you hours of build headaches and shrink your app images!

The Scenario

Imagine you want to create a Docker image for your Spring Boot app. You first build the app inside the image, then copy the result to a smaller image for running. Doing this manually means writing separate Dockerfiles and copying files back and forth.

The Problem

Manually managing multiple Dockerfiles or copying build artifacts is slow and error-prone. You might forget to copy files, end up with huge images full of build tools, or have inconsistent builds that are hard to debug.

The Solution

Multi-stage Docker builds let you write one Dockerfile with multiple steps. You build your app in one stage, then copy only the final result to a clean, small image. This keeps images small, builds consistent, and simplifies your workflow.

Before vs After

✗ Before

FROM openjdk:17 AS builder
WORKDIR /app
COPY . .
RUN ./gradlew build

FROM openjdk:17
COPY build/libs/app.jar /app.jar
CMD ["java", "-jar", "/app.jar"]

✓ After

FROM openjdk:17 AS builder
WORKDIR /app
COPY . .
RUN ./gradlew build

FROM openjdk:17-jdk-slim
COPY --from=builder /app/build/libs/app.jar /app.jar
CMD ["java", "-jar", "/app.jar"]

What It Enables

It enables creating small, efficient Docker images that build reliably and run faster in production.

Real Life Example

A developer builds a Spring Boot app with tests and dependencies in the first stage, then produces a tiny image with just the app jar and runtime for deployment to Kubernetes.

Key Takeaways

Manual Docker builds can be slow and create large images.

Multi-stage builds combine build and runtime steps in one Dockerfile.

This results in smaller images and simpler, more reliable builds.