Multi-stage Docker builds in Spring Boot

Introduction

Multi-stage Docker builds help you create smaller and faster Docker images by separating the build and runtime steps.

When you want to build a Spring Boot app and keep the final image small.

When you want to avoid including build tools like Maven or Gradle in the final image.

When you want faster deployment by reducing image size.

When you want to keep your Dockerfile clean and organized.

When you want to improve security by excluding unnecessary files from the final image.

Syntax

Spring Boot

FROM <build-image> AS build
# build steps here

FROM <runtime-image>
COPY --from=build <source> <destination>
CMD ["java", "-jar", "app.jar"]

The AS build names the first stage so you can copy files from it later.

The final stage uses a smaller image to run the app, copying only needed files.

Examples

This example builds a Spring Boot app using Maven, then copies the jar to a smaller OpenJDK image.

Spring Boot

FROM maven:3.8.6-openjdk-17 AS build
WORKDIR /app
COPY pom.xml .
COPY src ./src
RUN mvn clean package -DskipTests

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

This example uses Gradle to build the app, then copies the jar to a lightweight runtime image.

Spring Boot

FROM gradle:7.5-jdk17 AS build
WORKDIR /app
COPY build.gradle settings.gradle ./
COPY src ./src
RUN gradle build --no-daemon

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

Sample Program

This Dockerfile first builds the Spring Boot app using Maven in the build stage. Then it copies the built jar file into a smaller OpenJDK image to run the app. This keeps the final image small and clean.

Spring Boot

FROM maven:3.8.6-openjdk-17 AS build
WORKDIR /app
COPY pom.xml .
COPY src ./src
RUN mvn clean package -DskipTests

FROM openjdk:17-jdk-slim
WORKDIR /app
COPY --from=build /app/target/demo-0.0.1-SNAPSHOT.jar ./app.jar
CMD ["java", "-jar", "app.jar"]

Important Notes

Always use a smaller runtime image like openjdk:17-jdk-slim for the final stage to reduce image size.

Use --from=build to copy files from the build stage by its name.

Skipping tests during build can speed up image creation but only do this if tests are run elsewhere.

Summary

Multi-stage builds separate building and running your app to keep images small.

Use a full image with build tools first, then copy only needed files to a smaller runtime image.

This method improves deployment speed, security, and image cleanliness.

Practice

1. What is the main benefit of using multi-stage Docker builds for a Spring Boot application?

easy

A. It creates smaller and cleaner Docker images by separating build and runtime stages.

B. It allows running multiple containers from the same image simultaneously.

C. It automatically updates the Spring Boot version inside the container.

D. It enables running the application without a Dockerfile.

Solution

1. Step 1: Understand multi-stage build purpose

   Multi-stage builds separate the build environment from the runtime environment to reduce image size.

2. Step 2: Identify benefit for Spring Boot apps

   By copying only the needed files from build to runtime stage, the final image is smaller and cleaner.

3. Final Answer:

   It creates smaller and cleaner Docker images by separating build and runtime stages. -> Option A

4. Quick Check:

   Multi-stage builds = smaller images [OK]

Hint: Multi-stage builds reduce image size by splitting build and run [OK]

Common Mistakes:

• Confusing multi-stage builds with running multiple containers
• Thinking it updates Spring Boot version automatically
• Believing Dockerfile is not needed

2. Which of the following is the correct syntax to start a new build stage named 'builder' in a Dockerfile?

easy

A. NEW STAGE builder FROM openjdk:17

B. FROM openjdk:17 AS builder

C. BUILD openjdk:17 AS builder

D. STAGE builder FROM openjdk:17

Solution

1. Step 1: Recall Dockerfile multi-stage syntax

   The correct syntax to start a build stage is 'FROM <image> AS <name>'.

2. Step 2: Match syntax to options

   Only FROM openjdk:17 AS builder uses 'FROM openjdk:17 AS builder' which is valid.

3. Final Answer:

   FROM openjdk:17 AS builder -> Option B

4. Quick Check:

   Multi-stage start = FROM ... AS ... [OK]

Hint: Use 'FROM image AS name' to start a build stage [OK]

Common Mistakes:

• Using 'STAGE' or 'BUILD' keywords incorrectly
• Placing 'AS' after the stage name
• Writing 'NEW STAGE' which is invalid

3. Given this Dockerfile snippet for a Spring Boot app:

FROM maven:3.8.6-openjdk-17 AS builder
WORKDIR /app
COPY pom.xml .
RUN mvn dependency:go-offline
COPY src ./src
RUN mvn package -DskipTests

FROM openjdk:17-jdk-slim
COPY --from=builder /app/target/myapp.jar /app/myapp.jar
ENTRYPOINT ["java", "-jar", "/app/myapp.jar"]

What will be the size impact of this multi-stage build compared to a single-stage build including Maven?

medium

A. The final image will be smaller because Maven and source files are excluded.

B. The final image will be larger because it contains both Maven and JDK.

C. The final image size is the same as a single-stage build.

D. The final image will fail to build due to missing source files.

Solution

1. Step 1: Analyze multi-stage build steps

   The first stage builds the app with Maven, the second stage copies only the jar file.

2. Step 2: Understand image size impact

   Since Maven and source code are not copied to the final image, it is smaller than a single-stage build including Maven.

3. Final Answer:

   The final image will be smaller because Maven and source files are excluded. -> Option A

4. Quick Check:

   Copy only jar = smaller image [OK]

Hint: Final image excludes build tools, so it's smaller [OK]

Common Mistakes:

• Assuming Maven stays in final image
• Thinking source files are copied to final image
• Believing multi-stage builds cause build failure here

4. Identify the error in this Dockerfile snippet for a multi-stage build:

FROM openjdk:17 AS builder
WORKDIR /app
COPY . .
RUN ./mvnw package

FROM openjdk:17-jdk-slim
COPY /app/target/app.jar /app/app.jar
ENTRYPOINT ["java", "-jar", "/app/app.jar"]

medium

A. The first stage should use a Maven image, not openjdk.

B. The ENTRYPOINT syntax is incorrect and will cause a runtime error.

C. The WORKDIR command is missing in the second stage.

D. The COPY command in the second stage uses an absolute path instead of a relative path from the builder stage.

Solution

1. Step 1: Check COPY command in second stage

   The COPY command uses '/app/target/app.jar' which is an absolute path on the host, not from the builder stage.

2. Step 2: Correct COPY syntax for multi-stage

   It should be 'COPY --from=builder /app/target/app.jar /app/app.jar' to copy from the builder stage.

3. Final Answer:

   The COPY command in the second stage uses an absolute path instead of a relative path from the builder stage. -> Option D

4. Quick Check:

   Use COPY --from=builder for multi-stage files [OK]

Hint: Use COPY --from=stage to copy files between stages [OK]

Common Mistakes:

• Forgetting --from=builder in COPY
• Assuming ENTRYPOINT syntax is wrong
• Thinking WORKDIR is required in second stage

5. You want to optimize a Spring Boot Dockerfile using multi-stage builds. Which approach best reduces the final image size while ensuring the app runs correctly?

hard

A. Build the app and run it in the same stage using a full Maven and JDK image.

B. Copy the entire source code and Maven cache to the final image for faster startup.

C. Use a Maven build stage to compile, then copy only the fat jar to a minimal JRE base image in the final stage.

D. Use multiple runtime stages with different JDK versions to support various environments.

Solution

1. Step 1: Understand multi-stage build optimization

   Building with Maven in one stage and copying only the final jar to a minimal runtime image reduces size.

2. Step 2: Evaluate options for best practice

   Use a Maven build stage to compile, then copy only the fat jar to a minimal JRE base image in the final stage. uses a build stage with Maven and a minimal JRE image for runtime, which is optimal.

3. Final Answer:

   Use a Maven build stage to compile, then copy only the fat jar to a minimal JRE base image in the final stage. -> Option C

4. Quick Check:

   Build then minimal runtime = smallest image [OK]

Hint: Build with Maven, run with minimal JRE for smallest image [OK]

Common Mistakes:

• Running build and runtime in one large image
• Copying unnecessary source code to final image
• Using multiple runtime stages unnecessarily