Multi-stage Docker builds in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a multi-stage Docker build?

A multi-stage Docker build uses multiple steps in one Dockerfile to create smaller, efficient images by separating build and runtime environments.

beginner

Why use multi-stage builds for Spring Boot applications?

They help create smaller images by compiling the app in one stage and copying only the needed files to the final image, improving speed and security.

intermediate

In a multi-stage Dockerfile, what does the AS keyword do?

It names a build stage so you can refer to it later to copy files from that stage into another stage.

intermediate

What is copied from the build stage to the final stage in a Spring Boot multi-stage Docker build?

Usually the compiled JAR file and any necessary runtime files are copied to keep the final image small and clean.

advanced

How does multi-stage build improve security?

By excluding build tools and source code from the final image, it reduces attack surface and leaks of sensitive data.

What is the main benefit of using multi-stage Docker builds?

A More layers in the image

B Faster internet download

C Automatic code debugging

D Smaller and cleaner final images

Multi-stage builds help create smaller images by separating build and runtime stages.

In a Dockerfile, how do you name a build stage?

A Using the AS keyword

B Using the FROM keyword twice

C Using the COPY command

D Using the RUN command

The AS keyword names a stage so you can copy files from it later.

What file is typically copied from the build stage in a Spring Boot multi-stage build?

A The compiled JAR file

B The source code files

C The Dockerfile

D The Maven settings file

The compiled JAR is the runnable artifact copied to the final image.

Which of these is NOT a reason to use multi-stage builds?

A Reduce image size

B Improve build speed

C Include build tools in final image

D Enhance security by excluding source code

Multi-stage builds exclude build tools from the final image to keep it clean.

What command copies files from a named build stage to another stage?

A RUN --from=stageName

B COPY --from=stageName

C ADD --from=stageName

D FROM --copy=stageName

COPY --from=stageName copies files from a previous build stage.

Explain how a multi-stage Docker build works for a Spring Boot application.

Think about building first, then packaging only what is needed.

Describe the benefits of using multi-stage Docker builds in terms of image size and security.

Focus on what is left out of the final image.

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