Multi-stage Docker builds in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the main benefit of using multi-stage Docker builds in Spring Boot applications?

Consider a Spring Boot app Dockerfile using multi-stage builds. What is the primary advantage of this approach?

A It automatically scales the application based on CPU usage.

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

C It reduces the final image size by separating build and runtime environments.

D It enables hot-reloading of code changes inside the container.

💡 Hint

Think about what happens when you separate build tools from the runtime environment.

Multi-stage builds let you compile your app in one stage with all tools, then copy only the needed files to a smaller runtime image. This keeps the final image small and efficient.

📝 Syntax

intermediate

Which Dockerfile snippet correctly copies only the built Spring Boot jar to the final image?

Given a multi-stage Dockerfile, which COPY command correctly copies the built jar from the build stage named 'builder'?

A COPY /app/target/myapp.jar /app/myapp.jar

B COPY --from=builder /app/target/myapp.jar /app/myapp.jar

C COPY --stage=builder /app/myapp.jar /app/myapp.jar

D COPY --from=builder /myapp.jar /app/myapp.jar

💡 Hint

Remember the syntax for copying files from a named build stage.

The correct syntax uses --from=builder to copy from the build stage named 'builder'. The source path must match where the jar is built.

🔧 Debug

advanced

Why does this multi-stage Docker build fail to run the Spring Boot app?

Look at this Dockerfile snippet:

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

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

Why might the container fail to start?

A The base image openjdk:17-jdk-slim does not support running Java apps.

B The ENTRYPOINT syntax is invalid and causes a syntax error.

C The Maven build stage is missing the JDK, so the jar is not created.

D The wildcard *.jar in COPY does not work; it needs the exact jar filename.

💡 Hint

Consider how Docker COPY handles wildcards.

Docker COPY does not expand wildcards like *.jar. You must specify the exact jar filename or rename it during build to a fixed name.

❓ state_output

advanced

What is the size difference between single-stage and multi-stage Spring Boot Docker images?

You build two Docker images for the same Spring Boot app:

• Image A uses a single stage with Maven and JDK.
• Image B uses multi-stage: Maven build stage and a slim JRE runtime stage.

Which statement best describes their size difference?

A Image B is significantly smaller because it excludes build tools and source files.

B Image A is smaller because it has everything in one layer.

C Both images have the same size because the jar file is the same.

D Image B is larger because it has two stages increasing size.

💡 Hint

Think about what is included in the final image layers.

Multi-stage builds copy only the final jar and runtime dependencies, excluding build tools and source code, making the image smaller.

🧠 Conceptual

expert

Which statement about multi-stage Docker builds for Spring Boot apps is FALSE?

Choose the false statement about multi-stage Docker builds in Spring Boot projects.

A Multi-stage builds require the final image to include the entire source code for debugging.

B Multi-stage builds can reduce attack surface by minimizing installed packages in the runtime.

C Multi-stage builds allow caching dependencies separately to speed up rebuilds.

D Multi-stage builds improve security by excluding build tools from the runtime image.

💡 Hint

Consider what is included in the final runtime image.

The final image usually excludes source code to keep it small and secure; including full source is not required and is discouraged.