Multi-stage Docker builds in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the base image for building the Spring Boot app.

Spring Boot

FROM [1] AS build

Drag options to blanks, or click blank then click option'

Aubuntu:latest

Bopenjdk:17-jdk-slim

Cnode:18-alpine

Dpython:3.12-slim

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-Java base image like Ubuntu or Node.js causes build errors.

Choosing a JDK version incompatible with the app.

The base image openjdk:17-jdk-slim provides the Java 17 environment needed to build the Spring Boot app.

2fill in blank

medium

Complete the code to copy the Maven wrapper and project files into the build image.

Spring Boot

COPY [1] ./

Drag options to blanks, or click blank then click option'

Asrc

BDockerfile

Cmvnw pom.xml

Dtarget

Attempts:

3 left

💡 Hint

Common Mistakes

Copying only the source folder without build files.

Copying the Dockerfile instead of project files.

Copying mvnw and pom.xml is necessary to run Maven build commands inside the image.

3fill in blank

hard

Fix the error in the build command to package the Spring Boot app.

Spring Boot

RUN ./mvnw clean [1] -DskipTests

Drag options to blanks, or click blank then click option'

Ainstall

Bcompile

Cspring-boot:run

Dverify

Attempts:

3 left

💡 Hint

Common Mistakes

Using compile only compiles but does not package the jar.

Using spring-boot:run runs the app instead of building.

The install goal compiles and packages the app and installs it to the local Maven repository, which is suitable for building the jar.

4fill in blank

hard

Fill both blanks to specify the runtime image and copy the built jar from the build stage.

Spring Boot

FROM [1] AS runtime
COPY --from=build [2] /app/app.jar

Drag options to blanks, or click blank then click option'

Aopenjdk:17-jre-slim

B/target/*.jar

C/build/libs/app.jar

Dopenjdk:17-jdk-slim

Attempts:

3 left

💡 Hint

Common Mistakes

Using the JDK image for runtime unnecessarily increases image size.

Copying from wrong build output folder.

The runtime stage uses a lightweight JRE image openjdk:17-jre-slim. The built jar is usually in the target folder after Maven build.

5fill in blank

hard

Fill all three blanks to complete the command that runs the Spring Boot jar with memory options.

Spring Boot

ENTRYPOINT ["java", [1], [2], "-jar", [3]]

Drag options to blanks, or click blank then click option'

A"-Xms256m"

B"-Xmx512m"

C"/app/app.jar"

D"-Dspring.profiles.active=prod"

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting memory options can cause performance issues.

Placing the jar path before -jar causes errors.

The ENTRYPOINT runs the jar with initial and max heap size options -Xms256m and -Xmx512m, followed by the path to the jar.

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