Dockerfile for Spring Boot in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Docker build command

You have this Dockerfile for a Spring Boot app:

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

What will be the output of docker build -t springboot-app . if the target/app.jar file is missing?

A Successfully builds the image with tag springboot-app

B Build completes but image is empty

C Warning about missing file but image builds

D Error: COPY failed: stat target/app.jar: no such file or directory

💡 Hint

Think about what happens if the file to copy does not exist during build.

Docker build fails if a file specified in COPY does not exist. The error shows the missing file path.

❓ Configuration

intermediate

Correct Dockerfile for multi-stage build

Which Dockerfile correctly uses a multi-stage build to compile a Spring Boot app with Maven and produce a minimal final image?

A

FROM maven:3.8.6-openjdk-17
COPY . /app
WORKDIR /app
RUN mvn clean package
ENTRYPOINT ["java", "-jar", "target/app.jar"]

B

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
COPY --from=build /app/target/app.jar app.jar
ENTRYPOINT ["java", "-jar", "app.jar"]

C

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

D

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

💡 Hint

Multi-stage builds separate build environment and runtime environment.

Option B uses Maven image to build and then copies only the jar to a slim image for runtime, reducing image size.

❓ Troubleshoot

advanced

Why does the Spring Boot app fail to start in Docker?

You built a Docker image for your Spring Boot app but when running the container, it exits immediately with no logs. The Dockerfile is:

FROM openjdk:17-jdk-slim
COPY target/app.jar app.jar
ENTRYPOINT java -jar app.jar

What is the most likely cause?

A The base image openjdk:17-jdk-slim does not support Spring Boot

B The app.jar file is missing in the image

C ENTRYPOINT should be an array of strings, not a single string

D The container needs environment variables to start

💡 Hint

How Docker interprets ENTRYPOINT differs between string and array syntax.

ENTRYPOINT as a string runs in shell form, which can cause the process to not stay in foreground. Using exec form (array) is recommended.

🔀 Workflow

advanced

Order of steps to build and run Spring Boot Docker container

Put these steps in the correct order to build and run a Spring Boot app Docker container:
1. Run the container with docker run
2. Write the Dockerfile
3. Build the jar with Maven
4. Build the Docker image with docker build

A 1,2,3,4

B 2,1,3,4

C 1,3,2,4

D 2,3,1,4

💡 Hint

Think about what you need before building the image and running the container.

You first write the Dockerfile, then build the jar, then build the image, then run the container.

✅ Best Practice

expert

Best practice for reducing Spring Boot Docker image size

Which option is the best practice to reduce the size of a Spring Boot Docker image?

A Use a multi-stage build with Maven and copy only the jar to a slim OpenJDK base image

B Use the full OpenJDK image and copy all source code into the image

C Build the jar inside the container every time it runs

D Use a base image with JRE instead of JDK and copy the jar

💡 Hint

Think about separating build and runtime environments.

Multi-stage builds allow compiling in a full environment and then copying only the needed jar to a small runtime image, reducing size.