Why containerization matters in Spring Boot - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a Spring Boot application main class.

Spring Boot

public class Application {
    public static void main(String[] args) {
        SpringApplication.[1](Application.class, args);
    }
}

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

Alaunch

Bstart

Cexecute

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' or 'launch' instead of 'run' causes errors.

The SpringApplication.run() method starts the Spring Boot application.

2fill in blank

medium

Complete the Dockerfile line to specify the base image for a Spring Boot app.

Spring Boot

FROM [1]

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

Anode:latest

Bubuntu:20.04

Copenjdk:17-jdk-slim

Dpython:3.12

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing non-Java images like Node or Python causes build failures.

Spring Boot apps run on Java, so the base image should be a Java JDK image like openjdk:17-jdk-slim.

3fill in blank

hard

Fix the error in the Dockerfile command to copy the jar file.

Spring Boot

COPY target/[1] /app/app.jar

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

Aapp.jar

Bapplication.jar

Cmyapp.jar

Dspringboot.jar

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong jar file names causes Docker build errors.

The jar file built by Spring Boot is commonly named app.jar in this example, so the COPY command must match the actual jar filename.

4fill in blank

hard

Fill both blanks to complete the Dockerfile command that runs the Spring Boot app.

Spring Boot

ENTRYPOINT ["java", "[1]", "[2]"]

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

A-jar

Bapp.jar

C-cp

Dtarget/app.jar

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-cp' instead of '-jar' or wrong jar file path.

The ENTRYPOINT runs the jar with java -jar app.jar.

5fill in blank

hard

Fill all three blanks to create a Docker command that builds and runs the container.

Spring Boot

docker [1] -t springboot-app . && docker [2] -d -p [3]:8080 springboot-app

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

Abuild

Brun

C8080

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' instead of 'run' or wrong port mapping.

First, build the image with docker build. Then run it detached with docker run -d mapping port 8080.

Practice

1. Why is containerization important for a Spring Boot application?

easy

A. It automatically writes the application code for you.

B. It makes the app run only on Windows machines.

C. It replaces the need for a database in your app.

D. It packages the app with all dependencies for consistent running anywhere.

Solution

1. Step 1: Understand containerization purpose

   Containerization bundles the app with all its dependencies and environment settings.

2. Step 2: Identify benefits for Spring Boot apps

   This bundling ensures the app runs the same on any machine, avoiding "it works on my PC" issues.

3. Final Answer:

   It packages the app with all dependencies for consistent running anywhere. -> Option D

4. Quick Check:

   Containerization = consistent app environment [OK]

Hint: Containers bundle app + dependencies for consistent runs [OK]

Common Mistakes:

• Thinking containers write code automatically
• Believing containers replace databases
• Assuming containers limit OS compatibility

2. Which of the following is the correct Dockerfile instruction to set the base image for a Spring Boot app container?

easy

A. FROM openjdk:17-jdk-slim

B. RUN openjdk 17

C. COPY openjdk:17-jdk-slim

D. BASE openjdk:17-jdk-slim

Solution

1. Step 1: Recall Dockerfile syntax for base image

   The correct instruction to specify the base image is FROM.

2. Step 2: Match the option with correct syntax

   FROM openjdk:17-jdk-slim uses FROM openjdk:17-jdk-slim, which is valid and common for Java apps.

3. Final Answer:

   FROM openjdk:17-jdk-slim -> Option A

4. Quick Check:

   Base image set by FROM [OK]

Hint: Base image in Dockerfile always starts with FROM [OK]

Common Mistakes:

• Using RUN instead of FROM for base image
• Confusing COPY with base image instruction
• Using non-existent BASE keyword

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

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

What happens when you run this container?

medium

A. The Spring Boot app starts inside the container using Java 17.

B. The container will fail because the ENTRYPOINT syntax is wrong.

C. The app.jar file is deleted after copying.

D. The container runs but does not start the app.

Solution

1. Step 1: Analyze Dockerfile commands

   The base image provides Java 17. The app.jar is copied into the container. ENTRYPOINT runs the jar with Java.

2. Step 2: Understand container behavior on run

   Running the container executes the ENTRYPOINT command, starting the Spring Boot app inside.

3. Final Answer:

   The Spring Boot app starts inside the container using Java 17. -> Option A

4. Quick Check:

   ENTRYPOINT runs app.jar with Java [OK]

Hint: ENTRYPOINT runs the app inside container [OK]

Common Mistakes:

• Thinking ENTRYPOINT syntax is invalid
• Assuming files are deleted after COPY
• Believing container runs without starting app

4. You built a Docker image for your Spring Boot app but when running the container, it exits immediately. Which is the most likely cause?

medium

A. The app.jar file is too large to run in a container.

B. The ENTRYPOINT command is missing or incorrect, so the app never starts.

C. The Dockerfile uses FROM openjdk:8 instead of 17.

D. Docker does not support Spring Boot apps.

Solution

1. Step 1: Understand container exit behavior

   Containers exit immediately if no running process is specified or if the process ends quickly.

2. Step 2: Identify cause related to ENTRYPOINT

   If ENTRYPOINT is missing or wrong, the container has no command to keep running, so it exits.

3. Final Answer:

   The ENTRYPOINT command is missing or incorrect, so the app never starts. -> Option B

4. Quick Check:

   Missing ENTRYPOINT = container exits immediately [OK]

Hint: Always set ENTRYPOINT to start your app [OK]

Common Mistakes:

• Blaming Java version mismatch for immediate exit
• Thinking file size causes container exit
• Believing Docker can't run Spring Boot apps

5. You want to deploy multiple instances of your Spring Boot app using containers to handle more users smoothly. What containerization feature helps achieve this easily?

hard

A. Containerization removes the need for load balancers.

B. Containers automatically rewrite your app code for scaling.

C. Container isolation allows running many app copies independently on the same host.

D. Containers force apps to run only one instance at a time.

Solution

1. Step 1: Understand container isolation

   Containers isolate apps so multiple copies can run on the same machine without conflicts.

2. Step 2: Connect isolation to scaling

   This isolation lets you start many instances to share user load smoothly.

3. Final Answer:

   Container isolation allows running many app copies independently on the same host. -> Option C

4. Quick Check:

   Isolation enables multiple app instances [OK]

Hint: Isolation lets many containers run side-by-side [OK]

Common Mistakes:

• Thinking containers rewrite app code automatically
• Believing containers replace load balancers
• Assuming containers limit to one instance