Dockerfile for Spring Boot in Spring Boot - Step-by-Step Execution

Concept Flow - Dockerfile for Spring Boot

Start: Write Dockerfile

↓

Choose base image

↓

Copy jar file into image

↓

Set entrypoint to run jar

↓

Build Docker image

↓

Run container

↓

Spring Boot app runs inside container

This flow shows how a Dockerfile is created and used to build and run a Spring Boot application inside a container.

Execution Sample

Spring Boot

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

This Dockerfile uses an OpenJDK base image, copies the Spring Boot jar, and sets the command to run it.

Execution Table

Step	Dockerfile Instruction	Action	Result
1	FROM openjdk:17-jdk-slim	Select base image with Java 17	Base image ready with Java environment
2	COPY target/app.jar app.jar	Copy Spring Boot jar into image	app.jar available inside image
3	ENTRYPOINT ["java", "-jar", "/app.jar"]	Set container start command	Container will run the jar on start
4	docker build -t springboot-app .	Build Docker image from Dockerfile	Image 'springboot-app' created
5	docker run -p 8080:8080 springboot-app	Run container exposing port 8080	Spring Boot app runs and listens on port 8080

💡 Container runs until stopped; Spring Boot app serves requests on port 8080

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
Docker Image	None	openjdk:17-jdk-slim	openjdk + app.jar	Configured with ENTRYPOINT	Built as springboot-app	Running container with app

Key Moments - 3 Insights

Why do we use 'COPY target/app.jar app.jar' instead of copying the whole project?

Because the Docker image only needs the compiled jar file to run the app, not the source code. This is shown in execution_table step 2.

What does ENTRYPOINT do in the Dockerfile?

ENTRYPOINT sets the command that runs when the container starts, here it runs the Spring Boot jar. See execution_table step 3.

Why do we expose port 8080 when running the container?

Spring Boot listens on port 8080 by default, so exposing it allows access from outside the container. See execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the base image used in step 1?

A openjdk:8-jdk

B ubuntu:latest

C openjdk:17-jdk-slim

D springboot:latest

💡 Hint

Check the Dockerfile Instruction column in step 1 of the execution_table.

At which step is the Spring Boot jar copied into the Docker image?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for the COPY instruction in the execution_table.

If we change the ENTRYPOINT to run a different jar, which step would change?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

ENTRYPOINT is set in step 3 according to the execution_table.

Concept Snapshot

Dockerfile for Spring Boot:
FROM openjdk:17-jdk-slim
COPY target/app.jar app.jar
ENTRYPOINT ["java", "-jar", "/app.jar"]
Build image with 'docker build -t name .'
Run container with 'docker run -p 8080:8080 name'
Spring Boot app runs inside container on port 8080

Full Transcript

This visual execution shows how to create a Dockerfile for a Spring Boot application. First, we select a base image with Java 17. Then we copy the compiled Spring Boot jar file into the image. Next, we set the ENTRYPOINT to run the jar when the container starts. After writing the Dockerfile, we build the Docker image and then run a container exposing port 8080. The Spring Boot app runs inside the container and listens on port 8080. Key points include copying only the jar file, setting the correct ENTRYPOINT, and exposing the right port for access.

Practice

1. What is the main purpose of a Dockerfile in a Spring Boot project?

easy

A. To write the Java code for the Spring Boot application

B. To define how to package and run the Spring Boot app inside a container

C. To configure the database connection settings

D. To create unit tests for the Spring Boot app

Solution

1. Step 1: Understand the role of Dockerfile

   A Dockerfile contains instructions to build a Docker image that packages the app and its environment.

2. Step 2: Identify its use in Spring Boot

   For Spring Boot, it defines how to copy the app jar, expose ports, and run the app in a container.

3. Final Answer:

   To define how to package and run the Spring Boot app inside a container -> Option B

4. Quick Check:

   Dockerfile = Package & run app [OK]

Hint: Dockerfile builds container image for your app [OK]

Common Mistakes:

• Confusing Dockerfile with source code files
• Thinking Dockerfile configures app logic
• Assuming Dockerfile is for testing only

2. Which of the following is the correct syntax to copy the Spring Boot jar file into the Docker image in a Dockerfile?

easy

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

B. MOVE target/app.jar /app.jar

C. ADD target/app.jar

D. COPY target/app.jar /app.jar

Solution

1. Step 1: Understand COPY command syntax

   The COPY command syntax is: COPY source_path destination_path inside the image.

2. Step 2: Apply to Spring Boot jar

   We copy the built jar from local 'target/app.jar' to '/app.jar' inside the image.

3. Final Answer:

   COPY target/app.jar /app.jar -> Option D

4. Quick Check:

   COPY source destination [OK]

Hint: COPY source then destination path [OK]

Common Mistakes:

• Using MOVE instead of COPY
• Missing destination path in ADD
• Reversing source and destination paths

3. Given this Dockerfile snippet:

FROM openjdk:17-jdk-slim
COPY target/app.jar /app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "/app.jar"]

What happens when you run the built Docker image?

medium

A. The container runs but does not start the app

B. The container fails because EXPOSE is missing

C. The container runs the Spring Boot app on port 8080

D. The container runs the app but on port 80

Solution

1. Step 1: Analyze Dockerfile commands

   FROM sets base image with Java 17. COPY adds app.jar. EXPOSE 8080 declares port. ENTRYPOINT runs the jar.

2. Step 2: Understand container behavior

   When run, container starts Java app listening on port 8080 as declared.

3. Final Answer:

   The container runs the Spring Boot app on port 8080 -> Option C

4. Quick Check:

   ENTRYPOINT runs app, EXPOSE 8080 [OK]

Hint: ENTRYPOINT runs app; EXPOSE shows port [OK]

Common Mistakes:

• Thinking EXPOSE runs the app
• Assuming default port 80 without config
• Ignoring ENTRYPOINT command

4. You wrote this Dockerfile:

FROM openjdk:17-jdk-slim
COPY app.jar /app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "/app.jar"]

But when running the container, it fails with 'file not found'. What is the likely cause?

medium

A. The jar file is not named 'app.jar' in the build folder

B. EXPOSE 8080 is missing

C. ENTRYPOINT syntax is incorrect

D. Base image openjdk:17-jdk-slim does not support Java

Solution

1. Step 1: Check COPY source path

   COPY expects a local file named 'app.jar' but likely the jar is named differently or in a different folder.

2. Step 2: Understand error cause

   If the jar is missing or misnamed, container cannot find '/app.jar' to run, causing failure.

3. Final Answer:

   The jar file is not named 'app.jar' in the build folder -> Option A

4. Quick Check:

   COPY source must match actual jar name [OK]

Hint: Check jar filename matches COPY source [OK]

Common Mistakes:

• Assuming EXPOSE affects file copying
• Miswriting ENTRYPOINT array syntax
• Blaming base image for missing file

5. You want to optimize your Spring Boot Dockerfile to use a smaller image and faster startup. Which Dockerfile snippet achieves this best?

hard

A. FROM openjdk:17-jre-slim
COPY target/app.jar /app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "/app.jar"]

B. FROM alpine
COPY target/app.jar /app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "/app.jar"]

C. FROM openjdk:17-jdk-slim
COPY target/app.jar /app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "/app.jar"]

D. FROM openjdk:17-jdk
COPY target/app.jar /app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "/app.jar"]

Solution

1. Step 1: Compare base images

   openjdk:17-jre-slim has only Java runtime, smaller than jdk images which include compilers.

2. Step 2: Evaluate options for size and startup

   Using jre-slim reduces image size and speeds startup. Alpine lacks Java by default, causing errors.

3. Final Answer:

   FROM openjdk:17-jre-slim COPY target/app.jar /app.jar EXPOSE 8080 ENTRYPOINT ["java", "-jar", "/app.jar"] -> Option A

4. Quick Check:

   Use JRE slim for smaller, faster images [OK]

Hint: Use JRE slim base image for smaller Docker images [OK]

Common Mistakes:

• Using full JDK image unnecessarily
• Choosing Alpine without Java setup
• Ignoring image size impact