Expense Tracker Workshop — Session 2

SQLite + Repository + Validation

A follow-up workshop to Session 1. Participants will add a SQLite database, a Repository layer, validation, and error handling. The Controller will not change a single line — validating the architecture from Session 1.

By the end of the workshop you will have a production-ready API where data survives a restart.

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Prerequisites

• Completed Session 1 (expense-tracker-workshop-01) or knowledge of Spring Boot basics
• IntelliJ IDEA Community + JDK 21
• sqlite3 CLI (brew install sqlite3 / apt install sqlite3)

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Workshop structure

Step	Topic	What we build
Step 1	SQLite setup and schema	expenses.db file on disk
Step 2	Repository: INSERT and SELECT	ExpenseRepository with JdbcTemplate
Step 3	Repository: DELETE and findById	Complete CRUD Repository
Step 4	Service → Repository connection	Data survives a restart!
Step 5	Request validation @Valid	API rejects invalid data
Step 6	Error handling	Consistent error responses
Step 7	OpenAPI finalization	Production-ready Swagger documentation

Each step has:

• start — project with TODO comments (your tasks)
• final — complete solution with explanatory comments

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How to work with this repository

Option A — Folders (recommended for beginners)

No git knowledge required. Just download and open folders.

1. Download the repository:

Click the green Code button → Download ZIP on this page. Unzip the file.

2. Getting started — Step 1:

• Open IntelliJ IDEA
• File → Open → select the step-01-start folder
• Wait for dependencies to download
• Fill in the code where you see // TODO: comments
• Run: ./mvnw spring-boot:run

3. When you're done or stuck:

Open the step-01-final/ folder in IntelliJ — that's the complete solution.

4. Continue to the next step:

Close the project and open step-02-start/.

Step-by-step flow:

step-01-start/  →  fill in TODO  →  compare with  →  step-01-final/
step-02-start/  →  fill in TODO  →  compare with  →  step-02-final/
step-03-start/  →  fill in TODO  →  compare with  →  step-03-final/
step-04-start/  →  fill in TODO  →  compare with  →  step-04-final/
step-05-start/  →  fill in TODO  →  compare with  →  step-05-final/
step-06-start/  →  fill in TODO  →  compare with  →  step-06-final/
step-07-start/  →  fill in TODO  →  compare with  →  step-07-final/  ← FINAL STATE

Tip — verifying data in the database:

sqlite3 expenses.db ".tables"              # shows tables
sqlite3 expenses.db "SELECT * FROM expenses;"  # shows data

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Option B — Git branches (for advanced users)

Each step has its own branch. The branch contains ONLY the files for that step (at root level) — a complete Maven project.

1. Clone the repository:

git clone https://github.com/UnityInFlow/expense-tracker-workshop-02.git
cd expense-tracker-workshop-02

2. Switch to the first step:

git checkout step-01-start

You now see the complete Maven project at the root. Open it in IntelliJ.

3. Work on the tasks:

Edit the files directly — fill in the code where you see // TODO:. Run: ./mvnw spring-boot:run

4. When you're done or stuck:

git stash                     # saves your changes aside
git checkout step-01-final    # switches to the solution

5. Continue to the next step:

git checkout step-02-start    # next step

All available branches:

step-01-start    step-01-final
step-02-start    step-02-final
step-03-start    step-03-final
step-04-start    step-04-final    ← data survives a restart!
step-05-start    step-05-final
step-06-start    step-06-final
step-07-start    step-07-final    ← final state

Tip — comparing your work with the solution:

# On step-02-start after your work:
git diff step-02-final -- src/

Tip — jumping to any step (when you fall behind):

git checkout step-04-start    # skip to Step 4
# Continue from here — all previous steps are done

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Key moments

Step	What happens
Step 4 final	Data survives a restart! HashMap replaced by SQLite. Controller did not change.
Step 5 final	API rejects empty descriptions and negative amounts — validation works.
Step 6 final	All errors in the same JSON format — consistent.
Step 7 final	Swagger UI with example values and error codes — production ready.

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Tools

Tool	Usage
IntelliJ IDEA Community	All steps
JDK 21	Compilation and execution
sqlite3 CLI	Verifying data in the database
Swagger UI	Testing the API in the browser

Detailed installation guide: SETUP.md

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Related

Session 1 — Kotlin & Spring Boot Basics https://github.com/UnityInFlow/expense-tracker-workshop-01

Next steps after the workshop:

• W01 Kotlin Idioms — idiomatic Kotlin for backend
• W02 Spring Boot Basics — advanced Spring Boot patterns