Spring Boot — From Language To Backend
Open any Java backend posting in India — Bengaluru product startup, fintech, GCC, services giant — and Spring Boot is on it. Not usually. Effectively always. Java is the language you learned; Spring is the job you get hired for. This track closes that gap with one move: SplitEase, the terminal expense-splitter you built as the Core Java capstone, becomes a real backend — a secured, tested REST API on PostgreSQL that you can run, demo, and defend line by line in an interview.
The De-Magic Promise
Spring has a reputation, and it earned it: annotations as magic. Tutorials make it worse — “just add @Service and it works.” That style produces developers who can assemble a CRUD app in an afternoon and cannot debug it in a week. The annotation worked or it didn’t; they have no third option.
This track makes one promise: every annotation gets de-magicked before you’re allowed to use it. You’ll see — and usually write — the plain-Java machinery underneath first:
| The annotation | The plain Java underneath it |
|---|---|
@Autowired | Constructor calls in a main method. You will write that main by hand before Spring ever runs. |
@Transactional | A try / commit / catch / rollback wrapper, added by a proxy object standing in front of your class. |
| Auto-configuration | If-statements over the classpath: “H2 is on the classpath and no DataSource bean exists, so I’ll make one.” |
When you know the machinery, error messages stop being walls of red and start being maps. That’s the whole difference between a developer who restarts the app and hopes, and one who reads the failure and fixes it.
The Journey
One project, nine layers, each module adding exactly one:
flowchart LR
A[CLI Ledger] --> B[Beans and DI]
B --> C[REST endpoints]
C --> D[PostgreSQL via JPA]
D --> E[DTOs and validation]
E --> F[Services and transactions]
F --> G[JWT security]
G --> H[Tests]
H --> I[Deploy ready API]
The capstone’s Ledger becomes the service layer. The CLI commands become REST endpoints. The file save becomes a PostgreSQL database. By the end, the WhatsApp-group-math problem you solved in a terminal is solved by an API any frontend could call.
One Project, Not Nine Exercises
Every module’s Build This advances the same codebase, splitease-api. Module 03’s controller is rewired in module 04. Module 05’s DTOs wrap module 04’s entities. Module 07 locks down every endpoint the earlier modules built.
That means something blunt: if you skip a module’s build, the next module’s build won’t compile. That is intentional. Real backend work is never a fresh folder — it’s changing a system that already exists without breaking what already works. This track makes you practice that from day one.
The Modules
| # | Module | What it gives you |
|---|---|---|
| 01 | What Spring Actually Solves | Beans, DI, the container — de-magicked with a ten-line main you write yourself |
| 02 | Your First App, Demystified | Generate splitease-api on start.spring.io and account for every file in it |
| 03 | REST Controllers | CLI commands become HTTP endpoints — friends over JSON, no DB yet |
| 04 | Data & JPA | The file save becomes PostgreSQL — entities, repositories, H2 as a stepping stone |
| 05 | DTOs & Validation | Records at the boundary; bad input rejected before it touches your logic |
| 06 | Services, Transactions & Errors | The Ledger math moves in; @Transactional de-magicked; one consistent error JSON |
| 07 | Security & JWT | Register and login issue tokens; everything else locked down |
| 08 | Testing That Proves Something | Unit, slice, and integration tests — and what not to test |
| 09 | Capstone: The SplitEase API | Groups, the settle endpoint, pagination, README — finished and demoable |
Before You Start
- The Core Java track, done. Especially the SplitEase capstone — this track’s project picks up exactly where that one ends. If you don’t have a working CLI Ledger with balances and settle-up, go build it; everything here stands on it.
- The sprint schedule already points here. Week 4 and Week 6 sketch DI, DTOs, validation, and JWT at survey level — they’re the calendar view of this material. This track is the textbook view. Same content, full depth.
- Stack for the whole track, no drift: Java 21, Spring Boot 3.x, Maven (
./mvnw), PostgreSQL, money in paise aslong, records for DTOs, constructor injection only.
Before Photo — No Shame In Zero
Take this now, before module 01. Scoring zero is the honest baseline — the entire point of the next nine modules is to make these three questions feel insultingly easy.
Wherever you landed: same next step. Start at Spring Boot 01 — What Spring Actually Solves.