Architecture

MVC Pattern

SpringBootLibrary follows the classic Model-View-Controller (MVC) pattern, adapted for a REST API:

HTTP Client (Browser / Postman)
        │
        ▼
┌───────────────────┐
│    Controller     │  ← Handles HTTP requests & responses
│  (REST endpoints) │    com.example.library.controller
└────────┬──────────┘
         │
         ▼
┌───────────────────┐
│     Service       │  ← Business logic
│                   │    com.example.library.service
└────────┬──────────┘
         │
         ▼
┌───────────────────┐
│   Repository      │  ← Spring Data JPA interfaces
│                   │    com.example.library.repository
└────────┬──────────┘
         │
         ▼
┌───────────────────┐
│  Database (MySQL) │  ← Entities via Hibernate / JPA
│  H2 (tests)       │    com.example.library.model
└───────────────────┘

Package Structure

Package	Responsibility
controller/	REST endpoints — BookController, BorrowingController, UserController
service/	Business logic — BookService, BorrowingService, UserService
repository/	JPA repositories — CRUD + custom queries
model/	JPA entities — Book, Borrowing, User

Key Design Decisions

Spring Data JPA is used for the persistence layer. Repository interfaces extend JpaRepository, providing standard CRUD operations without boilerplate code.

H2 in-memory database is activated automatically in the test Spring profile, so unit and integration tests run without requiring a real MySQL instance (except when using @SpringBootTest with the integration Maven profile, which spins up a MySQL Docker container via GitHub Actions).

Springdoc OpenAPI auto-generates an interactive Swagger UI from the controller annotations at runtime, always reflecting the current API surface.

Static Resources

The frontend is a plain HTML + JavaScript + CSS single-page application located at:

src/main/resources/static/
├── index.html
├── app.js
└── style.css

It communicates with the backend over REST, keeping the view layer decoupled from the server.