🚗 Spring Boot OOP & JPA Demo

A small hands-on project built to demonstrate how object-oriented programming concepts (classes, inheritance, polymorphism) translate into a real Spring Boot + JPA/Hibernate application — and to actually watch Hibernate work through console logs.

This project accompanies a blog series on programming paradigms and OOP. It shows how a simple Car entity gets persisted, queried, and updated, and how Hibernate's dirty checking mechanism triggers an UPDATE without ever writing raw SQL.

🛠️ Tech Stack

• ☕ Java 17
• 🌱 Spring Boot 4.1.x
• 🗃️ Spring Data JPA (Hibernate)
• 💾 H2 (in-memory database)
• 🐘 Gradle (Groovy DSL)

📁 Project Structure

demo/
├── build.gradle
├── settings.gradle
└── src/
    ├── main/
    │   ├── java/com/example/demo/
    │   │   ├── DemoApplication.java       # Entry point + CommandLineRunner demo logic
    │   │   ├── entity/
    │   │   │   └── Car.java               # JPA entity
    │   │   └── repository/
    │   │       └── CarRepository.java     # Spring Data JPA repository
    │   └── resources/
    │       └── application.yml            # DB + logging configuration
    └── test/
        └── java/com/example/demo/
            └── DemoApplicationTests.java  # @DataJpaTest slice tests

✨ What This Demonstrates

• 🏷️ @Entity / @Id / @GeneratedValue — how a plain Java class becomes a table-mapped object (ORM)
• 🔌 Repository interfaces — how Spring Data JPA generates a runtime proxy implementation with zero hand-written SQL
• 🕵️ Dirty checking — how modifying a managed entity's field (car.accelerate(20)) produces an UPDATE only once the persistence context is flushed, without ever calling save() again
• ⚡ Dynamic dispatch in practice — how DI containers (Spring's ApplicationContext) decide at runtime which implementation actually runs, conceptually mirroring vtable-based dispatch at the JVM level

🚀 Getting Started

✅ Prerequisites

• JDK 17 or later
• No local database needed — H2 runs in-memory

▶️ Run the app

./gradlew bootRun

Watch the console — you'll see Hibernate-generated SQL logged in real time as the app runs through: save → find → mutate → flush.

🧪 Run the tests

./gradlew test

⚙️ Configuration Notes

application.yml enables verbose Hibernate logging so the underlying SQL and dirty-checking behavior is visible:

logging:
  level:
    org.hibernate.SQL: DEBUG
    org.hibernate.orm.jdbc.bind: TRACE

⚠️ Turn these off (or drop to INFO) for anything beyond local experimentation — this level of logging is not meant for production.

📚 Related Reading

• 📝 Blog post:Programming Paradigm and where it came from

📄 License

Personal learning project — no license restrictions, feel free to reuse or adapt. 🙌