Java Data JPA Playground

A hands-on learning project for practicing Spring Boot and Spring Data JPA fundamentals — entity mapping, relationship types, inheritance strategies, composite/embedded keys, and repository query methods.

This isn't a production app — it's a sandbox for experimenting with how JPA/Hibernate behaves, using a small "online course platform" domain model (authors, courses, sections, lectures, resources).

Tech Stack

• Java 17
• Spring Boot 3.5.9
• Spring Data JPA (Hibernate)
• Spring Web
• PostgreSQL
• Lombok — reduces boilerplate on entities
• Java Faker — for generating fake test data
• Maven (with wrapper — no local install required)

Domain Model

The core model represents a simple course platform:

erDiagram
    AUTHOR }o--o{ COURSE : "authors_courses (many-to-many)"
    COURSE ||--o{ SECTION : "one-to-many"
    SECTION ||--o{ LECTURE : "one-to-many"
    LECTURE ||--|| RESOURCE : "one-to-one"

Resource is a base class extended by three concrete resource types, using table-per-class inheritance:

Resource (id, name, size, url)
 ├── Video  (+ length)
 ├── Text   (+ content)
 └── File   (+ type)

There's also a standalone example under models/embedded/ demonstrating a composite primary key:

• OrderId — an @Embeddable class combining username + orderDate, used as Order's @EmbeddedId
• Address — an @Embeddable value object embedded inside Order

JPA Concepts Practiced

Repository query methods

AuthorRepository demonstrates several ways Spring Data derives queries from method names, plus a custom JPQL update:

List<Author> findAllByFirstName(String fn);
List<Author> findAllByFirstNameIgnoreCase(String fn);
List<Author> findAllByFirstNameContainingIgnoreCase(String fn);
List<Author> findAllByFirstNameStartsWithIgnoreCase(String fn);
List<Author> findAllByFirstNameEndsWithIgnoreCase(String fn);
List<Author> findAllByFirstNameInIgnoreCase(List<String> firstNames);

@Modifying
@Transactional
@Query("UPDATE Author a SET a.age = :age WHERE a.id = :id")
void updateAuthor(int age, int id);

Project Structure

src/main/java/com/devlab/jpa/
├── JpaApplication.java              # entry point + CommandLineRunner for experiments
├── models/
│   ├── BaseEntity.java              # @MappedSuperclass with audit-style fields
│   ├── Author.java
│   ├── Course.java
│   ├── Section.java
│   ├── Lecture.java
│   ├── Resource.java                # table-per-class inheritance root
│   ├── Video.java
│   ├── Text.java
│   ├── File.java
│   └── embedded/
│       ├── Address.java             # @Embeddable
│       ├── Order.java               # entity with composite key
│       └── OrderId.java             # @EmbeddedId
├── repositories/
│   └── AuthorRepository.java
└── resources/
    └── application.yml

Prerequisites

• Java 17+
• PostgreSQL running locally (Maven wrapper is included, so a separate Maven install isn't required)

Getting Started

1. Clone the repo

   ShellCopy code

   git clone https://github.com/theShihamAhamed/java-data-jpa_playground.git
   cd java-data-jpa_playground
   

2. Create the database

   sqlCopy code

   CREATE DATABASE data_jpa;
   

3. Configure the connection

   Database credentials live in src/main/resources/application.yml. Update them to match your local Postgres setup:

   yamlCopy code

   spring:
     datasource:
       url: jdbc:postgresql://localhost:5432/data_jpa
       username: postgres
       password: your_password
   

   This file currently has local defaults committed for convenience — if you make this a real project, move credentials to environment variables and add application.yml to .gitignore instead.

4. Run the application

   ShellCopy code

   ./mvnw spring-boot:run
   

   On Windows, use mvnw.cmd spring-boot:run instead.

Notes

• show-sql: true is enabled, so generated SQL is printed to the console — useful for seeing exactly what Hibernate does for each mapping/query.
• JpaApplication's CommandLineRunner bean is where new repository calls get tried out during learning — swap out its contents to experiment with different queries or entity relationships.

License

Personal learning project — feel free to use it as a reference.