Chapter 4.1☕ 16 min read

JPA Basics: @Entity, @Id, @GeneratedValue

Your Java class becomes a database table with just a few annotations.

01The Concept: ORM and Entities

Until now, we saved our API data inside an ArrayList. But if you restart your Spring Boot app, all data is lost. A real backend needs a database. Spring Boot uses Spring Data JPA to talk to SQL databases (like MySQL, PostgreSQL) using Java objects instead of writing raw SQL queries.

The Hyderabad IT Employee ID Card Analogy:

When a new developer joins an IT firm in HITEC City, they don't just exist as a name on a piece of paper. HR creates a profile for them in the company database and assigns a unique Employee ID badge (like EMP-101).

In Spring Boot, ORM (Object-Relational Mapping) works the same way. You write a normal Java class, but you mark it as an @Entity. This tells Spring: "This Java class is a Database Table." The @Id annotation acts as the unique Employee ID badge (Primary Key), and @GeneratedValue tells the database to automatically increment the ID number whenever a new record is created.

02Technical Explanation

@Entity: Tells Spring Boot/JPA to map this Java class to a database table. By default, the table name matches the class name.
@Id: Marks a field as the Primary Key.
@GeneratedValue(strategy = GenerationType.IDENTITY): Tells the database to auto-increment the ID column (e.g., 1, 2, 3...) automatically. You don't pass an ID when creating an object; the database gives it one.

03Full Working Code: The H2 Database Setup

First, we need to configure H2 in application.properties.

# Enable H2 Console in the browser
spring.h2.console.enabled=true
# URL to connect to the in-memory DB
spring.datasource.url=jdbc:h2:mem:devinhyderabad
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=
# Hibernate should auto-create tables based on Entity classes
spring.jpa.hibernate.ddl-auto=update

04The Java Entity Code (Book.java)

package com.devinhyderabad;

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;

// 1. Maps this class to a database table named "Book"
@Entity
public class Book {

    // 2. This is the Primary Key
    @Id
    // 3. Database will auto-generate IDs (1, 2, 3...)
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;
    private String author;

    // Default constructor is REQUIRED by JPA/Hibernate
    public Book() {}

    public Book(String title, String author) {
        this.title = title;
        this.author = author;
    }

    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getTitle() { return title; }
    public void setTitle(String title) { this.title = title; }
    public String getAuthor() { return author; }
    public void setAuthor(String author) { this.author = author; }
}

If you run this app, Spring Boot will automatically create a table called BOOK in the H2 database with columns id, title, and author, even though we never wrote a CREATE TABLE SQL command!

05Why It Matters / Interview Note

Interview Question: "What is the difference between GenerationType.IDENTITY and GenerationType.SEQUENCE?"

Answer: IDENTITY relies on the database table's auto-increment feature (like MySQL AUTO_INCREMENT). The database assigns the ID only at the time of the actual INSERT SQL execution. SEQUENCE uses a separate database object (like a counter) to generate IDs before the insert. Hibernate prefers SEQUENCE because it allows pre-allocation of IDs, which improves batch-insert performance.

Enterprise Note: In enterprise apps, we use spring.jpa.hibernate.ddl-auto=update only for local development. In production, using Hibernate to create/alter tables is dangerous and banned. We use Flyway or Liquibase (covered in Chapter 7) to version-control database changes manually.

Key Takeaways

✅ @Entity maps a Java class to a database table
✅ @Id marks the Primary Key field
✅ @GeneratedValue auto-increments the ID column
✅ ORM bridges Java objects and SQL tables seamlessly

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