🗄️ Spring Data JPA & Hibernate — Complete Interview Notes

Stack: Spring Boot 3.x · Spring Data JPA · Hibernate 6.x · PostgreSQL

1. JPA vs Hibernate vs Spring Data JPA

Conceptual Hierarchy

Spring Data JPA   (abstraction layer — repositories, query derivation)
      ↓
   JPA API        (javax.persistence / jakarta.persistence — specification)
      ↓
  Hibernate       (JPA implementation — actual ORM engine)
      ↓
   JDBC           (raw DB connection)

Feature	JPA	Hibernate	Spring Data JPA
Type	Specification (interface)	Implementation	Abstraction
Package	`jakarta.persistence`	`org.hibernate`	`org.springframework.data`
Provides	Annotations, JPQL	HQL, Session, Caching	Repositories, Query derivation
Standalone	❌	✅	❌ (needs JPA impl)

application.properties — Baseline Config

# DataSource
spring.datasource.url=jdbc:postgresql://localhost:5432/mydb
spring.datasource.username=postgres
spring.datasource.password=secret
spring.datasource.driver-class-name=org.postgresql.Driver

# JPA / Hibernate
spring.jpa.database-platform=org.hibernate.dialect.PostgreSQLDialect
spring.jpa.hibernate.ddl-auto=validate        # prod: validate | dev: update
spring.jpa.show-sql=true
spring.jpa.properties.hibernate.format_sql=true
spring.jpa.properties.hibernate.use_sql_comments=true

# HikariCP
spring.datasource.hikari.maximum-pool-size=10
spring.datasource.hikari.minimum-idle=2
spring.datasource.hikari.connection-timeout=30000

2. Entity Lifecycle & States

Four Entity States

New / Transient  ──persist()──▶  Managed / Persistent
                                        │
                              flush() → SQL sent to DB
                                        │
                              detach() / close()
                                        ↓
                                    Detached  ──merge()──▶  Managed

Managed ──remove()──▶  Removed  ──commit()──▶  Deleted from DB

State	In PersistenceContext	DB Row Exists	Changes Tracked
Transient	❌	❌	❌
Managed	✅	✅ (after flush)	✅ (dirty check)
Detached	❌	✅	❌
Removed	✅	✅ (until commit)	✅

Code Example

@Service
@Transactional
public class UserService {

    @PersistenceContext
    private EntityManager em;

    public void demonstrateLifecycle() {
        // TRANSIENT — no ID, not tracked
        User user = new User("Aditya", "[email protected]");

        // MANAGED — tracked by persistence context
        em.persist(user);

        // Dirty checking — no explicit save needed!
        user.setEmail("[email protected]"); // UPDATE will be issued at flush

        // DETACHED
        em.detach(user);
        user.setEmail("[email protected]"); // NOT tracked

        // Back to MANAGED
        User mergedUser = em.merge(user);

        // REMOVED
        em.remove(mergedUser);
    }
}

⚠️ Key Interview Point: In Spring Data JPA, save() calls persist() for new entities and merge() for detached ones. It checks id == null OR @Version field.