💾 Hibernate

Hibernate is a framework used for persisting (saving) Java objects to a database.

• Handles low-level SQL operations internally
• Minimizes the amount of JDBC code developers need to write
• Provides Object-to-Relational Mapping (ORM) between Java classes and database tables

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📘 What is JPA?

JPA stands for Jakarta Persistence API (previously Java Persistence API).

• A standard specification for ORM (Object-to-Relational Mapping)
• Defines a set of interfaces
• Requires a concrete implementation (like Hibernate, EclipseLink, etc.) to work

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✅ Benefits of Using JPA

• Promotes portability and flexibility by coding to a standard API
• Avoids vendor lock-in — you can switch implementations if needed
• Example: If Vendor ABC discontinues their JPA implementation, you can switch to Vendor XYZ with minimal code changes

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🏷️ Common JPA Annotations

JPA provides a set of annotations to map Java classes to database tables and manage relationships.

📄 Entity & Table Mapping

Annotation	Purpose
@Entity	Marks the class as a JPA entity (table in DB)
@Table(name="...")	Specifies the table name in the database

🔑 Primary Key & Generation

Annotation	Purpose
@Id	Marks the primary key field
@GeneratedValue	Specifies how the primary key should be generated
@Column(name="...")	Maps a field to a column in the database

🔁 Relationships

Annotation	Purpose
@OneToOne	One-to-one relationship
@OneToMany	One-to-many relationship
@ManyToOne	Many-to-one relationship
@ManyToMany	Many-to-many relationship
@JoinColumn	Defines the foreign key column for a relationship

🕵️ Other Useful Annotations

Annotation	Purpose
@Transient	Excludes a field from being persisted
@Lob	Marks a field as Large Object (e.g. BLOB, CLOB)
@Temporal	Specifies date/time precision (for Date fields)

These annotations allow you to control how Java objects map to database tables and manage data relationships efficiently.

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🔑 ID Generation Strategies in JPA

JPA provides multiple strategies for automatically generating primary key values using the @GeneratedValue annotation.

🧩 Syntax:

@Id
@GeneratedValue(strategy = GenerationType.TYPE)
private Long id;

Strategy	Description
AUTO	Default strategy; lets the persistence provider choose the best strategy
IDENTITY	Uses the database's auto-increment feature for primary key generation
SEQUENCE	Uses a database sequence object to generate primary keys
TABLE	Uses a separate table to generate and manage primary key values

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🆚 EntityManager vs JpaRepository

Feature	EntityManager	JpaRepository
Type	Low-level API	High-level abstraction built on top of JPA
Origin	Part of JPA Specification	Part of Spring Data JPA
Method Definitions	Requires manual creation of queries and methods	Comes with predefined CRUD methods
Query Writing	Manual with JPQL or Criteria API	Supports method naming conventions and custom queries
Boilerplate Code	More code required	Minimal code required
Use Case	Fine-grained control over persistence operations	Rapid development with common database operations
Example Usage	entityManager.persist(entity)	jpaRepository.save(entity)

✅ Summary:

• Use EntityManager when you need custom, low-level control.
• Use JpaRepository for standard CRUD operations and faster development.

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⚙️ spring.jpa.hibernate.ddl-auto

The spring.jpa.hibernate.ddl-auto property in application.properties controls how the database schema is managed at runtime.

🔧 Common Values:

Value	Description
none	No action will be performed (default if not set explicitly)
create	Drops and recreates the database schema each time the app starts
create-drop	Same as create, but also drops schema when the app shuts down
update	Updates the existing schema without losing data
validate	Validates that the schema matches the entities, but does not modify it

📝 Example:

spring.jpa.hibernate.ddl-auto=update

⚠️ Use create or create-drop only in development, as they will delete existing data.