HB Core Concepts

Hibernate provides a powerful ORM (Object-Relational Mapping) framework that simplifies database interactions in Java applications. In this section, we'll explore the core components of Hibernate:

✅ Session & SessionFactory

✅ Transaction Management

✅ Entity Classes and Mappings

✅ Hibernate Dialects

Session & SessionFactory:

What is `SessionFactory`?

A heavyweight object that creates Session instances.
It is thread-safe and should be created once per application.
Uses a configuration file (hibernate.cfg.xml) to connect to the database.

What is `Session`?

A lightweight object used to interact with the database.
It is not thread-safe and should be created per database operation.
Used to perform CRUD (Create, Read, Update, Delete) operations.

Main.java

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.cfg.Configuration;

public class Main {

public static void main(String[] args) {

// Create and save Student object

Student student = new Student("John Doe", "john@example.com");

// Create Hibernate session

SessionFactory sessionFactory = new Configuration().configure("hibernate.cfg.xml").addAnnotatedClass(Student.class).buildSessionFactory();

Session session = sessionFactory.openSession();

// Close session

session.close();

sessionFactory.close();

System.out.println("Hibernate setup successful!");

}

Transaction Management

Why Use Transactions?

Transactions ensure data consistency. If an error occurs in the middle of an operation, the changes should be rolled back.

Transaction Methods

beginTransaction() → Starts a new transaction
commit() → Saves changes to the database
rollback() → Reverts changes if an error occurs

Main.java

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.cfg.Configuration;

public class TransactionExample {

public static void main(String[] args) {

Transaction transaction = null;

// Create a new student

Student student = new Student("Bob", "bob@example.com");

// Create Hibernate session

SessionFactory sessionFactory = new Configuration().configure("hibernate.cfg.xml").addAnnotatedClass(Student.class).buildSessionFactory();

Session session = sessionFactory.openSession();

try {

transaction = session.beginTransaction();

session.save(student);

// Commit transaction

transaction.commit();

System.out.println("Transaction Successful!");

} catch (Exception e) {

if (transaction != null) {

transaction.rollback(); // Rollback in case of error

System.out.println("Transaction Rolled Back!");

}

e.printStackTrace();

} finally {

session.close();

sessionFactory.close();

}

Entity Classes and Mappings

What is an Entity?

An entity is a Java class that is mapped to a database table using annotations or XML mappings.

Common Hibernate Annotations

Annotation	Description
@Entity	Marks the class as an Entity (Table in DB)
@Table(name="table_name")	Defines a custom table name
@Id	Marks a field as Primary Key
@GeneratedValue(strategy = GenerationType.IDENTITY)	Enables Auto-Increment for the ID
@Column(name="column_name")	Maps a field to a specific column name
@Transient	Prevents a field from being saved in the DB
@Temporal(TemporalType.DATE)	Stores Date-only values
@OneToOne,@OneToMany,@ManyToOne,@ManyToMany	Defines Relationships

Example: Entity Class with Mappings:

Student.java

import jakarta.persistence.*;

@Entity

@Table(name = "students")

public class Student {

@Id

@GeneratedValue(strategy = GenerationType.IDENTITY)

private Long id;

@Column(name = "student_name", nullable = false)

private String name;

@Column(unique = true)

private String email;

// Constructors

public Student() {}

public Student(String name, String email) {

this.name = name;

this.email = email;

}

// Getters and Setters

public Long getId() { return id; }

public void setId(Long id) { this.id = id; }

public String getName() { return name; }

public void setName(String name) { this.name = name; }

public String getEmail() { return email; }

public void setEmail(String email) { this.email = email; }

}

Hibernate Dialects

What is a Hibernate Dialect?

A dialect helps Hibernate translate HQL (Hibernate Query Language) into SQL for the specific database.

Common Hibernate Dialects

Database	Hibernate Dialect
MySQL	org.hibernate.dialect.MySQLDialect
PostgreSQL	org.hibernate.dialect.PostgreSQLDialect
Oracle	org.hibernate.dialect.OracleDialect
SQL Server	org.hibernate.dialect.SQLServerDialect
H2 (In-Memory)	org.hibernate.dialect.H2Dialect

Example: Setting Dialect in `hibernate.cfg.xml`

<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>

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Session & SessionFactory:

What is SessionFactory?

What is Session?