Prerequisites

1. Java Development Kit (JDK) 21 or higher: Ensure JDK is installed and configured on your system.
2. Integrated Development Environment (IDE): IntelliJ IDEA, Eclipse, or any other IDE.
3. Maven: Ensure Maven is installed and configured on your system.

Step 1: Create a Maven Project

1. Open your IDE and create a new Maven project.
2. Update the pom.xml file to include Hibernate and other required dependencies.

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.example</groupId> <artifactId>hibernate-many-to-many-example</artifactId> <version>1.0-SNAPSHOT</version> <dependencies> <dependency> <groupId>org.hibernate.orm</groupId> <artifactId>hibernate-core</artifactId> <version>6.2.0.Final</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-api</artifactId> <version>1.7.30</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-simple</artifactId> <version>1.7.30</version> </dependency> <dependency> <groupId>com.h2database</groupId> <artifactId>h2</artifactId> <version>1.4.200</version> </dependency> </dependencies> </project> 

Explanation

• Hibernate Core Dependency: Includes the main Hibernate framework.
• SLF4J Dependencies: Used for logging.
• H2 Database Dependency: An in-memory database for testing purposes.

Step 2: Create Hibernate Configuration File

Create a file named hibernate.cfg.xml in the src/main/resources directory.

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <property name="hibernate.dialect">org.hibernate.dialect.H2Dialect</property> <property name="hibernate.connection.driver_class">org.h2.Driver</property> <property name="hibernate.connection.url">jdbc:h2:mem:testdb</property> <property name="hibernate.connection.username">sa</property> <property name="hibernate.connection.password"></property> <property name="hibernate.hbm2ddl.auto">update</property> <property name="hibernate.show_sql">true</property> <property name="hibernate.format_sql">true</property> </session-factory> </hibernate-configuration> 

Explanation

• Dialect: Specifies the SQL dialect (H2 in this case).
• Connection Properties: Configure the JDBC connection to the H2 database.
• hbm2ddl.auto: Automatically manages the database schema (update existing schema).
• show_sql: Prints SQL statements to the console.
• format_sql: Formats SQL statements.

Step 3: Create the Student Entity Class

Create a package named com.example.entity and a class named Student.

package com.example.entity; import jakarta.persistence.*; import java.util.HashSet; import java.util.Set; @Entity public class Student { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; @ManyToMany(cascade = CascadeType.ALL) @JoinTable( name = "student_course", joinColumns = @JoinColumn(name = "student_id"), inverseJoinColumns = @JoinColumn(name = "course_id") ) private Set<Course> courses = new HashSet<>(); public Student() {} public Student(String name) { this.name = name; } 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 Set<Course> getCourses() { return courses; } public void setCourses(Set<Course> courses) { this.courses = courses; } public void addCourse(Course course) { courses.add(course); course.getStudents().add(this); } public void removeCourse(Course course) { courses.remove(course); course.getStudents().remove(this); } @Override public String toString() { return "Student{id=" + id + ", name='" + name + '\'' + '}'; } } 

Explanation

• @Entity: Marks the class as an entity.
• @Id: Marks the field as the primary key.
• @GeneratedValue: Specifies the strategy for generating values for the primary key.
• @ManyToMany: Defines a many-to-many relationship with the Course entity.
• @JoinTable: Specifies the join table.
• @JoinColumn: Specifies the foreign key column for the current entity.
• @inverseJoinColumns: Specifies the foreign key column for the other entity in the relationship.

Step 4: Create the Course Entity Class

Create a class named Course in the same package.

package com.example.entity; import jakarta.persistence.*; import java.util.HashSet; import java.util.Set; @Entity public class Course { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; @ManyToMany(mappedBy = "courses") private Set<Student> students = new HashSet<>(); public Course() {} public Course(String name) { this.name = name; } 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 Set<Student> getStudents() { return students; } public void setStudents(Set<Student> students) { this.students = students; } @Override public String toString() { return "Course{id=" + id + ", name='" + name + '\'' + '}'; } } 

Explanation

• @Entity: Marks the class as an entity.
• @Id: Marks the field as the primary key.
• @GeneratedValue: Specifies the strategy for generating values for the primary key.
• @ManyToMany: Defines a many-to-many relationship with the Student entity.
• mappedBy: Specifies the field in the Student entity that owns the relationship.

Step 5: Create a Hibernate Utility Class

Create a package named com.example.util and a class named HibernateUtil.

package com.example.util; import org.hibernate.SessionFactory; import org.hibernate.boot.registry.StandardServiceRegistryBuilder; import org.hibernate.cfg.Configuration; import org.hibernate.service.ServiceRegistry; public class HibernateUtil { private static SessionFactory sessionFactory; static { try { Configuration configuration = new Configuration().configure(); ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder() .applySettings(configuration.getProperties()).build(); sessionFactory = configuration.buildSessionFactory(serviceRegistry); } catch (Throwable ex) { throw new ExceptionInInitializerError(ex); } } public static SessionFactory getSessionFactory() { return sessionFactory; } } 

Explanation

• Configuration: Loads Hibernate configuration from hibernate.cfg.xml.
• ServiceRegistry: Builds the service registry from the configuration settings.
• SessionFactory: Provides sessions to interact with the database.

Step 6: Create Main Class

Create a package named com.example and a class named Main.

package com.example; import com.example.entity.Course; import com.example.entity.Student; import com.example.util.HibernateUtil; import org.hibernate.Session; import org.hibernate.Transaction; public class Main { public static void main(String[] args) { // Initialize session and transaction Session session = HibernateUtil.getSessionFactory().openSession(); Transaction transaction = session.beginTransaction(); // Create students Student student1 = new Student("John Doe"); Student student2 = new Student("Jane Doe"); // Create courses Course course1 = new Course("Math"); Course course2 = new Course("Science"); // Add courses to students student1.addCourse(course1); student1.addCourse(course2); student2.addCourse(course1); // Save students (this will also save courses due to cascade) session.save(student1); session.save(student2); transaction.commit(); session.close(); // Retrieve and display students session = HibernateUtil.getSessionFactory().openSession(); Student retrievedStudent1 = session.get(Student.class, student1.getId()); Student retrievedStudent2 = session.get(Student.class, student2.getId()); System.out.println("Retrieved Student 1: " + retrievedStudent1); System.out.println("Courses: " + retrievedStudent1.getCourses()); System.out.println("Retrieved Student 2: " + retrievedStudent2); System.out.println("Courses: " + retrievedStudent2.getCourses()); session.close(); // Update a student's course session = HibernateUtil.getSessionFactory().openSession(); transaction = session.beginTransaction(); Student studentToUpdate = session.get(Student.class, student1.getId()); studentToUpdate.removeCourse(course2); session.update(studentToUpdate); transaction.commit(); session.close(); // Delete a student session = HibernateUtil.getSessionFactory().openSession(); transaction = session.beginTransaction(); Student studentToDelete = session.get(Student.class, student2.getId()); session.delete(studentToDelete); transaction.commit(); session.close(); // Retrieve and display updated students session = HibernateUtil.getSessionFactory().openSession(); Student updatedStudent1 = session.get(Student.class, student1.getId()); System.out.println("Updated Student 1: " + updatedStudent1); System.out.println("Courses: " + updatedStudent1.getCourses()); session.close(); // Close the SessionFactory HibernateUtil.getSessionFactory().close(); } } 

Explanation

• Session: Opens a session to interact with the database.
• Transaction: Begins and commits a transaction for database operations.
• Save: Persists the entity to the database.
• Retrieve: Fetches the entity from the database using its ID.
• Update: Updates the entity in the database.
• Delete: Removes the entity from the database.

Step 7: Run the Application

1. Run the Main class.
2. The output in the console should be:

Hibernate: create table Course (id bigint generated by default as identity, name varchar(255), primary key (id)) Hibernate: create table Student (id bigint generated by default as identity, name varchar(255), primary key (id)) Hibernate: create table student_course (student_id bigint not null, course_id bigint not null, primary key (student_id, course_id)) Hibernate: alter table student_course add constraint FKk8ue48wh82jw46hmhp8u6up1o foreign key (course_id) references Course Hibernate: alter table student_course add constraint FKp0fhshy3x4bdojl88kxwsh7hf foreign key (student_id) references Student Hibernate: insert into Course (name) values (?) Hibernate: insert into Course (name) values (?) Hibernate: insert into Student (name) values (?) Hibernate: insert into Student (name) values (?) Hibernate: insert into student_course (student_id, course_id) values (?, ?) Hibernate: insert into student_course (student_id, course_id) values (?, ?) Hibernate: insert into student_course (student_id, course_id) values (?, ?) Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_ from Student student0_ where student0_.id=? Hibernate: select courses0_.student_id as student_1_3_0_, courses0_.course_id as course_i2_3_0_, course1_.id as id1_0_1_, course1_.name as name2_0_1_ from student_course courses0_ inner join Course course1_ on courses0_.course_id=course1_.id where courses0_.student_id=? Retrieved Student 1: Student{id=1, name='John Doe'} Courses: [Course{id=1, name='Math'}, Course{id=2, name='Science'}] Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_ from Student student0_ where student0_.id=? Hibernate: select courses0_.student_id as student_1_3_0_, courses0_.course_id as course_i2_3_0_, course1_.id as id1_0_1_, course1_.name as name2_0_1_ from student_course courses0_ inner join Course course1_ on courses0_.course_id=course1_.id where courses0_.student_id=? Retrieved Student 2: Student{id=2, name='Jane Doe'} Courses: [Course{id=1, name='Math'}] Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_ from Student student0_ where student0_.id=? Hibernate: select courses0_.student_id as student_1_3_0_, courses0_.course_id as course_i2_3_0_, course1_.id as id1_0_1_, course1_.name as name2_0_1_ from student_course courses0_ inner join Course course1_ on courses0_.course_id=course1_.id where courses0_.student_id=? Hibernate: delete from student_course where student_id=? and course_id=? Hibernate: update Student set name=? where id=? Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_ from Student student0_ where student0_.id=? Hibernate: select courses0_.student_id as student_1_3_0_, courses0_.course_id as course_i2_3_0_, course1_.id as id1_0_1_, course1_.name as name2_0_1_ from student_course courses0_ inner join Course course1_ on courses0_.course_id=course1_.id where courses0_.student_id=? Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_ from Student student0_ where student0_.id=? Hibernate: select courses0_.student_id as student_1_3_0_, courses0_.course_id as course_i2_3_0_, course1_.id as id1_0_1_, course1_.name as name2_0_1_ from student_course courses0_ inner join Course course1_ on courses0_.course_id=course1_.id where courses0_.student_id=? Hibernate: delete from student_course where student_id=? Hibernate: delete from Student where id=? Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_ from Student student0_ where student0_.id=? Hibernate: select courses0_.student_id as student_1_3_0_, courses0_.course_id as course_i2_3_0_, course1_.id as id1_0_1_, course1_.name as name2_0_1_ from student_course courses0_ inner join Course course1_ on courses0_.course_id=course1_.id where courses0_.student_id=? Updated Student 1: Student{id=1, name='John Doe'} Courses: [Course{id=1, name='Math'}] 

Conclusion

You have successfully created an example using Hibernate to demonstrate a many-to-many mapping with annotations. This tutorial covered setting up a Maven project, configuring Hibernate, creating entity classes with a many-to-many relationship, and performing CRUD operations.