Steps to Implement Many-To-Many Mapping in JPA

1. Define the entities involved in the relationship.

We can identify the entities that participate in many-to-many relationships. For example, we can consider the entities of the Student and Course.

Student Entity:

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String name;
    
    @ManyToMany(mappedBy = "students")
    private Set<Course> courses = new HashSet<>();
    
    // Getters and setters
}

Course Entity:

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String name;
    
    @ManyToMany
    @JoinTable(
        name = "course_student",
        joinColumns = @JoinColumn(name = "course_id"),
        inverseJoinColumns = @JoinColumn(name = "student_id")
    )
    private Set<Student> students = new HashSet<>();
    
    // Getters and setters
}

2. Configure the Relationships of the Entities

We can use the annotations such as @ManyToMany and it defines the relationship between the entities. In Course entity, we can use the @JoinTable to specify the join table names and the columns.

3. Map to the Join Table

In the course entity, we can use @JoinTable annotation to the map the join table that can links the Course and Student entities of the application.

 @ManyToMany
    @JoinTable(
            name = "course_student", // Name of the join table
            joinColumns = @JoinColumn(name = "course_id"), // Column in the join table that references Course
            inverseJoinColumns = @JoinColumn(name = "student_id") // Column in the join table that references Student
    )
    private Set<Student> students = new HashSet<>();

4. Access and Manipulate the Associated Entities

Once the entites are properly can be configured and persisted. We can access and manipulate the associated entities as needed to the application.

// Creating a new student
Student student = new Student();
student.setName("John");

// Creating a new course
Course course = new Course();
course.setName("Math");

// Associating the student with the course
student.getCourses().add(course);
course.getStudents().add(student);

// Saving the entities
entityManager.getTransaction().begin();
entityManager.persist(student);
entityManager.persist(course);
entityManager.getTransaction().commit();

// Retrieving a student and accessing associated courses
Student retrievedStudent = entityManager.find(Student.class, student.getId());
Set<Course> courses = retrievedStudent.getCourses();
for (Course c : courses) {
    System.out.println("Course Name: " + c.getName());
}

// Retrieving a course and accessing associated students
Course retrievedCourse = entityManager.find(Course.class, course.getId());
Set<Student> students = retrievedCourse.getStudents();
for (Student s : students) {
    System.out.println("Student Name: " + s.getName());
}

JPA Many-To-Many Mapping provides the many-to-many relationship representing the association between the two entities where each instance of the one entity can be associated with the multiple instances of the other entity.

Many-to-many mapping is used when multiple instances of one entity are associated with multiple instances of another entity, and it forms the many-to-many relationship. In database terms, it can be typically implemented using the join table that links the primary keys of both entities.