Hibernate - Many to Many using Annotations
Just like 'One to One' and 'One to Many', Many to Many can also be achieved using annotations in an easier way.
An 'Employee' can be assigned to many 'Project'. At the same time a 'Project' can also have multiple 'Employee' objects or employees.
Let us see the 'Employee' class:
class Employee {
@Id
@Column(name="ID")
String id;
@Column(name="NAME")
String name;
@ManyToMany(mappedBy="employee", Cascade=CascadeType.ALL)
Set <Project> projectSet = new HashSet <Project>();
---Getters & Setters---
}
An 'Employee' can be assigned to various 'Project'. So, in the 'Employee' class we have put:
Set <Project> projectSet = new HashSet <Project>();
And just above it we have put the @ManyToMany annotation.
@ManyToMany(mappedBy="employeeSet", Cascade=CascadeType.ALL)
mappedBy="employeeSet" tells Hibernate that Many to Many mapping has to be done with the 'employeeSet' object of the 'Project' class.
Cascade=CascadeType.ALL - If the Employee object is saved, updated or deleted, the Project objects associated to it should also be saved, updated or deleted.
The 'Project' class is defined below.
class Project{
@Id
@Column(name="PROJECT_ID")
int projectId;
@Column(name="PROJECT_NAME")
String projectName;
@ManyToMany
Set <Employee> employeeSet = new HashSet <Employee>();
---Getters & Setters---
}
Since, we are establishing a bi-directional relationship, we have put @ManyToMany annotation on top of
Set <Employee> employeeSet = new HashSet <Employee>();
So, we will be writing the main class where we will be saving the 'Employee' data and its corresponding 'Project' data to the database using Hibernate.
import org.hibernate.Session;
import org.hibernate.Transaction;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class HibernateSave {
public static void main(String[] args) {
static SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
Session session = sessionFactory.openSession();
session.beginTransaction();
Set <Project> projectSet = new Set <Project>();
Project projectPhills = new Project();
projectPhills.setProjectId("A001");
projectPhills.setProjectName("Phills Project");
projectSet.add(projectPhills);
Project projectONeal = new Project();
projectONeal.setProjectId("B001");
projectONeal.setProjectName("O Neals Project");
projectSet.add(projectONeal);
Employee employee = new Employee();
employee.setId(1);
employee.setName("Joe");
employee.setProjectSet(projectSet);
session.save(employee);
session.getTransaction().commit();
session.close();
sessionFactory.close();
}
}
EMPLOYEE
PROJECT
| PROJECT_ID |
PROJECT_NAME |
| A001 |
Phills Project |
| B001 |
O Neals Project |
EMPLOYEE_PROJECT
| ID |
PROJECT_ID |
| 1 |
A001 |
| 1 |
B001 |
Note : If mappedBy="employeeSet" was not given with @ManyToMany annotation in the Employee class, Hibernate would have created an extra table 'PROJECT_EMPLOYEE'. Which we don't want.
