Many to Many Mapping

Event.java

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;

@Entity
public class Event {

 private int eventID;
 private String eventName;
 private List<Delegate> delegates = new ArrayList<Delegate>();

 @Id
 @GeneratedValue
 public int getEventID() {
  return eventID;
 }

 public void setEventID(int eventID) {
  this.eventID = eventID;
 }

 public String getEventName() {
  return eventName;
 }

 public void setEventName(String eventName) {
  this.eventName = eventName;
 }

 @ManyToMany
 @JoinTable(name="Join_Delegate_event",
 joinColumns={@JoinColumn(name="eventID")},
 inverseJoinColumns={@JoinColumn(name="delegateID")}
 )
 public List<Delegate> getDelegates() {
  return delegates;
 }

 public void setDelegates(List<Delegate> delegates) {
  this.delegates = delegates;
 }

}

Delegate.java

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;

@Entity
public class Delegate {

 private int delegateID;
 private String delegateName;
 private List<Event> events = new ArrayList<Event>();

 @Id
 @GeneratedValue
 public int getDelegateID() {
  return delegateID;
 }

 public void setDelegateID(int delegateID) {
  this.delegateID = delegateID;
 }

 public String getDelegateName() {
  return delegateName;
 }

 public void setDelegateName(String delegateName) {
  this.delegateName = delegateName;
 }

 @ManyToMany
 @JoinTable(name="Join_Delegate_event",
 joinColumns={@JoinColumn(name="delegateID")},
 inverseJoinColumns={@JoinColumn(name="eventID")}
 )
 public List<Event> getEvents() {
  return events;
 }

 public void setEvents(List<Event> events) {
  this.events = events;
 }

}

Test.java

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;


public class Test {

 /**
  * @param args
  */
 public static void main(String[] args) {
  AnnotationConfiguration config=new AnnotationConfiguration();
  config.addAnnotatedClass(Event.class);
  config.addAnnotatedClass(Delegate.class);
  config.configure("hibernate.cfg.xml");
  
  new SchemaExport(config).create(true, true);
  SessionFactory factory=config.buildSessionFactory();
  Session session=factory.getCurrentSession();
  session.beginTransaction();
  
  Delegate del1=new Delegate();
  del1.setDelegateName("Kanishka Dilshan");

  Delegate del2=new Delegate();
  del2.setDelegateName("Sajith Athukorala");
  
  Delegate del3=new Delegate();
  del3.setDelegateName("Vinod De Silva");
  
  Delegate del4=new Delegate();
  del4.setDelegateName("Ravindy Madhuwan");
  
  Event ev1=new Event();
  ev1.setEventName("Java");
  
  Event ev2=new Event();
  ev2.setEventName("C++");
  
  Event ev3=new Event();
  ev3.setEventName("Assembly");
  
  ev1.getDelegates().add(del1);
  ev1.getDelegates().add(del2);
  ev1.getDelegates().add(del3);
  
  ev2.getDelegates().add(del2);
  ev2.getDelegates().add(del4);
  
  ev3.getDelegates().add(del3);
  
  session.save(del1);
  session.save(del2);
  session.save(del3);
  session.save(del4);
  
  session.save(ev1);
  session.save(ev2);
  session.save(ev3);
  
  session.getTransaction().commit();
 }

}

New Annotations :

• @ManyToMany : denotes that the entity has many to many relationship
• @JoinTable(name="Join_Delegate_event",
      joinColumns={@JoinColumn(name="eventID")},
      inverseJoinColumns={@JoinColumn(name="delegateID")}
      )   : Here the name parameter denotes the table name which is created to maintain many to many relationship.

Result :