dineshonjava

Many to Many Mapping in Hibernate

Many to many mapping in hibernate is required when each record in an entity may have many linked records in another entity and vice-versa.

In this tutorial you will learn how to map many-to-many relationship using Hibernate. Consider the following relationship between Vehicle and UserDetails entity.

 
Many to Many Mapping in Hibernate

According to the relationship a user can have in any number of vehicles and the vehicle can have any number of users.

UserDetail.java

package com.sdnext.hibernate.tutorial.dto;

import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table (name="USER")
public class UserDetails 
{
    @Id
    @Column(name="USER_ID")
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int    userId;
    
    @Column(name="USER_NAME") 
    private String userName;
    
    @ManyToMany
    private Collection<Vehicle> vehicle = new ArrayList<Vehicle>();
    
    public int getUserId() {
        return userId;
    }
    public Collection<Vehicle> getVehicle() {
        return vehicle;
    }
    public void setVehicle(Collection<Vehicle> vehicle) {
        this.vehicle = vehicle;
    }
    public void setUserId(int userId) {
        this.userId = userId;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
}

Vehicle.java


package com.sdnext.hibernate.tutorial.dto;

import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="VEHICLE")
public class Vehicle 
{
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="VEHICLE_ID")
    private int vehicleId;
    
    @Column(name="VEHICLE_NAME")
    private String vehicleName;
    
    @ManyToMany(mappedBy="vehicle")
    private Collection<UserDetails> user = new ArrayList<UserDetails>();
    
    public Collection<UserDetails> getUser() {
        return user;
    }
    public void setUser(Collection<UserDetails> user) {
        this.user = user;
    }
    public int getVehicleId() {
        return vehicleId;
    }
    public void setVehicleId(int vehicleId) {
        this.vehicleId = vehicleId;
    }
    public String getVehicleName() {
        return vehicleName;
    }
    public void setVehicleName(String vehicleName) {
        this.vehicleName = vehicleName;
    }
}

hibernate.cfg.xml:


<hibernate-configuration>
    <session-factory>
        <!-- Database connection settings -->
         <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
         <property name="connection.url">jdbc:mysql://localhost:3306/hibernateDB</property>
         <property name="connection.username">root</property>
         <property name="connection.password">root</property>

        <!-- JDBC connection pool (use the built-in) -->
         <property name="connection.pool_size">1</property>

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

        <!-- Enable Hibernate's automatic session context management -->
          <property name="current_session_context_class">thread</property>
       
        <!-- Disable the second-level cache -->
         <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
         <property name="show_sql">true</property>
       
        <!-- Drop and re-create the database schema on startup -->
         <property name="hbm2ddl.auto">create</property>
        
        
         <mapping class="com.sdnext.hibernate.tutorial.dto.UserDetails">
         <mapping class="com.sdnext.hibernate.tutorial.dto.Vehicle">
        
     </mapping></mapping></session-factory>
 </hibernate-configuration>
HibernateTestDemo.java

package com.sdnext.hibernate.tutorial;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;

import com.sdnext.hibernate.tutorial.dto.UserDetails;
import com.sdnext.hibernate.tutorial.dto.Vehicle;


public class HibernateTestDemo {

    /**
     * @param args
     */
    public static void main(String[] args)
    {
        UserDetails user = new UserDetails();
        UserDetails user2 = new UserDetails();
        
        Vehicle vehicle = new Vehicle();
        Vehicle vehicle2 = new Vehicle();
        
        vehicle.setVehicleName("Car");
        vehicle.getUser().add(user);
        vehicle.getUser().add(user2);
        
        vehicle2.setVehicleName("Jeep");
        vehicle2.getUser().add(user2);
        vehicle2.getUser().add(user);
        
        user.setUserName("First User");
        user2.setUserName("Second User");
        user.getVehicle().add(vehicle);
        user.getVehicle().add(vehicle2);
        user2.getVehicle().add(vehicle);
        user2.getVehicle().add(vehicle2);
        
        SessionFactory sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
        Session session = sessionFactory.openSession();
        session.beginTransaction();
        session.save(vehicle);
        session.save(vehicle2);
        session.save(user);
        session.save(user2);
        session.getTransaction().commit();
        session.close();
    }
}


Using Mapping files instead of Annotation

userDetail.hbm.xml
<hibernate-mapping>
    <class name="com.sdnext.hibernate.tutorial.dto.UserDetails" table="USER">
         <id column="ID" name="userId" type="long">
             <generator class="assigned">
          </generator></id>
          <property column="column" name="UserName">
          <list cascade="all" name="vehicle" table="USER_VEHICLE">
              <key column="USER_ID">
              <many-to-many class="com.sdnext.hibernate.tutorial.dto.Vehicle" column="VEHICLE_ID">
          </many-to-many></key></list>
    </property></class> 
</hibernate-mapping>
vehicle.hbm.xml

<hibernate-mapping>
     <class name="com.sdnext.hibernate.tutorial.dto.Vehicle" table="VEHICLE">
         <id column="ID" name="userId" type="long">
              <generator class="assigned">
         </generator></id>
         <property column="VEHICLE_NAME" name="vehicleName">
     </property></class>
</hibernate-mapping>



In Next Chapter we will discuss about the Cascade Type.

                    <<Previous Chapter 19<<    >>Next Chapter21>>