Hibernate JPA Many To Many Relation Mapping Example

By admin | November 15, 2013 | Updated : May 23, 2016 hibernate, jpa, No Comments

Create Employee & Event model classes as per the above table structure

Employee.java

package com.jpa.manytomany;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
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 Employee implements java.io.Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue
    @Column(name = "EMPLOYEEID")
    private long employeeId;
    @Column(name = "EMPLOYEENAME")
    private String employeeName;
    @ManyToMany(cascade = { CascadeType.ALL })
    @JoinTable(name = "EMPLOYEE_EVENT", joinColumns = { @JoinColumn(name = "EMPLOYEEID") },
    inverseJoinColumns = { @JoinColumn(name = "EVENTID") })
    private Set<Event> events = new HashSet<Event>();

    public Employee() {
        super();
    }

    public Employee(String employeeName) {
        super();
        this.employeeName = employeeName;
    }

    public long getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(long employeeId) {
        this.employeeId = employeeId;
    }

    public String getEmployeeName() {
        return employeeName;
    }

    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    public Set<Event> getEvents() {
        return events;
    }

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

}

Event.java

package com.jpa.manytomany;

import java.util.HashSet;
import java.util.Set;

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

@Entity
public class Event implements java.io.Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue
    @Column(name = "EVENTID")
    private long eventID;
    @Column(name = "EVENTNAME")
    private String eventName;

    @ManyToMany(mappedBy="events")
    private Set<Employee> employees = new HashSet<Employee>();

    public Event() {
        super();
    }

    public Event(String eventName) {
        this.eventName = eventName;
    }

    public long getEventID() {
        return eventID;
    }

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

    public String getEventName() {
        return eventName;
    }

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

    public Set<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(Set<Employee> employees) {
        this.employees = employees;
    }



}

@ManyToMany(cascade = { CascadeType.ALL }) annotation is used for linking each record of Employee table with Event table and vice versa
@JoinTable(name = "EMPLOYEE_EVENT", joinColumns = { @JoinColumn(name = "EMPLOYEEID") }, inverseJoinColumns = { @JoinColumn(name = "EVENTID") }) is used to define the join table, here it is "EMPLOYEE_EVENT", which is connecting 2 columns ie; EMPLOYEEID belongs to employee table & EVENTID belongs to event table
@Entity declares the class as an entity (i.e. a persistent POJO class)
@Table is set at the class level; it allows you to define the table, catalog, and schema names for your entity mapping. If no @Table is defined the default values are used: the unqualified class name of the entity.
@Id declares the identifier property of this entity.
@GeneratedValue annotation is used to specify the primary key generation strategy to use. If the strategy is not specified by default AUTO will be used.
@Column annotation is used to specify the details of the column to which a field or property will be mapped. If the @Column annotation is not specified by default the property name will be used as the column name.

EntityManagerUtil.java

package com.jpa.manytomany;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class EntityManagerUtil {
private static final EntityManagerFactory entityManagerFactory;
static {
    try {
      entityManagerFactory = Persistence.createEntityManagerFactory("test");

    } catch (Throwable ex) {
      System.err.println("Initial SessionFactory creation failed." + ex);
      throw new ExceptionInInitializerError(ex);
    }
}

public static EntityManager getEntityManager() {
    return entityManagerFactory.createEntityManager();

}
}

persistence.xml

persistence.xml file must be under src/META-INF (Please check the screenshot(project structure)

<persistence xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
	version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
	<persistence-unit name="test" transaction-type="RESOURCE_LOCAL">
		<properties>
			<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
			<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/test" />
			<property name="javax.persistence.jdbc.user" value="root" />
			<property name="javax.persistence.jdbc.password" value="root" />
			<property name="hibernate.hbm2ddl.auto" value="create-drop" />
			<property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect" />
		</properties>
	</persistence-unit>
</persistence>

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You might also like following tutorials :

Employee.java

Event.java

EntityManagerUtil.java

persistence.xml

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