Hibernate many-to-many relationship với MySQL trên Eclipse

create table EMPLOYEE (
   id INT NOT NULL auto_increment,
   first_name VARCHAR(20) default NULL,
   last_name  VARCHAR(20) default NULL,
   salary     INT  default NULL,
   PRIMARY KEY (id)
);

create table CERTIFICATE (
   id INT NOT NULL auto_increment,
   certificate_name VARCHAR(30) default NULL,
   PRIMARY KEY (id)
);

create table EMP_CERT (
   employee_id INT NOT NULL,
   certificate_id INT NOT NULL,
   PRIMARY KEY (employee_id,certificate_id)
);

public class Certificate {
   private int id;
   private String name; 

   public Certificate() {}
   public Certificate(String name) {
      this.name = name;
   }
   public int getId() {
      return id;
   }
   public void setId( int id ) {
      this.id = id;
   }
   public String getName() {
      return name;
   }
   public void setName( String name ) {
      this.name = name;
   }
   public boolean equals(Object obj) {
      if (obj == null) return false;
      if (!this.getClass().equals(obj.getClass())) return false;

      Certificate obj2 = (Certificate)obj;
      if((this.id == obj2.getId()) && (this.name.equals(obj2.getName())))
      {
         return true;
      }
      return false;
   }
   public int hashCode() {
      int tmp = 0;
      tmp = ( id + name ).hashCode();
      return tmp;
   }
}

import java.util.*;

public class Employee {
   private int id;
   private String firstName; 
   private String lastName;   
   private int salary;
   private Set certificates;

   public Employee() {}
   public Employee(String fname, String lname, int salary) {
      this.firstName = fname;
      this.lastName = lname;
      this.salary = salary;
   }
   public int getId() {
      return id;
   }
   public void setId( int id ) {
      this.id = id;
   }
   public String getFirstName() {
      return firstName;
   }
   public void setFirstName( String first_name ) {
      this.firstName = first_name;
   }
   public String getLastName() {
      return lastName;
   }
   public void setLastName( String last_name ) {
      this.lastName = last_name;
   }
   public int getSalary() {
      return salary;
   }
   public void setSalary( int salary ) {
      this.salary = salary;
   }

   public Set getCertificates() {
      return certificates;
   }
   public void setCertificates( Set certificates ) {
      this.certificates = certificates;
   }
}

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
 "-//Hibernate/Hibernate Mapping DTD//EN"
 "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="vn.viettuts.hibernate.entity.Employee" table="EMPLOYEE">
        <id name="id" type="int" column="id">
            <generator class="native" />
        </id>
        <set name="certificates" cascade="save-update" table="EMP_CERT">
            <key column="employee_id" />
            <many-to-many column="certificate_id" 
            class="vn.viettuts.hibernate.entity.Certificate" />
        </set>
        <property name="firstName" column="first_name" type="string" />
        <property name="lastName" column="last_name" type="string" />
        <property name="salary" column="salary" type="int" />
    </class>

    <class name="vn.viettuts.hibernate.entity.Certificate" table="CERTIFICATE">
        <id name="id" type="int" column="id">
            <generator class="native" />
        </id>
        <property name="name" column="certificate_name" type="string" />
    </class>
</hibernate-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration SYSTEM 
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
    <session-factory>
        <property name="hibernate.dialect">
            org.hibernate.dialect.MySQLDialect
        </property>
        <property name="hibernate.connection.driver_class">
            com.mysql.jdbc.Driver
        </property>

         
        <property name="hibernate.connection.url">
            jdbc:mysql://localhost/testdb
        </property>
        <property name="hibernate.connection.username">
            root
        </property>
        <property name="hibernate.connection.password">
            1234567890
        </property>

         
        <mapping resource="Employee.hbm.xml" />
        <mapping resource="Certificate.hbm.xml" />

    </session-factory>
</hibernate-configuration>

import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

import vn.viettuts.hibernate.entity.Certificate;
import vn.viettuts.hibernate.entity.Employee;

public class ManageEmployee {
    private static SessionFactory factory;

    public static void main(String[] args) {
        try {
            factory = new Configuration().configure().buildSessionFactory();
        } catch (Throwable ex) {
            System.err.println("Failed to create sessionFactory object." + ex);
            throw new ExceptionInInitializerError(ex);
        }
        ManageEmployee ME = new ManageEmployee();
        // Let us have a set of certificates for the first employee
        HashSet certificates = new HashSet();

        certificates.add(new Certificate("MCA"));
        certificates.add(new Certificate("MBA"));
        certificates.add(new Certificate("PMP"));

        // Add employee records in the database
        Integer empID1 = ME.addEmployee("Ho", "Van A", 4000, certificates);

        // Add another employee record in the database
        Integer empID2 = ME.addEmployee("Vo", "Van B", 3000, certificates);

        // List down all the employees
        ME.listEmployees();

        // Update employee's salary records
        ME.updateEmployee(empID1, 5000);

        // Delete an employee from the database
        ME.deleteEmployee(empID2);

        // List down all the employees
        ME.listEmployees();

    }

    // Method to add an employee record in the database
    public Integer addEmployee(String fname, String lname, int salary, Set cert) {
        Session session = factory.openSession();
        Transaction tx = null;
        Integer employeeID = null;
        try {
            tx = session.beginTransaction();
            Employee employee = new Employee(fname, lname, salary);
            employee.setCertificates(cert);
            employeeID = (Integer) session.save(employee);
            tx.commit();
        } catch (HibernateException e) {
            if (tx != null)
                tx.rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
        return employeeID;
    }

    // Method to list all the employees detail
    public void listEmployees() {
        Session session = factory.openSession();
        Transaction tx = null;
        try {
            tx = session.beginTransaction();
            List employees = session.createQuery("FROM Employee").list();
            for (Iterator iterator1 = employees.iterator(); iterator1.hasNext();) {
                Employee employee = (Employee) iterator1.next();
                System.out.print("First Name: " + employee.getFirstName());
                System.out.print("  Last Name: " + employee.getLastName());
                System.out.println("  Salary: " + employee.getSalary());
                Set certificates = employee.getCertificates();
                for (Iterator iterator2 = certificates.iterator(); iterator2.hasNext();) {
                    Certificate certName = (Certificate) iterator2.next();
                    System.out.println("Certificate: " + certName.getName());
                }
            }
            tx.commit();
        } catch (HibernateException e) {
            if (tx != null)
                tx.rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }

    // Method to update salary for an employee
    public void updateEmployee(Integer EmployeeID, int salary) {
        Session session = factory.openSession();
        Transaction tx = null;
        try {
            tx = session.beginTransaction();
            Employee employee = (Employee) session.get(Employee.class, EmployeeID);
            employee.setSalary(salary);
            session.update(employee);
            tx.commit();
        } catch (HibernateException e) {
            if (tx != null)
                tx.rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }

    // Method to delete an employee from the records
    public void deleteEmployee(Integer EmployeeID) {
        Session session = factory.openSession();
        Transaction tx = null;
        try {
            tx = session.beginTransaction();
            Employee employee = (Employee) session.get(Employee.class, EmployeeID);
            session.delete(employee);
            tx.commit();
        } catch (HibernateException e) {
            if (tx != null)
                tx.rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }
}