Error inserting data with Hibernate

I'm trying to insert data in two tables with a one-to-many mapped with Hibernate in Netbeans, but when I run the application I get the following error:

  

run:   Mar 11, 2017 12:28:20 AM org.hibernate.annotations.common.reflection.java.JavaReflectionManager   INFO: HCANN000001: Hibernate Commons Annotations {4.0.4.Final}   Mar 11, 2017 12:28:20 AM org.hibernate.Version logVersion   INFO: HHH000412: Hibernate Core {4.3.1.Final}   Mar 11, 2017 12:28:20 AM org.hibernate.cfg.Environment   INFO: HHH000206: hibernate.properties not found   Mar 11, 2017 12:28:20 AM org.hibernate.cfg.Environment buildBytecodeProvider   INFO: HHH000021: Bytecode provider name: javassist   Mar 11, 2017 12:28:20 AM org.hibernate.cfg.Configuration configure   INFO: HHH000043: Configuring from resource: /hibernate.cfg.xml   Mar 11, 2017 12:28:20 AM org.hibernate.cfg.Configuration getConfigurationInputStream   INFO: HHH000040: Configuration resource: /hibernate.cfg.xml   Mar 11, 2017 12:28:20 AM org.hibernate.internal.util.xml.DTDEntityResolver resolveEntity   WARN: HHH000223: Recognized obsolete hibernate namespace link . Use namespace link instead. Refer to Hibernate 3.6 Migration Guide!   Mar 11, 2017 12:28:20 AM org.hibernate.cfg.Configuration addResource   INFO: HHH000221: Reading mappings from resource: Entity / Certificate.hbm.xml   Mar 11, 2017 12:28:20 AM org.hibernate.cfg.Configuration addResource   INFO: HHH000221: Reading mappings from resource: Entity / Employee.hbm.xml   Mar 11, 2017 12:28:20 AM org.hibernate.cfg.Configuration doConfigure   INFO: HHH000041: Configured SessionFactory: null   Mar 11, 2017 12:28:20 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure   WARN: HHH000402: Using Hibernate built-in connection pool (not for production use!)   Mar 11, 2017 12:28:20 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator   INFO: HHH000401: using driver [com.mysql.jdbc.Driver] at URL [jdbc: mysql: // localhost: 3306 / java_util_set? ZeroDateTimeBehavior = convertToNull]   Mar 11, 2017 12:28:20 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator   INFO: HHH000046: Connection properties: {user = root, password = ****}   Mar 11, 2017 12:28:20 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator   INFO: HHH000006: Autocommit mode: false   Mar 11, 2017 12:28:20 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure   INFO: HHH000115: Hibernate connection pool size: 20 (min = 1)   Mar 11, 2017 12:28:20 AM org.hibernate.dialect.Dialect   INFO: HHH000400: Using dialect: org.hibernate.dialect.MySQL5Dialect   Failed to create sessionFactory object.org.hibernate.MappingException: Association references unmapped class: Certificate   Exception in thread "main" java.lang.ExceptionInInitializerError       at java_util_set.Java_util_Set.main (Java_util_Set.java:36)   Caused by: org.hibernate.MappingException: Association references unmapped class: Certificate       at org.hibernate.cfg.HbmBinder.bindCollectionSecondPass (HbmBinder.java:2557)       at org.hibernate.cfg.HbmBinder $ CollectionSecondPass.secondPass (HbmBinder.java:2808)       at org.hibernate.cfg.CollectionSecondPass.doSecondPass (CollectionSecondPass.java:70)       at org.hibernate.cfg.Configuration.originalSecondPassCompile (Configuration.java:1695)       at org.hibernate.cfg.Configuration.secondPassCompile (Configuration.java:1424)       at org.hibernate.cfg.Configuration.buildSessionFactory (Configuration.java:1844)       at org.hibernate.cfg.Configuration.buildSessionFactory (Configuration.java:1928)       at java_util_set.Java_util_Set.main (Java_util_Set.java:33)

Could you tell me how to solve it or tell me where I can get good examples to work all kinds of relationships in Hibernate with Netbeans. These are the codes I have:

Database:

CREATE DATABASE IF NOT EXISTS 'java_util_set' /*!40100 DEFAULT CHARACTER SET latin1 */;
USE 'java_util_set';
CREATE TABLE IF NOT EXISTS 'certificate' (
'id' int(11) NOT NULL AUTO_INCREMENT,
'certificate_name' varchar(30) DEFAULT NULL,
'employee_id' int(11) DEFAULT NULL,
PRIMARY KEY ('id')
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE IF NOT EXISTS 'employee' (
'id' int(11) NOT NULL AUTO_INCREMENT,
'first_name' varchar(20) DEFAULT NULL,
'last_name' varchar(20) DEFAULT NULL,
'salary' int(11) DEFAULT NULL,
PRIMARY KEY ('id')
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

This is the Employee class:

package Entity;
// Generated 10/03/2017 11:27:32 PM by Hibernate Tools 4.3.1

import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.Table;

/**
 * Employee generated by hbm2java
 */
@Entity
@Table(name="employee"
    ,catalog="java_util_set"
)
public class Employee {

     private Integer id;
     private String firstName;
     private String lastName;
     private Integer salary;
     private Set certificates;

    public Employee() {
    }

    public Employee(String firstName, String lastName, Integer salary) {
       this.firstName = firstName;
       this.lastName = lastName;
       this.salary = salary;
    }

     @Id @GeneratedValue(strategy=IDENTITY)

    @Column(name="id", unique=true, nullable=false)
    public Integer getId() {
        return this.id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name="first_name", length=20)
    public String getFirstName() {
        return this.firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    @Column(name="last_name", length=20)
    public String getLastName() {
        return this.lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @Column(name="salary")
    public Integer getSalary() {
        return this.salary;
    }

    public void setSalary(Integer salary) {
        this.salary = salary;
    }

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

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

This is the Certificate class:

package Entity;
// Generated 10/03/2017 11:27:32 PM by Hibernate Tools 4.3.1

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.Table;

/**
 * Certificate generated by hbm2java
 */
@Entity
@Table(name = "certificate", catalog = "java_util_set"
)
public class Certificate  {

    private Integer id;
    private String certificateName;
    private Integer employeeId;

    public Certificate() {
    }

    public Certificate(String certificateName) {
        this.certificateName = certificateName;
        //this.employeeId = employeeId;
    }

    @Id
    @GeneratedValue(strategy = IDENTITY)

    @Column(name = "id", unique = true, nullable = false)
    public Integer getId() {
        return this.id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name = "certificate_name", length = 30)
    public String getCertificateName() {
        return this.certificateName;
    }

    public void setCertificateName(String certificateName) {
        this.certificateName = certificateName;
    }

   /* @Column(name = "employee_id")
    public Integer getEmployeeId() {
        return this.employeeId;
    }

    public void setEmployeeId(Integer employeeId) {
        this.employeeId = employeeId;
    }*/

    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.certificateName.equals(obj2.getCertificateName()))) {
            return true;
        }
        return false;
    }

    public int hashCode() {
        int tmp = 0;
        tmp = (id + certificateName).hashCode();
        return tmp;
    }
}

Mapping of employee:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<!-- Generated 10/03/2017 11:27:33 PM by Hibernate Tools 4.3.1 -->
<hibernate-mapping>
    <class name="Entity.Employee"  table="EMPLOYEE">
      <meta attribute="class-description">
         This class contains the employee detail. 
      </meta>
      <id name="id" type="int" column="id">
         <generator class="identity"/>
      </id>
      <set name="certificates" cascade="all">
         <key column="employee_id"/>
         <one-to-many class="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>
</hibernate-mapping>

Certificate Mapping:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<!-- Generated 10/03/2017 11:27:33 PM by Hibernate Tools 4.3.1 -->
<hibernate-mapping>
      <class name="Entity.Certificate" table="CERTIFICATE">
      <meta attribute="class-description">
         This class contains the certificate records. 
      </meta>
      <id name="id" type="int" column="id">
         <generator class="identity"/>
      </id>
      <property name="certificateName" column="certificate_name" type="string"/>
   </class>
</hibernate-mapping>

Configuration and reverse reengineering:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/java_util_set?zeroDateTimeBehavior=convertToNull</property>
    <property name="hibernate.connection.username">root</property>
    <property name="hibernate.connection.password">*******</property>
    <mapping resource="Entity/Certificate.hbm.xml"/>
    <mapping resource="Entity/Employee.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

Reingenieria:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-reverse-engineering PUBLIC "-//Hibernate/Hibernate Reverse Engineering DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-reverse-engineering-3.0.dtd">
<hibernate-reverse-engineering>
  <schema-selection match-catalog="java_util_set"/>
  <table-filter match-name="employee"/>
  <table-filter match-name="certificate"/>
</hibernate-reverse-engineering>

This is the java_util_set class:

package java_util_set;

import Entity.Certificate;
import Entity.Employee;
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.Transaction;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

/**
 *
 * @author AFAL3D
 */
public class Java_util_Set {

    /**
     * @param args the command line arguments
     */
    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);
        }
        Java_util_Set ME = new Java_util_Set();
        /* Let us have a set of certificates for the first employee  */
        HashSet set1 = new HashSet();
        set1.add(new Certificate("MCA"));
        set1.add(new Certificate("MBA"));
        set1.add(new Certificate("PMP"));

        /* Add employee records in the database */
        Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, set1);

        /* Another set of certificates for the second employee  */
        HashSet set2 = new HashSet();
        set2.add(new Certificate("BCA"));
        set2.add(new Certificate("BA"));

        /* Add another employee record in the database */
        Integer empID2 = ME.addEmployee("Dilip", "Kumar", 3000, set2);

        /* 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.getCertificateName());
                }
            }
            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();
        }
    }
}