<?xml version="1.0" encoding="utf-8"?>
    <!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://www.hibernate.org/dtd/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/pruebahibernate</property>
    <property name="hibernate.connection.username">root</property>
    <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="hibernate.search.autoregister_listeners">false</property>
    <property name="hibernate.show_sql">true</property>
    <mapping resource="src/entidades/Contacto.hbm.xml"/>
    </session-factory>
    </hibernate-configuration>

public class HibernateUtil
    {
       private static final SessionFactory sessionFactory;

       static
       {
          try
          {
           sessionFactory = new Configuration().configure().buildSessionFactory();
          } catch (HibernateException he)
          {
             System.err.println("Ocurrió un error en la inicialización de la SessionFactory: " + he);
             throw new ExceptionInInitializerError(he);
          }
      }

      public static SessionFactory getSessionFactory()
      {
         return sessionFactory;
      }
    }

public class ContactoDAO
    {
    private Session sesion;
    private Transaction tx;

    public long guardaContacto(Contacto contacto) throws HibernateException
    {
      long id = 0;

      try
      {
        iniciaOperacion();
        id = (Long) sesion.save(contacto);
        tx.commit();
      } catch (HibernateException he)
      {
        manejaExcepcion(he);
        throw he;
      } finally
      {
        sesion.close();
      }

      return id;
      }

    private void iniciaOperacion() throws HibernateException
    {
      sesion = HibernateUtil.getSessionFactory().openSession();
      tx = sesion.beginTransaction();
    }
    }

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
    <!-- Generated 30/01/2017 23:44:31 by Hibernate Tools 3.5.0.Final -->
    <hibernate-mapping>
    <class name="entidades.Contacto" table="CONTACTO">
      <id name="id" type="long">
        <column name="ID" />
        <generator class="assigned" />
      </id>
      <property name="nombre" type="java.lang.String">
        <column name="NOMBRE" />
      </property>
      <property name="email" type="java.lang.String">
        <column name="EMAIL" />
      </property>
      <property name="telefono" type="java.lang.String">
        <column name="TELEFONO" />
      </property>
    </class>
    </hibernate-mapping>

Exception in thread "main" java.lang.ExceptionInInitializerError
at entidades.ContactoDAO.iniciaOperacion(ContactoDAO.java:102)
at entidades.ContactoDAO.guardaContacto(ContactoDAO.java:20)
at ejemplo.hibernate.PruebaHibernate.main(PruebaHibernate.java:23)
Caused by: java.lang.NullPointerException
at org.hibernate.cfg.Configuration.addResource(Configuration.java:560)
at org.hibernate.cfg.Configuration.parseMappingElement(Configuration.java:1587)
at org.hibernate.cfg.Configuration.parseSessionFactory(Configuration.java:1555)
at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1534)
at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1508)
at org.hibernate.cfg.Configuration.configure(Configuration.java:1428)
at org.hibernate.cfg.Configuration.configure(Configuration.java:1414)
at entidades.HibernateUtil.<clinit>(HibernateUtil.java:17)

private Transaction iniciaOperacion() throws HibernateException {
      sesion = HibernateUtil.getSessionFactory().openSession();
      tx = sesion.beginTransaction();
      return tx
    }