public class MyGenerator implements IdentifierGenerator
{
  @Override
  public Serializable generate(SessionImplementor session, Object object) throws HibernateException
  {
    String prefix = "id-";
    int id = 0;

    // Is this the right way or do I have to use session argument somehow?
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("myPersistenceUnit");
    EntityManager em = emf.createEntityManager();

    em.getTransaction().begin();

    try
    {
      Query selectQuery = em.createQuery("SELECT counter AS next_value FROM sequence_numbers WHERE sequence='my_id'");
      List<Integer> idList = (List<Integer>)selectQuery.getResultList(); // should work up to here

      if (idList.isEmpty())
      {
        id = 0;
      }
      else
      {
        id = idList.get(0) + 1;
      }

      Query updateQuery = em.createQuery("UPDATE sequence_numbers SET counter=" + id + " WHERE sequence='my_id'");
      // how do I send UPDATE query?

      String code = prefix + StringUtils.leftPad("" + id, 27, '0');

      return code;

     // end transaction, close em and emf, end try..catch and so on
  }
}