public Object simulate(...) {
  Object ret = null;
  Connection con = null;
  Transaction trans = null;
  Session session = null; 
  beginSession();
  try {
    trans = session.beginTransaction();
    con = session.connection();
    ret = calculate(con); // <<<<< legacy method that uses a Connection object
  } catch (Exception e) {
    e.printStackTrace();
  } finally {
    try {
      trans.rollback(); // <<<<< the exception occurs here
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      endSession();
    }
  }
  return ret;
}

public Object calculate(Connection con) {
  // performs all the processing over db
}