public T mapRow(ResultSet rs, int rowNum) throws SQLException {
// 验证列数
ResultSetMetaData rsmd = rs.getMetaData();
int nrOfColumns = rsmd.getColumnCount();
if (nrOfColumns != 1) {
throw new IncorrectResultSetColumnCountException("incorrect column count, expected 1 but " + nrOfColumns);
}
Object result = getColumnValue(rs, 1, this.mappedClass);
return (T) result;
}