if (nit == null) {
throw new SQLException("No hay elemento clave de la clase proveedor");
}
ResultSet rs = null;
PreparedStatement pst = null;
Proveedor proveedor = null;
try {
pst = con.prepareStatement("select * from proveedor where nit = ?");
pst.setString(1, nit.trim());
rs = pst.executeQuery();
while(rs.next()) {