JDBC，DatabaseMetaData获取外键有关问题

public static Set getForeignKeyColumns(Connection c, String catalog, String schema, String table) throws SQLException {                      DatabaseMetaData dmd = c.getMetaData();                      ResultSet rs = null;                      HashSet columns = new HashSet();                   try {                          rs = dmd.getImportedKeys(catalog, schema, table);                                                while (rs.next()) {                              columns.add(rs.getString(8));                         }                                              }catch(Exception e){                    e.printStackTrace();                }                finally {                          if (rs != null) rs.close();                    return columns;                 }                  }