如何在java中檢查結果集是空的還是null

Question

我有一個sql查詢在我的java類（Servlet）中運行，如果數據庫中沒有該查詢的數據然后想要做其他事情，我試圖做什么。

簡單來說，我正在檢查結果集中是否沒有數據比想要做其他事情，但這不起作用

我試過的

String str = null;
    Gson gson = new Gson();
    LinkedHashMap<Object, Object> lhm = null;
    LinkedList<LinkedHashMap<Object, Object>> mainList = new LinkedList<LinkedHashMap<Object, Object>>();

    String sql;

    try {
        Connection con = DBConnection.createConnection();
        Statement statement = con.createStatement();

        sql = "select distinct a.DISPLAYCOUNTERNAME from DISPLAYCOUNTERNAMES a,DISPLAYCOUNTER b where a.DISPLAYCOUNTERCODE=b.DISPLAYCOUNTERCODE and USERNAME='"
                + userName + "'";

        System.out.println(sql);
        ResultSet resultSet = statement.executeQuery(sql);

        if (!resultSet.isBeforeFirst()) { // if there is no data
            lhm = new LinkedHashMap<Object, Object>();
            lhm.put("outlet", "NoData");
            mainList.add(lhm);
            str = gson.toJson(mainList);
        }

            while (resultSet.next()) { // if there is data
                lhm = new LinkedHashMap<Object, Object>();
                counterName = resultSet.getString("DISPLAYCOUNTERNAME");
                 System.out.println("counternam"+counterName);
                lhm.put("Counter name", counterName);

                mainList.add(lhm);
                str = gson.toJson(mainList);

            }


        System.out.println(str);
        response.setContentType("application/json");
        response.getWriter().write(str);

    } catch (SQLException e) {
        System.out.println("SQL Issues 2...");
        e.printStackTrace();
    }

上面的代碼拋出錯誤，因為SQL Issues 2... java.sql.SQLException: This method should only be called on ResultSet objects that are scrollable (type TYPE_SCROLL_INSENSITIVE).

我不知道我在這里做錯了什么，任何一種幫助都將受到贊賞

Answer 1

您可以嘗試修改該行：

Statement statement = con.createStatement();

至

Statement statement = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, 
                                          ResultSet.CONCUR_READ_ONLY);

Answer 2

您可以使用next但切換到使用do / while循環，如果第一次調用next返回true，則行指針指向結果集中的第一行，因此您必須在再次調用next之前讀取它

if (!resultSet.next()) { 
   // do no data stuff
} else { 
    do { 
      //handle result set data
    } while (rs.next()); 
}