java - 准备好的语句似乎在 java servlet 中总是返回 null

Question

我正在做一个项目，用户应该在表单中填写他想要的标准，然后根据这些标准，servlet 从数据库返回适当的结果；我正在使用 mysql 服务器。我的问题是我为此使用的准备好的语句(因为用户可以使用可变数量的条件进行搜索)总是返回 null，所以我得到一个

Exception: null java.lang.NullPointerException at    
Servlet_GrowerResults.doGet(Servlet_GrowerResults.java:102)

这是 GetData 类中的 fetchGrowers 方法，它将 ResultSet 返回给 Servlet_GrowerResults

private PreparedStatement stmt_growers=null;
private ResultSet rs_growers;
private final String getGrowersQuery="select * from grower where name like ? and surname like ? and area like ? and partnership like ?;";

public ResultSet fetchGrowers(String criteria_name,String criteria_surname,String criteria_area, String criteria_partnership){
    try{
        stmt_growers=dbcon.getConnection().prepareStatement(getGrowersQuery);
        stmt_growers.setString(1, criteria_name);
        stmt_growers.setString(3, criteria_surname);
        stmt_growers.setString(5, criteria_area);
        stmt_growers.setString(7, criteria_partnership);

        rs_growers=stmt_growers.executeQuery();
        return rs_growers;
    }
    catch(Exception e9){
        errorMessage = "Error while getting all growers from database!<br>" + e9.getMessage();
        return null;
    }

这是初始化 ResultSet 的行。还有其他 else if 来覆盖所有 criteria 提交的情况

   try{
        getData.getDBcon().open();
        ResultSet rsAreas=getData.fetchGrowerAreas();
        ResultSet rsPartnership=getData.fetchGrowerPartnership();
        ResultSet rsGrowersReturned;

        String sqlWildcard="%";

        //Cases of criteria submited by the user,
        //in order to send the apropriate parameters
        //to the PreparedStatement

        //1st case: searching without names criteria
        if( (criteriaNames.length()>0) && (!(criteriaAreas.length()>0)) && (!(criteriaPartnerships.length()>0)) ){
            rsGrowersReturned=getData.fetchGrowers(sqlWildcard,sqlWildcard,criteriaAreas,criteriaPartnerships);
        }

这是异常指向的行。更具体地说，第 102 行是:while(rsGrowersReturned.next())

 if((sorting.equals("περιοχή"))){
        while(rsAreas.next()){
                out.println("<table>");
                String area=rsAreas.getString("area");
                area = new String(area.getBytes("ISO-8859-1"), "ISO-8859-7");
                out.println("<tr><th>"+rsAreas.getString("area")+"</th></tr>");

    /*line 102*/ while(rsGrowersReturned.next()){
                    if(rsGrowersReturned.getString("area").equals(rsAreas.getString("area"))){
                        String name=rsGrowersReturned.getString("name");
                        name = new String(name.getBytes("ISO-8859-1"), "ISO-8859-7");
                        String surname=rsGrowersReturned.getString("surname");
                        surname = new String(surname.getBytes("ISO-8859-1"), "ISO-8859-7");
                        String partnership=rsGrowersReturned.getString("partnership");
                        partnership = new String(partnership.getBytes("ISO-8859-1"), "ISO-8859-7");

                        out.println("<tr><td>"+name+" "+surname+"</td>");
                        out.println("<td>"+partnership+"</td></tr>");
                    }//end of neaced if
                }//end of second while
            }//end of first while
            out.println("</table>");
        }//end of if

我可以看到确实出现了问题，因为在到达第二个区域之前，它会按预期打印第一个区域，然后出现异常!!!

我尝试了直接对mysql服务器进行sql查询，是可以的，就是找不到问题所在!每一个答案将不胜感激，谢谢。

Answer 1

您需要按顺序为您的 PrepareStatement 设置输入参数:

stmt_growers.setString(1, criteria_name);
stmt_growers.setString(2, criteria_surname);
stmt_growers.setString(3, criteria_area);
stmt_growers.setString(4, criteria_partnership);