Question

当我将输出从Java类重定向到JSP页面时，我得到一个 java.lang.NullPointerException ，并且无法弄清楚出错了什么。任何帮助将不胜感激。

这是我的JSP应用程序的第一个表单页面：

<body>
    <FORM METHOD=POST ACTION="formprocessing.jsp">
Enter 1st DATE: <INPUT TYPE=TEXT NAME=date1 SIZE=20><BR>
Enter 2nd DATE: <INPUT TYPE=TEXT NAME=date2 SIZE=20><BR>
Enter PARAM CODE: <INPUT TYPE=TEXT NAME=p_code SIZE=4>
<P><INPUT TYPE=SUBMIT>
</FORM>
  </body>

此Java类将输入此处的输入值 set 放到输出页面上：

package duplicaterecords;

public class UserData {

    String date1;
    String date2;
    String p_code;

    public void setDate1( String value )
    {
        date1 = value;
    }

    public void setDate2( String value )
    {
        date2 = value;
    }

    public void setP_code( String value )
    {
        p_code = value;
    }

    public String getDate1() { return date1; }

    public String getDate2() { return date2; }

    public String getP_code() { return p_code; }


}

这是我的主要 Java类，我尝试将输出重定向到JSP页面：

package duplicaterecords;

import java.io.* ;
import java.sql.*;
import java.text.*;
import javax.servlet.http.*;

 /*to find duplicate records and their time stamps*/
public class duplicaterecords extends HttpServlet {
    public static void main(String[] args,HttpServletRequest request,
               HttpServletResponse response) 
    {
    int l,x=0,y=0,tow[],i=0,z=0,tower1=0,t=0;
    String p_code,date[],date1,date2,getdate,date3,tower,t_split;

    String time2;
    tow=new int[1000];
    date=new String[100];

     DecimalFormat df = new DecimalFormat("#.##");
     try
         {  

                                  BufferedReader b = new BufferedReader(new InputStreamReader(System.in)); //input buffer 
                        Class.forName("com.mysql.jdbc.Driver");
                                  Connection
                                  con=DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "root");
                                  Statement stmt=con.createStatement();

              String query1="select distinct(date) FROM `report_data` WHERE date>= ? and date<= ? "; //Query one for date input 
               PreparedStatement ps=con.prepareStatement(query1);

               System.out.println("Enter the 1st DATE"); //Date 1 is entered 
                date1 = b.readLine();
                System.out.println("Enter the 2nd DATE"); //Date 2 is entered 
                date2=b.readLine();
                                    ps.setString(1,date1);
                ps.setString(2,date2);
                         System.out.println("enter the param_code"); // param_code is entered 
                p_code= b.readLine();

               ResultSet result=ps.executeQuery();  
                          System.out.print("Tow_id");
                         while(result.next() )
                        {
                          getdate=result.getString("date");
                          System.out.print("\t"+getdate);
                    date3='%'+getdate+'%';
                                    date[x]=date3;
                    x++;
                         }
                          l=x;


              String query2="SELECT distinct(tow_id) FROM  `tower_data` WHERE TIME_STAMP LIKE ? "; //query 2 for finding tower-id 
                                   PreparedStatement ps1=con.prepareStatement(query2);
                ps1.setString(1,date[0]);      
                ResultSet result1=ps1.executeQuery(); 



                while(result1.next())
                 { 
                  System.out.println("");
                  tower=result1.getString("tow_id");
                   tower1= Integer.parseInt(tower);
            tow[z]=tower1;

                 t=y;
                 System.out.print(tower1);
                           int count=0;
                 x=0;
                 request.getSession().setAttribute("tower_id", tow);

                           while(count<l)
                   {
                          String query3="SELECT time_stamp FROM tower_data WHERE `TIME_STAMP` LIKE ? AND `PARAM_CODE` = ? AND `TOW_ID`=? GROUP BY time_stamp HAVING count( * ) >1";
                                       //Query 3 for finding time stamps with duplicate data 
                    PreparedStatement ps2=con.prepareStatement(query3); 
                    ps2.setString(2,p_code);
                     ps2.setString(1,date[x]);
                    ps2.setInt(3,tower1);
                    ResultSet result2=ps2.executeQuery();

                                       int row=0;
                                      while(result2.next())
                  {

                  t_split=result2.getString("time_stamp");

                 String[] parts= t_split.split(" "); //splitting time_stamp to extract only time without date 
                                      time2=parts[1]; //time stored in time2
                System.out.println("\t"+time2);
                row++;

                }
               if(row==0)
                                    {
                                    System.out.println("\t"+"no duplicate");                        
                }
                       System.out.print("\t"+"\t");



                                  x++;
             count++;
                   }       




        }
        con.close();


                          }
                         catch (Exception e)
                        {
                        e.printStackTrace();

                        }
                        }



                       }

最后这是输出JSP页面，它必须显示Java类的输出：

<%@page import="java.net.*"%>
<%@page import="javax.servlet.*"%>
<html>
  <body>
    You entered<BR>
Date1: <%= duplicaterecords.getDate1() %><BR>
Date2: <%= duplicaterecords.getDate2() %><BR>
PARAM CODE: <%= duplicaterecords.getP_code() %><BR>
 <table border="1">
<tr><td><B>Tower ID:</B></td></tr>
<%
ArrayList towerArray = (ArrayList) session.getAttribute("tower_id");
String myString="";
for(int i = 0; i < towerArray.size(); i++)
{
myString = (String) towerArray.get(i);

}

%> 
<tr><td><%=myString%></td></tr>


</table>
</body>
</html>

任何帮助？

Answer 1

我没有看到实际的错误，所以这只是猜测，但你正在做：

request.getSession().setAttribute("tower_id", tow);

其中tow是一个int数组。

然后在JSP中，你说：

ArrayList towerArray = (ArrayList) session.getAttribute("tower_id");

所以这些类型不兼容。

此外，与您的问题无关，我会建议您，代码质量一般似乎非常糟糕。我建议阅读更多关于Java约定等的内容。

Answer 2

行动中 request.getSession（）。setAttribute（“tower_id”，tow）;

在jsp中

ArrayList towerArray = (ArrayList) session.getAttribute("tower_id");

但是tow是一个数组

int l,x=0,y=0,tow[],i=0,z=0,tower1=0,t=0;

但除此之外，为什么在servlet代码中有一个静态的main方法，doGet或doPost在哪里？

这就是为什么你看到NPE而不是类强制转换异常的原因。您的会话中没有tower_id属性，因此当您执行此操作时

towerArray.size()

你得到了NPE

我的Java / JSP应用程序中的java.lang.NullPointerException

2 个答案: