從servlet傳遞列表到jsp null

我想根據用戶在JSP頁面中選擇的日期選擇從數據庫檢索值。用戶將選擇兩個日期並查詢數據庫。從servlet傳遞列表到jsp null

在我的Servlet中，我正在這樣做。 for語句告訴我列表中的值是正確的。

VehiclesList list = dataManager.getVehiclesWhere(dateFrom, dateTo); 

request.setAttribute("vehiclesList", list); 

VehiclesList vList = (VehiclesList) request.getAttribute("vehiclesList"); 

for (int pos = 0; pos < vList.size(); pos++) { 
     Vehicles vehicles = vList.getVehicleAt(pos); 
     System.out.println(vehicles.getC02Emissions()); 
     System.out.println(vehicles.getManufacture()); 
     System.out.println(vehicles.getRegNumber()); 
     System.out.println(vehicles.getHireRate()); 
} 

RequestDispatcher dispatcher = request.getRequestDispatcher("/vehicles.jsp"); 
dispatcher.forward(request, response);

這是我的JSP頁面中的代碼。我正在做上述相同的過程，即請求屬性，創建一個新的列表並顯示數據，但我創建的新列表出現空，我收到一個空指針異常。

VehiclesList vList = new VehiclesList(); 
vList = (VehiclesList) request.getAttribute("vehiclesList"); 

for (int pos = 0; pos < vList.size(); pos++) { 
    Vehicles vehicle = vList.getVehicleAt(pos); 
    System.out.println(vehicle.getC02Emissions()); 
}

爲什麼JSP端的列表爲null，當它在servlet端擁有正確的值時？

編輯-----

這是我的堆棧跟蹤。

WARNING: StandardWrapperValve[jsp]: PWC1406: Servlet.service() for servlet jsp threw exception 
java.lang.NullPointerException 
    at org.apache.jsp.vehicles_jsp._jspService(vehicles_jsp.java from :257) 
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111) 
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:770) 
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:411) 
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:473) 
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:377) 
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:770) 
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1550) 
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281) 
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175) 
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655) 
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595) 
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:161) 
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:331) 
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231) 
    at com.sun.enterprise.v3.services.impl.ContainerMapper$AdapterCallable.call(ContainerMapper.java:317) 
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:195) 
    at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:860) 
    at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:757) 
    at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1056) 
    at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:229) 
    at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137) 
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104) 
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90) 
    at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79) 
    at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54) 
    at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59) 
    at com.sun.grizzly.ContextTask.run(ContextTask.java:71) 
    at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532) 
    at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513) 
    at java.lang.Thread.run(Thread.java:680)

EDIT 2 ------------

servlet代碼

package Oracle; 

import RentalRecords.Vehicles; 
import RentalRecords.VehiclesList; 
import java.io.IOException; 
import javax.servlet.RequestDispatcher; 
import javax.servlet.ServletException; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
import javax.servlet.http.HttpSession; 

public class VehiclesServlet extends HttpServlet { 

    private boolean dbOK = false;//boolean to check if data input is not null 
    private Vehicles vehicle;//used to get user data from bean 
    private DatabaseConnector dataManager;//used to interact with the database 

    /** 
    * 
    * Initialises the servlet by creating a new DatabaseConnector object which 
    * will be used to interface with the database 
    */ 
    public void init() { 
     dataManager = new DatabaseConnector(); 
    } 

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { 

     String dateFrom = request.getParameter("from");//get email submitted from form 
     String dateTo = request.getParameter("to");//get password submitted from form 
     System.out.println(dateFrom + dateTo); 



     if (dateFrom != null || dateTo != null) { 
      dbOK = true; 
     } else { 
      dbOK = false; 
     }//end if 

     HttpSession session = request.getSession(true);//set the session 


     if (dbOK) { 
      try { 
       System.out.println(dbOK); 
       VehiclesList list = dataManager.getVehiclesWhere(dateFrom, dateTo); 

       request.getSession().setAttribute("vehiclesList", list); 

       VehiclesList vList = (VehiclesList) request.getAttribute("vehiclesList"); 

       for (int pos = 0; pos < vList.size(); pos++) { 
         Vehicles vehicles = vList.getVehicleAt(pos); 
         System.out.println(vehicles.getC02Emissions()); 
         System.out.println(vehicles.getManufacture()); 
         System.out.println(vehicles.getRegNumber()); 
         System.out.println(vehicles.getHireRate()); 
       } 

       RequestDispatcher dispatcher = request.getRequestDispatcher("/vehicles.jsp"); 
       dispatcher.include(request, response); 
       //request.getRequestDispatcher("vehicles.jsp").forward(request, response); 
      } catch (Exception e) { 
       e.printStackTrace(); 
      }//end try 
     } else { 
      //User is not valid, redirect back to index.jsp and inform user 
      session.setAttribute("signupError", "Signup Not Successful"); 
      session.setAttribute("signupSuccess", null);//set this null if it is a resubmit 
      System.out.println(session.getAttribute("signupError")); 
      RequestDispatcher dispatcher = request.getRequestDispatcher("/vehicles.jsp"); 
      dispatcher.forward(request, response); 
     } 

    } 
}

JSP代碼

<%@page import="RentalRecords.Vehicles"%> 
<%@page import="RentalRecords.VehiclesList"%> 
<%@page contentType="text/html" pageEncoding="UTF-8"%> 
<!DOCTYPE html> 
<html> 
    <head> 
     <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 
     <title>JSP Page</title> 
     <script> 
      $(document).ready(function() { 
       var from = $("#from").datepicker({ 
        defaultDate: "+1w", 
        changeMonth: true, 
        numberOfMonths: 1, 
        minDate: 0, 
        dateFormat: 'd-M-y', 
        onClose: function(selectedDate) { 
         $("#to").datepicker("option", "minDate", selectedDate); 
        } 

       }); 
       var to = $("#to").datepicker({ 
        defaultDate: "+1w", 
        changeMonth: true, 
        numberOfMonths: 1, 
        minDate: 0, 
        dateFormat: 'd-M-y', 
        onClose: function(selectedDate) { 
         $("#from").datepicker("option", "maxDate", selectedDate); 
        } 
       }); 
      }); 
     </script> 
    </head> 
    <body> 
     <form id="getdates" method="get" action="getdates"> 
      <label for="from">From</label> 
      <input type="text" id="from" name="from" /> 
      <label for="to">to</label> 
      <input type="text" id="to" name="to" /> 
      <input id="submitdates" name="submitdates" type="submit" value="Search" /> 
     </form> 
     <div class="header_01">Vehicles</div> 

     <% 


      VehiclesList vList = new VehiclesList(); 
      vList = (VehiclesList) request.getSession().getAttribute("vehiclesList"); 

      for (int pos = 0; pos < vList.size(); pos++) { 
       Vehicles vehicle = vList.getVehicleAt(pos); 
       System.out.println(vehicle.getC02Emissions()); 


     %> 

     <table> 
      <tr> 
       <td class="label"><label id="ltable" for="table"></label> 
        <table> 
         <tr> 
          <td class="label"><label id="lbodytype" for="transmission">Body Type</label></td> 
          <td class="field"><input id="bodytype" name="transmission" type="text" value="<%= vehicle.getBodyType()%>" maxlength="50" readonly/></td> 
          <td class="status"></td> 
         </tr> 
         <tr> 
          <td class="label"><label id="ltransmission" for="transmission">Transmission</label></td> 
          <td class="field"><input id="transmission" name="transmission" type="text" value="<%= vehicle.getTransmission()%>" maxlength="50" readonly/></td> 
          <td class="status"></td> 
         </tr> 
        </table> 
       </td> 
      </tr> 
     </table> 
     <% }%> 
    </body> 
</html>

來源

2013-01-23 user667430

請發佈堆棧跟蹤。 –

Ok編輯發佈我的堆棧跟蹤 – user667430

@ user667430請嘗試使用'request.getSession（）。setAttribute（）'和'request.getSession（）。getAttribute（）'來測試它是否工作。只是爲了測試。我並不是說這是無狀態webapps的正確方法。 – Subin

檢查在JSP頁面

空

if(request.getAttribute("vehiclesList") !=null) 
{ 
VehiclesList vList = new VehiclesList(); 
vList = (VehiclesList) request.getAttribute("vehiclesList"); 

    for (int pos = 0; pos < vList.size(); pos++) { 
    Vehicles vehicle = vList.getVehicleAt(pos); 
    System.out.println(vehicle.getC02Emissions()); 
    } 
}

來源

2013-01-23 09:57:00

問題似乎是爲什麼request.getAttribute（「vehiclesList」）是null，儘管它在servlet中正確設置。 – Subin

這是第一個運行servlet或jsp的頁面？ –

如果它是servlet那麼不需要檢查其他是否爲jsp然後檢查是否爲null –

從servlet傳遞列表到jsp null

回答

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