Question

我尝试将值从servlet提取到我的JSP中，但它会抛出NullPointerException或其他错误。

这是从JSP获取值的servlet：

buildingprofilerequest.java

package asset.management.arms.buildingprofilemodule;

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;

import com.sun.xml.internal.ws.client.SenderException;
/**
 * Servlet implementation class buildingprofilerequest
 */
public class buildingprofilerequest extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        try{

            buildingservice building = new buildingservice();
            building.setBuilding_name(request.getParameter("combobox"));

            building = BuildingDAO.build(building);

            request.setAttribute("abcd", building);      


                 RequestDispatcher dispatcher =                  
                 getServletContext().getRequestDispatcher("/building_profile_details.jsp");

            dispatcher.include(request, response);

        }
    catch (Throwable theException)      
        {
             System.out.println(theException); 
        }

    }

我的bean看起来像这样简短：

buildingservice.java

package asset.management.arms.buildingprofilemodule;

public class buildingservice {

        private String building_name;

                public String getBuilding_name() {
            return building_name;
        }

        public void setBuilding_name(String newbuilding_name) {
            this.building_name = newbuilding_name;
        }

              //and has many more parameters and there getters and setters
        }

我的另一个班级是BuildingDAO.java，这里所有的计算都已完成：

package asset.management.arms.buildingprofilemodule;

import java.sql.*;
import asset.management.arms.loginmodule.ConnectionManager;

public class BuildingDAO {
    static Connection currentCon = null;
    static ResultSet rs = null;  

    public static buildingservice build(buildingservice bean) {

           String building_name = bean.getBuilding_name(); 

           String searchQuery = "select * from buildings";
         try{
            //connect to DB 
            currentCon = ConnectionManager.getConnection();
             stmt=currentCon.createStatement();
             rs = stmt.executeQuery(searchQuery);           

             while(rs.next()){

            //retreiving building parameters from database   
             String buildingname = rs.getString("building_name");
             String buildingnumber = rs.getString("building_number");
             int buildarea = rs.getInt("build_area");

                  //setting building parameters
                 bean.setBuilding_name(buildingname);
                 bean.setBuilding_number(buildingnumber);
                 bean.setBuild_area(buildarea);
                 bean.setBuilt_year(builtyear);
            catch (Exception ex) 
          {
             System.out.println(" " + ex);
          } 

            finally 
          {
             if (rs != null)    {
                try {
                   rs.close();
                } catch (Exception e) {}
                   rs = null;
                }

             if (stmt != null) {
                try {
                   stmt.close();
                } catch (Exception e) {}
                   stmt = null;
                }

             if (currentCon != null) {
                try {
                   currentCon.close();
                } catch (Exception e) {
                }

                currentCon = null;
             }
          }

        return bean;
    }

}

我的JSP是这样的：

building_profile_details.jsp

<%@ page language="java" contentType="text/html; charset=iso-8859-1"
    pageEncoding="ISO-8859-1" import="asset.management.arms.buildingprofilemodule.buildingservice"%>

<%  buildingservice hello = (buildingservice) request.getAttribute("abcd"); %>

<table width="1150" height="176" border="1" align="center" bordercolor="lightslategray">
      <tr>
        <td width="107"><div align="center"><b>Building Number</b> </div></td>
        <td width="325"><div align="center"><b>Building Name </b></div></td>
        <td width="70"><div align="center"><b>area</b></div></td>
        <td width="146"><div align="center"><b>built year</b></div></td> 
          </tr>

<tr>
    <td><div align="center"><%=hello.getBuilding_number()%></div></td>
    <td><div align="center"><%= hello.getBuilding_name()%></div></td>
<td><div align="center"><%=hello.getBuild_area()%></div></td>
 <td><div align="center"><%= hello.getBuilt_year()%></div></td>
</tr>
</table>

在JSP中我甚至尝试过其他方式：

<jsp:useBean id="hello" class="asset.management.arms.buildingprofilemodule.buildingservice">

然后在表的各个块中：

<jsp:getProperty name="hello" name="building_name">

但没有任何作用，它会抛出错误，说

org.apache.jasper.JasperException: Exception in JSP: /building_profile_details.jsp:82

82:     <td><div align="center"><%=hello.getBuilding_number()%></div></td>

和其他行类似。

这是如何引起的，我该如何解决？

Answer 1

在您的servlet中，替换

dispatcher.include(request, response);

通过

dispatcher.forward(request, response);

在JSP中，删除

<%  buildingservice hello = (buildingservice) request.getAttribute("abcd"); %>

并替换

<td><div align="center"><%=hello.getBuilding_number()%></div></td>
<td><div align="center"><%= hello.getBuilding_name()%></div></td>
<td><div align="center"><%=hello.getBuild_area()%></div></td>
<td><div align="center"><%= hello.getBuilt_year()%></div></td>

通过

<td><div align="center">${abcd.building_number}</div></td>
<td><div align="center">${abcd.building_name}</div></td>
<td><div align="center">${abcd.build_area}</div></td>
<td><div align="center">${abcd.built_year}</div></td>

另见：

您的代码中存在许多其他严重问题，但它们与当前的具体问题无关。然而，我会尝试总结最重要的一些：

代码将数据库资源保存为static个变量。这是一个主要的安全问题！
代码不以合理的方式处理异常。这不是开发人员，也不是用户友好的。
代码不尊重Java naming conventions。这会导致开发人员混淆和可维护性问题。
代码（特别是buildingservice和BuildingDAO）使用非常奇怪的方法/模式/流程。它看起来像是由一个不懂面向对象编程概念的程序程序员编写的。
JSP使用 scriptlet ，这是自2003年以来不鼓励的。让自己保持最新状态。 Java代码属于Java类，JSP应该只包含HTML，JSP标记和EL。
HTML使用不推荐使用的属性。让自己保持最新状态。学习CSS。

也可以这样做。

无法从jsp中的servlet获取值

1 个答案:

另见：