为什么报java.lang.NullPointerException

[已解决问题] 解决于 2018-08-27 14:10

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@page import="entity.Student"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
    <base href="<%=basePath%>">

    <title>My JSP 'index.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">

</head>

<body>
    <table align="center" border="1" cellpadding="0" cellspacing="0" width="650">
    <caption>
      <h2>学生信息</h2>
      <a href="insertStudent.jsp">添加学生</a>
    </caption>
    <tr style="background-color:green">
      <th>id</th>
      <th>name</th>
      <th>sex</th>
      <th>weight</th>
      <th>bri</th>
      <th>操作</th>
    </tr>
    <%
      Object obj=request.getAttribute("list");
         List<Student> stus=(List<Student>)obj;
         for(Student s : stus){
         %>
         <TR>
           <td><%=s.getSid()%></td>
           <td><%=s.getSname()%></td>
           <td><%=s.getSex()%></td>
           <td><%=s.getWeight()%></td>
           <td><%=s.getBri()%></td>
           <td>
            <a href="">删除</a>
            <a href="">修改</a>
           </td>
         </TR>
         <%
         }
    %>
    </table>
</body>
</html>

____________________________________________

package Dao;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;

import entity.Student;

public class StudentDao {
public List queryStudentAll() {
  Connection conn = BaseDao.getConn();
  Statement stat =null;
  ResultSet rs = null;
  ArrayList<Student> list = new ArrayList<Student>();
  try {
    stat = conn.createStatement();
    rs = stat.executeQuery("select * from student");
   while (rs.next()) {
    Student s = new Student();
    s.setSid(rs.getInt("sid"));
    s.setSname(rs.getString("sname"));
    s.setSex(rs.getString("sex"));
    s.setWeight(rs.getInt("weight"));
    s.setBri(rs.getDate("bri"));
    list.add(s);
   }
  } catch (Exception e) {
   e.printStackTrace();
  } finally {
   BaseDao.close(rs,stat, conn);
  }
  return list;
}

_________________________________________________

package servlet;

import java.io.IOException;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import Dao.StudentDao;

public class QueryStudentServlet extends HttpServlet {
  @Override
  protected void service(HttpServletRequest req, HttpServletResponse arg1)
    throws ServletException, IOException {
    StudentDao sd=new StudentDao();
    List list= sd.queryStudentAll();
    req.setAttribute("list",list);
    req.getRequestDispatcher("index.jsp").forward(req, arg1);
  }
}

为什么报java.lang.NullPointerException，oracle数据库也没事，是哪里没配置好吗？

大佬帮我解决一下。

错误———错误————错误————————错误————————错误

type Exception report

message java.lang.NullPointerException

description The server encountered an internal error that prevented it from fulfilling this request.

exception

org.apache.jasper.JasperException: java.lang.NullPointerException org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:556) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:477) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339) javax.servlet.http.HttpServlet.service(HttpServlet.java:731) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

root cause

java.lang.NullPointerException org.apache.jsp.index_jsp._jspService(index_jsp.java:128) org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) javax.servlet.http.HttpServlet.service(HttpServlet.java:731) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:439) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339) javax.servlet.http.HttpServlet.service(HttpServlet.java:731) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

寒兔 | 初学一级 | 园豆：183
提问于：2018-08-25 09:44

你倒是把异常信息贴出来啊，不贴出来怎么知道哪里有问题

– 青雉 5年前

@青雉: 我把错误放在题目下面了

– 寒兔 5年前

< >

最佳答案

Object obj=request.getAttribute("list");
List<Student> stus=(List<Student>)obj;
for(Student s : stus){
%>
<TR>
<td><%=s.getSid()%></td>
<td><%=s.getSname()%></td>
<td><%=s.getSex()%></td>
<td><%=s.getWeight()%></td>
<td><%=s.getBri()%></td>
<td>
<a href="">删除</a>
<a href="">修改</a>
</td>
</TR>
<%
}
%>
检查一下这里是不是出了问题吧，在Dao层设置断点于 return list;处，先看看数据是否正常或得到，如果断点能走到return list;你就看看list里的值是否正常，如果正常那基本就能肯定是你JSP里那段代码出问题了。

奖励园豆：5

山抹微云君^^ | 菜鸟二级 |园豆：274 | 2018-08-27 09:18

我已经解决了，我运行servlet文件就行了

寒兔 | 园豆：183 (初学一级) | 2018-08-27 10:57

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为什么报java.lang.NullPointerException

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