ArrayList求助

Question

求大神帮助小弟下，
开始定义了DaoVote类
package dao;
import java.sql.*;
import java.util.*;
import vo.Vote;
public class VoteDao {
    private Connection conn=null;
    public void initConnection() throws Exception{
     Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/school","root","mysqladmin");
    }
    public ArrayList<Vote> getAllVotes() throws Exception{
     ArrayList<Vote> al=new ArrayList<Vote>();
      
     initConnection();
     String sql="select teacherno,teachername,vote from t_vote";
     Statement stat=conn.createStatement();
     ResultSet rs=stat.executeQuery(sql);
     while(rs.next()){
     Vote vote=new Vote();
     vote.setTeacherno(rs.getString("teacherno"));
     vote.setTeachername(rs.getString("teachername"));
     vote.setVotenumber(rs.getInt("vote"));
     al.add(vote);
     }
     conn.close();
     return al;
    }
    public void updateVotes(String[] teacherno)throws Exception{
     initConnection();
     String sql="update t_vote set vote=vote+1 where teacherno=?";
     PreparedStatement ps=conn.prepareStatement(sql);
     for(int i=0;i<teacherno.length;i++){
     ps.setString(1,teacherno[i]);
     ps.executeUpdate();
     }
     conn.close();
    }
}
在JSP中使用
VoteDao vdao=new VoteDao();
                ArrayList<Vote> votes=vdao.getAllVotes();
                for(int i=0;i<votes.size();i++){
                    Vote vote=(Vote)votes.get(i);
总是出现错误，
org.apache.jasper.JasperException: An exception occurred processing JSP page /display.jsp at line 5148:                     //  String teachername = rs.getString("teachername");
49:                       //int vote = rs.getInt("vote");
50:                 VoteDao vdao=new VoteDao();
51:                 ArrayList<Vote> votes=vdao.getAllVotes();
52:                 for(int i=0;i<votes.size();i++){
53:                     Vote vote=(Vote)votes.get(i);
54:                 ArrayList  

Answer 1

51行应该没有错误，而且这个异常应该也不是正常的异常。。
有可能是jar包的问题
刚才搜了一下，说把jar包放到lib目录下或许可以解决

Answer 2

下断点看能不能进getAllVotes 一步步来吧  太笼统了 不知

Answer 3

楼主呀把完整的异常贴出来， ArrayList<Vote> votes=vdao.getAllVotes();不知道楼主用的jdk是什么版本，1.5以下是不支持泛型的，

Answer 4

51行上面吧arraylist 初始化下
ArrayList<Vote> votes=new ArrayList<Vote>();

Answer 5

这么明显的错误看不出来么， 你的成员变量private Connection conn=null;
压根就没赋值

Answer 6

楼上肯定没细看代码
 initConnection();
人家有实例化调用的方法好伐·
请认真

Answer 7

先对你的dao写个测试类，测试一下吧，看的话问题很难找到，debug调试着一点点找问题吧

Answer 8

String sql="select teacherno,teachername,vote from t_vote";
我怎么感觉是泛型的问题呢，Vote类成员变量只有这三个？实在不行就把泛型去掉吧，试试。

Answer 9

    private Connection conn=null;
    public void initConnection() throws Exception{
     Class.forName("com.mysql.jdbc.Driver");
        Connection conn =   DriverManager.getConnection("jdbc:mysql://localhost:3306/school","root","mysqladmin");
        this.conn = conn; //加上这个
    }

Answer 10

这个是jsp最终编译后会变成servlet和html，这个51行的报错有可能是java servlet里面的51行。不代表jsp中的51行，请楼主贴下全的jsp代码。
org.apache.jasper.JasperException: An exception occurred processing JSP page /display.jsp at line 51

Answer 11

 他初始化的connection是个局部变量，把initConnection方法里面的Connection去掉直接就是conn =  DriverManager.getConnection("jdbc:mysql://localhost:3306/school","root","mysqladmin");

Answer 12

我把我写的几部分代码全部贴上来，大家帮忙看看，灰常感谢
Vote.java:package vo;public class Vote {
    private String teacherno;
    private String teachername;
    private int votenumber;
    public String getTeacherno(){
     return teacherno;
    }
    public void setTeacherno(String teacherno){
     this.teacherno=teacherno;
    }
    public String getTeachername(){
     return teachername;
    }
    public void setTeachername(String teachername){
     this.teachername=teachername;
    }
    public int getVotenumber(){
     return votenumber;
    }
    public void setVotenumber(int votenumber){
     this.votenumber=votenumber;
    }
}VoteDao.java:
package dao;
import java.sql.*;
import java.util.*;
import vo.Vote;
public class VoteDao {
    private Connection conn=null;
    public void initConnection() throws Exception{
     Class.forName("com.mysql.jdbc.Driver");
        conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/school","root","mysqladmin");
        this.conn=conn;
    }
    public ArrayList<Vote> getAllVotes() throws Exception{
     ArrayList<Vote> al=new ArrayList<Vote>();
      
     initConnection();
     String sql="select teacherno,teachername,vote from t_vote";
     Statement stat=conn.createStatement();
     ResultSet rs=stat.executeQuery(sql);
     while(rs.next()){
     Vote vote=new Vote();
     vote.setTeacherno(rs.getString("teacherno"));
     vote.setTeachername(rs.getString("teachername"));
     vote.setVotenumber(rs.getInt("vote"));
     al.add(vote);
     }
     conn.close();
     return al;
    }
    public void updateVotes(String[] teacherno)throws Exception{
     initConnection();
     String sql="update t_vote set vote=vote+1 where teacherno=?";
     PreparedStatement ps=conn.prepareStatement(sql);
     for(int i=0;i<teacherno.length;i++){
     ps.setString(1,teacherno[i]);
     ps.executeUpdate();
     }
     conn.close();
    }
}vote.jsp:
<%@ page language="java" import="java.sql.*" pageEncoding="utf-8"%>
<%@ page import="dao.VoteDao" %>
<%
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 'vote.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">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->  </head>
  
  <body> 
    <% 
        String[] teacherno=request.getParameterValues("teacherno"); 
        //Class.forName(&quot;com.mysql.jdbc.Driver&quot;); 
        //Connection conn = DriverManager.getConnection(&quot;jdbc:mysql://localhost:3306/school&quot;,&quot;root&quot;,&quot;mysqladmin&quot;); 
        //String sql = &quot;update t_vote set vote=vote+1  where teacherno=?&quot;; 
        //PreparedStatement ps=conn.prepareStatement(sql); 
        //ps.setString(1,teacherno); 
        //ps.executeUpdate(); 
        //ps.close(); 
        //conn.close();         
        VoteDao vdao= new VoteDao();
        vdao.updateVotes(teacherno);
     %>
     <jsp:forward page="display.jsp"></jsp:forward>
  </body>
</html>display.jsp:
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@ page import="dao.VoteDao" %>
<%@ page import="vo.Vote" %>
<%
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 'display.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">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->  </head>
  
  <body>
        <form action="vote.jsp" method="post">
        <table align="center">
        <caption> 欢迎给教师投票   <input type="submit" value="提交投票"> </caption>
        <tr bgcolor = "yellow">
            <td> 编号 </td>
            <td> 姓名 </td>
            <td> 得票数 </td>
            <td> 投票 </td>
        </tr>
        <%
                //request.setCharacterEncoding("utf-8");
                //response.setContentType("text/html;charset=utf-8");
                //String Url=" jdbc:sqlserver://localhost:3306;DataBaseName=school;user=root&password=mysqladmin&useUnicode=true&characterEncoding=gb2312";　
                //Class.forName("com.mysql.jdbc.Driver");
                //Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/school","root","mysqladmin");
                //Statement stat = conn.createStatement();
                //String sql = "select teacherno,teachername,vote from t_vote";
                //ResultSet rs = stat.executeQuery(sql);
                //while(rs.next()){
                  //    String teacherno = rs.getString("teacherno");
                    //  String teachername = rs.getString("teachername");
                      //int vote = rs.getInt("vote");
                VoteDao vdao=new VoteDao();
                //ArrayList<Vote> votes=new ArrayList<Vote>();
                ArrayList<Vote> votes=vdao.getAllVotes();
                for(int i=0;i<votes.size();i++){
                    Vote vote=(Vote)votes.get(i);
                
         %>   
                <tr bgcolor = "pink">
                <td><%=  vote.getTeacherno()   %></td> 
                <td><%=  vote.getTeachername() %></td>
                <td><img  src="img/bar.jpg"  width="<%= vote.getVotenumber() %>" height="10"> <%=vote.getVotenumber() %></td>
                <td><input name="teacherno" type="checkbox" value="<%= vote.getTeacherno() %>"></td>
                </tr>
            <%       
                }
                //stat.close();
                //conn.close();
            %>
      </table>
      </form>
  </body>
</html>
最后是出现的错误：
HTTP Status 500 - An exception occurred processing JSP page /display.jsp at line 52type Exception reportmessage An exception occurred processing JSP page /display.jsp at line 52description The server encountered an internal error that prevented it from fulfilling this request.exceptionorg.apache.jasper.JasperException: An exception occurred processing JSP page /display.jsp at line 5249:                       //int vote = rs.getInt("vote");
50:                 VoteDao vdao=new VoteDao();
51:                 //ArrayList<Vote> votes=new ArrayList<Vote>();
52:                 ArrayList<Vote> votes=vdao.getAllVotes();
53:                 for(int i=0;i<votes.size();i++){
54:                     Vote vote=(Vote)votes.get(i);
55:                 
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:521)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:430)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
root causejava.lang.NullPointerException
dao.VoteDao.getAllVotes(VoteDao.java:16)
org.apache.jsp.display_jsp._jspService(display_jsp.java:111)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:388)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.37 logs.Apache Tomcat/6.0.37        
   

Answer 13

空指针异常
getAllVotes方法的16行Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/school","root","mysqladmin");
这行代码修改成 
conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/school","root","mysqladmin");
应该没问题了

Answer 14

java.lang.NullPointerException
 dao.VoteDao.getAllVotes(VoteDao.java:16)
很明显的空指针异常，你打个断点看一下，或者system.out看一下是个什么情况。。