500错误，不知道该如何解决

type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception java.lang.NullPointerException
files.UpdateUser.doPost(UpdateUser.java:45)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.20 logs.
--------------------------------------------------------------------------------Apache Tomcat/6.0.20

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java.lang.NullPointerException
files.UpdateUser.doPost(UpdateUser.java:45)
如上已经打出了错误代码段，在编程中你应该要有意识，加入对值的为空的判断
例：
if(value!=null){}
java.lang.NullPointerException
files.UpdateUser.doPost(UpdateUser.java:45)
UpdateUser.java:45行空指针啊
UpdateUser.java:
package files;import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import mysql.UserOpera;/**
* Servlet implementation class UpdateUser
*/
public class UpdateUser extends HttpServlet {
private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public UpdateUser() {
        super();
        // TODO Auto-generated constructor stub
    } /**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
//super.doGet(request, response);
doPost(request,response);
} /**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
//super.doPost(request, response);
request.setCharacterEncoding("gbk");
String username = request.getParameter("username");
String name = request.getParameter("name");
String pass = request.getParameter("pass");
String type = request.getParameter("type");
if(type.equals("0")){
type="admin";
}else if(type.equals("1")){
type="user";
}
if(name.equals("")||pass.equals("")||type.equals("")){
response.sendRedirect("usermain.jsp");
}
else{
UserOpera uo = new UserOpera();
int flag = uo.updateUser(username,name,pass,type);
if(flag!=0){
response.sendRedirect("GetAllUser");
}else{
response.sendRedirect("error.jsp");
}
}
}}UpdateUser.java:
<%@ page language="java" contentType="text/html; charset=gbk"
    pageEncoding="gbk"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<%
  //request.setCharacterEncoding("gbk");
  String username = (String)request.getParameter("username");
  username=new String(username.getBytes("iso-8859-1"),("gbk"));
  String name = (String)request.getParameter("name");
  name=new String(name.getBytes("iso-8859-1"),("gbk"));
  String pass = (String)request.getParameter("pass");
  String type = (String)request.getParameter("type");
%>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gbk">
<title>修改用户信息</title>
</head>
<body background="images/bj.jpg">
<table width="752"  cellpadding="0" cellspacing="0">
  <tr>
<td width="77"  valign="top"></td>
<td width="662" valign="top" cellpadding="0" cellspacing="0">
<form id="UpdateUser" name="UpdateUser" method="post" action="UpdateUser" onsubmit="return check()">
<table border="0" cellpadding="0" cellspacing="0" height="136" width="100%">
<tr>
  <td width="27"> </td>
  <td width="14"> </td>
<td align="center">修改用户信息</td>
</tr>
<tr>
  <td height="25"></td>
  <td> </td>
<td><input type="hidden" name="username" value="<%=username %>"></td>
  <td align="center" valign="middle">真实姓名</td>
  <td colspan="5" valign="top"><input name="name" value="<%=name %>" maxlength="18" size="18" /><font color="red">*</font></td>
  <td> </td>
  </tr>
<tr>
  <td height="14"></td>
  <td colspan="10" valign="top"> </td>
  <td></td>
  </tr>
<tr>
  <td height="25"></td>
  <td> </td>
          <td> </td>
          <td align="center" valign="middle">用户类型</td>
         <td colspan="5" valign="top"><input name="name" value="<%=name %>" maxlength="18" size="18" /><font color="red">*</font></td>
  <td> </td>
<%
if(type.equals("admin")){
%>
<select name="type" id="type">
<option value="0" selected>管理员</option>
<option value="1" >一般用户</option>
</select>
<%
}else if(type.equals("user")){
%>
<select name="type" id="type">
<option value="0" >管理员</option>
<option value="1" selected>一般用户</option>
</select>
<%
}
%>

</td>
          <td> </td>
          <td></td>
  </tr>
<tr>
  <td height="14"></td>
  <td colspan="10" valign="top"> </td>
  <td></td>
  </tr>
<tr>
  <td height="25"></td>
  <td> </td>
  <td> </td>
  <td align="center" valign="middle">密码</td>
  <td colspan="5" valign="top"><input name="pass" value="<%=pass %>" type="password" maxlength="18" size="18" /><font color="red">*</font></td>
  <td> </td>
  </tr>
<tr>
  <td height="14"></td>
  <td colspan="10" valign="top"> </td>
  <td></td>
  </tr>
<tr>
  <td height="25"></td>
  <td> </td>
  <td> </td>
  <td align="center" valign="middle">确认密码</td>
  <td colspan="5" valign="top"><input name="passag" value="<%=pass %>" type="password" maxlength="18" size="18" /><font color="red">*</font></td>
  <td> </td>
  </tr>
<tr>
  <td height="14"></td>
  <td colspan="10" valign="top" > </td>
  <td></td>
  </tr>
<tr>
  <td height="24"></td>
  <td> </td>
  <td> </td>
  <td align="center" valign="top"><label>
    <input name="save" type="submit" value="保存" />
  </label></td>
  <td colspan="2" valign="top"><label>
    <input name="back" type="button" value="返回"/>
  </label></td>
  <td> </td>
  <td> </td>
  <td></td>
  </tr>
</table>
</form>
</td>
</tr>
</table>
</body>
</html>
上面这个是我的Jsp页面的代码和servlet代码，，帮我看看该怎么改吧
打印一下看一看JSP页面中的 request.getParameter("") 赋给的变量有没有值
if(type.equals("0")){
type="admin";
}else if(type.equals("1")){
type="user";
}
if(name.equals("")||pass.equals("")||type.equals("")){
...改成
if("0".equals(type)){
type="admin";
}else if("1".equals(type)){
type="user";
}
if("".equals(name)||"".equals(pass)||"".equals(type)){
空指针呐空指针    貌似最近上csdn的人平率都少了
String type = request.getParameter("type");
看下你调用updateUser之前的那个页面是否传递了type值，这个值没有传过来，或者说你根本没有设置
目前跟这个没关系，现在出现的是另一个问题：
type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException: An exception occurred processing JSP page /UpdateUser.jsp at line 85: <%
6:   //request.setCharacterEncoding("gbk");
7:   String username = (String)request.getParameter("username");
8:   username=new String(username.getBytes("iso-8859-1"),"gbk");
9:   String name = (String)request.getParameter("name");
10:   name=new String(name.getBytes("iso-8859-1"),"gbk");
11:   String pass = (String)request.getParameter("pass");
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:505)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:416)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause java.lang.NullPointerException
org.apache.jsp.UpdateUser_jsp._jspService(UpdateUser_jsp.java:59)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.20 logs.
--------------------------------------------------------------------------------Apache Tomcat/6.0.20
UpdateUser.jsp
<%@ page language="java" contentType="text/html; charset=gbk"
    pageEncoding="gbk"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<%
  //request.setCharacterEncoding("gbk");
  String username = (String)request.getParameter("username");
  username=new String(username.getBytes("iso-8859-1"),"gbk");
  String name = (String)request.getParameter("name");
  name=new String(name.getBytes("iso-8859-1"),"gbk");
  String pass = (String)request.getParameter("pass");
  String type = (String)request.getParameter("type");
  System.out.println(username);
  System.out.println(name);
  System.out.println(pass);
  System.out.println(type);

%>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gbk">
<title>修改用户信息</title>
</head>
<body background="images/bj.jpg">
<table width="752"  cellpadding="0" cellspacing="0">
  <tr>
<td width="77"  valign="top"></td>
<td width="662" valign="top" cellpadding="0" cellspacing="0">
<form id="UpdateUser" name="UpdateUser" method="post" action="UpdateUser" onsubmit="return check()">
<table border="0" cellpadding="0" cellspacing="0" height="136" width="100%">
<tr>
  <td width="27"> </td>
  <td width="14"> </td>
<td align="center">修改用户信息</td>
</tr>
<tr>
  <td height="25"></td>
  <td> </td>
<td><input type="hidden" name="username" value="<%=username %>"></td>
  <td align="center" valign="middle">真实姓名</td>
  <td colspan="5" valign="top"><input name="name" value="<%=name %>" maxlength="18" size="18" /><font color="red">*</font></td>
  <td> </td>
  </tr>
<tr>
  <td height="14"></td>
  <td colspan="10" valign="top"> </td>
  <td></td>
  </tr>
<tr>
  <td height="25"></td>
  <td> </td>
          <td> </td>
          <td align="center" valign="middle">用户类型</td>
          <td colspan="5" valign="middle">
<%
if(type.equals("0")){
%>
<select name="type" id="type">
<option value="0" selected>管理员</option>
<option value="1" >一般用户</option>
</select>
<%
}else if(type.equals("1")){
%>
<select name="type" id="type">
<option value="0" >管理员</option>
<option value="1" selected>一般用户</option>
</select>
<%
}
%>
</td>
          <td> </td>
          <td></td>
  </tr>
<tr>
  <td height="14"></td>
  <td colspan="10" valign="top"> </td>
  <td></td>
  </tr>
<tr>
  <td height="25"></td>
  <td> </td>
  <td> </td>
  <td align="center" valign="middle">密码</td>
  <td colspan="5" valign="top"><input name="pass" value="<%=pass %>" type="password" maxlength="18" size="18" /><font color="red">*</font></td>
  <td> </td>
  </tr>
<tr>
  <td height="14"></td>
  <td colspan="10" valign="top"> </td>
  <td></td>
  </tr>
<tr>
  <td height="25"></td>
  <td> </td>
  <td> </td>
  <td align="center" valign="middle">确认密码</td>
  <td colspan="5" valign="top"><input name="passag" value="<%=pass %>" type="password" maxlength="18" size="18" /><font color="red">*</font></td>
  <td> </td>
  </tr>
<tr>
  <td height="14"></td>
  <td colspan="10" valign="top" > </td>
  <td></td>
  </tr>
<tr>
  <td height="24"></td>
  <td> </td>
  <td> </td>
  <td align="center" valign="top"><label>
    <input name="save" type="submit" value="保存" />
  </label></td>
  <td colspan="2" valign="top"><label>
    <input name="back" type="button" value="返回"/>
  </label></td>
  <td> </td>
  <td> </td>
  <td></td>
  </tr>
</table>
</form>
</td>
</tr>
</table>
</body>
</html>这是我的jsp代码，不知道该怎么改，麻烦各位英雄好汉帮我看看，非常感谢