register.jsp里的html代码：
<body>
<form action="com/servlet/UserRegServlet" method="post" onsubmit="register()">
用户名： <input type="text" name="username" /></br>
密 码： <input type="password" name="password"/></br>
确认密码：<input type="password" name="againpass"/></br>
性 别：<select name="gender"><option value="男">男</option>
<option value="女">女</option></select> </br>
密码保护问题<select name="question" >
<option value="您的真实姓名？" >您的真实姓名？</option>
<option value="您的高中班主任的名字？" >您的高中班主任的名字？</option>
<option value="您的手机号码？">您的手机号码？</option>
</select></br>
答 案：<input type="text" name="answer"/></br>
<input type="submit" name="submitReg" value="注册"/>
"
</form>
</body>
UserRegServlet类的代码：
package com.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.dao.UserDao;
import com.entity.Users;
import com.tool.Chinese;
//用户注册验证
public class UserRegServlet extends HttpServlet{
UserDao dao=null;
public void doPost(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException{
Users user=new Users();
String username = Chinese.chinese(request.getParameter("username")).trim();
String password=request.getParameter("password");
//验证用户输入的用户名是否已经存在
if(dao.selectPassword(username) == null || dao.selectPassword(username).equals("")){
user.setUserName(username);
user.setUserPwd(Chinese.chinese(request.getParameter("password")).trim());
user.setGender(request.getParameter("gender"));
user.setQuestion(Chinese.chinese(request.getParameter("question")).trim());
user.setAnswer(Chinese.chinese(request.getParameter("answer")).trim());
dao.insertUser(user);
request.getRequestDispatcher("RegSuccess.jsp").forward(request, response);
}else{
request.setAttribute("success", "该会员名称已经存在！！！");
}
}
public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException{
doPost(request,response);
}
}
web.xml里的代码：
<servlet>
<servlet-name>UserRegServlet</servlet-name>
<servlet-class>com.servlet.UserRegServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>UserRegServlet</servlet-name>
<url-pattern>/com/servlet/UserRegServlet</url-pattern>
</servlet-mapping>
本人今年大学大四，做毕业设计的时候遇到这个问题，请求哪位IT前辈帮忙看看，怎么解决啊，感激不尽！！！
------解决方案--------------------------------------------------------
很明显dao是null;
------解决方案--------------------------------------------------------