The server encountered an internal error () that prevented it from fulfilling this re

[mutyala prasad]

Hello to all,
when i running my url i got the error like below.
my webser is tomcat6.0.32.
pls give me the solution.


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

java.lang.NullPointerException
com.st.servlet.LoginServlet.service(LoginServlet.j ava:15)

this is my servlet:
******************

package com.st.servlet;
import javax.servlet.*;
import java.io.*;
import com.st.dao.UserDaoI;
import com.st.dao.UserDao;
public class LoginServlet extends GenericServlet{
private UserDaoI udao;
public void init(){
udao=new UserDao();
}
public void service(ServletRequest req,ServletResponse res)throws ServletException,IOException{
PrintWriter out=res.getWriter();
String un=req.getParameter("uname");
String pwd=req.getParameter("pass");
if(pwd.length()>=6){
boolean flag=udao.findUser(un,pwd);
if(flag){
out.println("valid user");
}
}
else{
out.println("password should be min 3 charecters");
}

this is my web.xml:
******************

<web-app>
<servlet>
<servlet-name>ls</servlet-name>
<servlet-class>com.st.servlet.LoginServlet</servlet-class>
<load-on-startup></load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>ls</servlet-name>
<url-pattern>/login</url-pattern>
</servlet-mapping>
</web-app>