为什么报错

package com.imooc.servlet;

import java.io.IOException;

import java.util.ArrayList;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import com.imooc.domain.User;

import com.imooc.service.UserService;

import com.imooc.serviceImpl.UserServiceImpl;

@WebServlet("/RegistServlet")

public class RegistServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

request.setCharacterEncoding("UTF-8");

response.setContentType("text/html;charset=utf-8");

String username=request.getParameter("username");

String password=request.getParameter("password");

User user=new User();

user.setUsername(username);

user.setPassword(password);

System.out.println(user);

UserService userService=new UserServiceImpl();

List<User> userList=(List<User>)getServletContext().getAttribute("userlist");

userService.regist(userList, user);

System.out.println(userList);

//System.out.println(userList);

//response.sendRedirect("login.jsp");

}

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

doGet(request, response);

}

}

package com.imooc.service;

import java.util.List;

import com.imooc.domain.User;

public interface UserService {

public void regist(List<User> userlist,User user);

}

package com.imooc.serviceImpl;

import java.util.ArrayList;

import java.util.List;

import com.imooc.domain.User;

import com.imooc.service.UserService;

public class UserServiceImpl implements UserService{

@Override

public void regist(List<User> userList, User user) {

// 保存用户信息

userList.add(user);

}

}

package com.imooc.domain;

public class User {

private String username;

private String password;

private String file;

public User(){

}

public User(String name, String password, String file) {

super();

this.username = name;

this.password = password;

this.file = file;

}

public String getName() {

return username;

}

public void setUsername(String name) {

this.username = name;

}

public String getPassword() {

return password;

}

public void setPassword(String password) {

this.password = password;

}

public String getFile() {

return file;

}

public void setFile(String file) {

this.file = file;

}

@Override

public String toString() {

return "User [name=" + username + ", password=" + password + ", file=" + file + "]";

}

}