为什么提示 空指针

[沁雅学院]

package com.edu.action;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.edu.bean.User;
import com.edu.dao.UserDaoImpl;

public class CommonServlet extends HttpServlet{
private UserDaoImpl userDao;

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException{
// TODO Auto-generated method stub
//操作标示
String type=request.getParameter("type");
if("login".equals(type)){

try {
this.login(request, response);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

}
}
/**
* 用户登录验证
* @param request
* @param response
* @throws Exception
*/
private void login(HttpServletRequest request,HttpServletResponse response) throws Exception{
Map<Object,String> map=new HashMap<Object,String>();
String username=request.getParameter("username");
String password=request.getParameter("password");
User user=userDao.findByLogin(username, password); //这句报错
if(user==null){
map.put("state", "error");
map.put("info", "用户不存在");
}
else{
HttpSession session=request.getSession();
session.setAttribute("user", user);
response.sendRedirect("main.jsp");
}

}


}

[沁雅学院]

debug 红色那句跳转不过去，直接报错 java.lang.NullPointerException 怎么回事啊？？ 求大神解释

[沁雅学院]

找到原因了 郁闷死了