servlet不能使用

这个是我的servlet
public class UserServlet extends HttpServlet {
   
    public void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        
    }

    public void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
        
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        User user = new User();
        user.setUsername(username);
        user.setPassword(password);        
        if(DAOFactory.getUserDAOImpl().isLogin(user)){        
            req.getRequestDispatcher("/main.jsp").forward(req, resp);
            }
    }
   
    public void doAdd(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
        System.out.println("操作成功");
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        User user = new User();
        user.setUsername(username);
        user.setPassword(password);
        if(DAOFactory.getUserDAOImpl().Add(user)){
            System.out.println("添加成功");
        }
    }
   
}

web.xml
    <servlet>
        <servlet-name>user</servlet-name>
        <servlet-class>servlet.UserServlet</servlet-class>
    </servlet>
   
    <servlet-mapping>
        <servlet-name>user</servlet-name>
        <url-pattern>/user.do</url-pattern>
    </servlet-mapping>
   
    <servlet-mapping>
        <servlet-name>user</servlet-name>
        <url-pattern>/Add.do</url-pattern>
    </servlet-mapping>


为什么这个<form action="user.do" method="post">能执行成功
而这个<form action="Add.do" method="post">就不能执行？