关于作业的拦截功能

拦截普通管理员 操作用户管理那个功能，我完全没有思考如何拦截 然后我只会拦截URL

<filter>
    <filter-name>UserFilter</filter-name>
    <filter-class>com.imooc.course.filter.UserFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>UserFilter</filter-name>
    <url-pattern>/addUser.jsp</url-pattern>
</filter-mapping>
<filter-mapping>
    <filter-name>UserFilter</filter-name>
    <url-pattern>/SelectUsers</url-pattern>
</filter-mapping>

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        if (!(req.getSession().getAttribute("code").equals(req.getParameter("checkCode")))){
            req.getRequestDispatcher("/index.jsp").forward(req,resp);
        }
        String username = (String) req.getParameter("username");
        String password = (String) req.getParameter("password");
        req.getSession().setAttribute("username",username);

        CourseService courseService=new CourseServiceImpl();
        int i=courseService.login(username,password);
        switch (i){
            case 1://超级管理员
                req.getRequestDispatcher("/server.jsp?i="+i).forward(req,resp);
                break;
            case 2://普通管理员
                req.getRequestDispatcher("/server.jsp?i="+i).forward(req,resp);
                break;
            case 0://登陆不成功，跳转到登陆页面
                req.getRequestDispatcher("/index.jsp").forward(req,resp);
                break;
        }
    }
}

public class UserFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest httpServletRequest= (HttpServletRequest) servletRequest;
        HttpServletResponse httpServletResponse= (HttpServletResponse) servletResponse;

        System.out.println("doFilter");
        int i= (int) httpServletRequest.getAttribute("i");
        System.out.println("i="+i);
        if(i!=1){
            System.out.println("doFilter   if(i!=1)");
            httpServletResponse.sendRedirect("/server.jsp?i="+i);
            return;
        }
        filterChain.doFilter(servletRequest,servletResponse);
        return;
    }

    @Override
    public void destroy() {

    }
}

然后我点击查询和添加管理员的时候 就给我报空指针异常了  可否给我讲解下思路怎么拦截什么的