1、创建过滤器包，创建LoginFilter类

2.在LoginFilter类中写过滤代码

    //白名单
    List<String> whitelist = Arrays.asList("/login.jsp");
    //用来存放配置文件中Action节点的属性
    List<Action> actionList = new ArrayList<>();

public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    HttpServletRequest request = (HttpServletRequest) servletRequest;
    HttpServletResponse response = (HttpServletResponse) servletResponse;
    String path = request.getServletPath();
    Object username = request.getSession().getAttribute("user");
   
    if (username != null) {
        //有登录，直接放行
        filterChain.doFilter(request, response);
        return;
    } else if (whitelist.contains(path)) {//在白名单中
        //放行
        filterChain.doFilter(servletRequest, servletResponse);
        return;
    } else {
        //没有有登录，重定向登录页面
        response.sendRedirect("login.jsp");
    }
}

Filter层中的LoginFilter类

Servlet层中的user （从登录页面获取的用户值，并存入Session中）

完成以上操作就可以实现不管进入那个页面都会跳到登录页面！ ！ ！