过滤器、监听器常见操作

用户登录之后才能进入主页，用户注销后就不能进入主页了
1.用户登录之后，向Ssession中放入用户的数据
2.进入主页的时候要判断用户是否登录，要求：在过滤器中实现

 //HttpServletRequest   ServletRequest

    HttpServletRequest request = (HttpServletRequest) servletRequest;
    HttpServletResponse response = (HttpServletResponse) servletResponse;

    Object user_session = request.getSession().getAttribute("USER_SESSION");
    if (user_session == null) {
        response.sendRedirect("/javaweb_filter_war_exploded/Login.jsp");
    }
    filterChain.doFilter(servletRequest,servletResponse);
}