public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2) throws IOException, ServletException

{

HttpServletRequest request = (HttpServletRequest) arg0;

HttpServletResponse response = (HttpServletResponse) arg1;

HttpSession session = request.getSession();

    if(session.getAttribute("username")!=null)

{

arg2.doFilter(arg0, arg1);//允许放行!

}

    else

{

response.sendRedirect(request.getContextPath()+"/login.jsp");

}

}