jsp filter 过滤器判断用户是否登录

   
  import   javax.servlet.Filter;  
  import   javax.servlet.FilterConfig;  
  import   javax.servlet.ServletException;  
  import   javax.servlet.ServletRequest;  
  import   javax.servlet.ServletResponse;  
  import   javax.servlet.FilterChain;  
  import   java.io.IOException;  
  import   javax.servlet.http.HttpServletRequest;  
  import   javax.servlet.http.HttpSession;  
  import   javax.servlet.http.HttpServletResponse;  
   
  public   class   RightFilter  
          implements   Filter   {  
      public   void   init(FilterConfig   filterConfig)   throws   ServletException   {  
      }  
   
      public   void   doFilter(ServletRequest   request,   ServletResponse   response,  
                                                FilterChain   chain)   throws   IOException,   ServletException   {  
          HttpServletRequest   req   =   (HttpServletRequest)   request;  
          HttpServletResponse   res   =   (HttpServletResponse)   response;  
           
          HttpSession   session   =   req.getSession(true);  
   
          //从session里取的用户名信息  
          String   username   =   (String)   session.getAttribute("username");  
           
          //判断如果没有取到用户信息,就跳转到登陆页面  
          if   (username   ==   null   ||   "".equals(username))   {  
              //跳转到登陆页面