求解：关于Filter在判断无登录状态下的问题。一直想不通！

package fkpi.ilter;

import

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class LoginFilter extends HttpFilter{
    @Override
    protected void doFilter(HttpServletRequest req, HttpServletResponse resp, FilterChain chain) {
        
            if(req.getSession().getAttribute("userName")==null){//拿取到sesson判断，如果空了
                try {
                    resp.sendRedirect(req.getContextPath()+"/index.jsp");//跳转到首页
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }else{
                try {
                    chain.doFilter(req, resp);//如果不空，说明已经登录了，就直接放行
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (ServletException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
    }
}