Java过滤器doFilter里chain.doFilter()函数的理解

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req=(HttpServletRequest)request;
        HttpServletResponse resp=(HttpServletResponse)response;
        String servletPath= req.getServletPath();
        HttpSession session=req.getSession();
        String flag=(String)session.getAttribute("flag");
        if(servletPath!=null&&(servletPath.equals("/03/index.jsp")||servletPath.equals("/03/login.jsp")||servletPath.equals("/loginServlet17"))){
            chain.doFilter(request, response);
        }else{
            if(flag!=null&&flag.equals("login_success")){
                chain.doFilter(request, response);  
            }else if(flag!=null&&flag.equals("login_error")){
                req.setAttribute("msg", "登录失败，请重新登录");
                req.setAttribute("return_uri", servletPath);
                RequestDispatcher rd=req.getRequestDispatcher("/03/login.jsp");
                rd.forward(req, resp);
            }else{
                req.setAttribute("msg","你尚未登录");
                req.setAttribute("return_uri", servletPath);
                RequestDispatcher rd=request.getRequestDispatcher("/03/login.jsp");
                rd.forward(req, resp);
            }
        }
        System.out.println(servletPath);
    }

<servlet-mapping>
      <servlet-name>myServlet</servlet-name>
      <url-pattern>/foo*</url-pattern>
  </servlet-mapping>
  
  <filter-mapping>
      <filter-name>servletFilter</filter-name>
      <url-pattern>myServlet</url-pattern>
  </filter-mapping>
  
  <filter-mapping>
      <filter-name>myFilter</filter-name>
      <url-pattern>/foo*</url-pattern>
  </filter-mapping>
  
  <filter-mapping>
      <filter-name>anotherFilter</filter-name>
      <url-pattern>/foo/bar</url-pattern>
  </filter-mapping>