JAVA判断session是否连接

String pid = "";

try{
  if(request.getSession(false)==null){
   response.sendRedirect("./../error_session.jsp");
    }
   
   pid  = (String)session.getAttribute("personid");

 if(pid == null){
 response.sendRedirect("../../error_session.jsp");
 } 
  } catch (Exception e) {
 e.printStackTrace();
  }