浏览器取不到后台传过来的数据

Question

后台传参代码：

public class LoginAction extends HttpServlet{
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
try{
String uname=req.getParameter("uname");
String upass=req.getParameter("upass");
//service帮助验证

                UserService us=new UserService();
                User user=us.checkLogin(uname, upass);

                if(user==null){
                    resp.sendRedirect("login.jsp?f=9");

                }else{
                     req.getSession().setAttribute("currentUser", user);
                     resp.sendRedirect("main.jsp");
                }

                }catch(Exception e){
                    e.printStackTrace();
                }
    }

}

浏览器端接收参数代码：

运行程序，前面都 正常运行，到了浏览器却报空指针错误，取不到参数f的值
什么原因？

错误日志：

严重: Servlet.service() for servlet jsp threw exception

java.lang.NullPointerException
at org.apache.jsp.login_jsp._jspService(login_jsp.java:68)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Thread.java:662)
null