哪里为空

servletjsp做登录的程序

try
  {
  Statement sql1=conn.createStatement();
  PreparedStatement sql=conn.prepareStatement(searchinfo);
 
  if(boo)
  {
  sql.setString(1,username);
  sql.setString(2,pwd);
 
  ResultSet rs=sql.executeQuery();
  boolean m=rs.next();
 
  if(m==true)
  {
  out.println("登陆成功");
  session.setMaxInactiveInterval(60000);
  session.setAttribute("username",username);
  String userid = rs.getString("userid");
  session.setAttribute("userid", "userid");
  session.setAttribute("fullname", rs.getString("fullname"));
  session.setAttribute("logins", rs.getString("logins"));
  session.setAttribute("rights", rs.getString("rights"));
  session.setAttribute("depid", rs.getString("depid"));
  session.setAttribute("mygroups", rs.getString("groups"));
 
  String lastip = req.getRemoteAddr();
 
  String dateTime = dateformat.format(date);
 
  String update = "UPDATE userinfo SET lastlogin=to_date('"+dateTime+"','yyyy-MM-dd hh:mi:ss'),logins=logins+1 where userid=" + userid;
  String update1 = "INSERT into Log(username,uptime,lastip,content) VALUES('" + username + "',to_date('"+dateTime+"','yyyy-MM-dd hh:mi:ss'),'" + lastip + "','登陆系统')";
 
  int n = sql1.executeUpdate(update1);
  int f = sql1.executeUpdate(update);
 
  System.out.println(userid+username);
 
  resp.sendRedirect("index.jsp");
  }
  else
  {
  out.println("<SCRIPT LANGUAGE=javascript>");
  out.println("alert('用户名或密码错误！');");
  out.println("window.location.href='default.jsp'; ");
  out.println("</script>");
  }
  }
  sql1.close();
  sql.close();
  conn.close();
  }
  catch(SQLException e2)
  {
  e2.printStackTrace();
  } 
  } 



调试整个代码都可以运行，但是后面就报错为

java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Integer.java:415)
at java.lang.Integer.parseInt(Integer.java:497)
at org.apache.jsp.index_jsp._jspService(index_jsp.java:72)
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)

我不清楚哪个String为空了，该怎么找？

求助