mysql+webservice错误，请问是怎么回事

请问这是怎么回事

加个username的isnull判断，估计没传回值，类似下面代码，不然提示吓人一跳

        if(request.getParameter("flag")!=null)
        if(request.getParameter("flag")!="")
        where = where +" and  flag = '"+request.getParameter("flag").toString()+"' ";