jsp redirects the address bar without changing the instance

<%@ page contentType="text/html;charset=gb2312"%>
<%
    String s = request.getParameter("l");
    if(s!=null)
    {
        String url = "http://p.ofstack.com/R/v";
%>
        <frameset cols="*" frameborder="NO" border="0" framespacing="0">
        <frame src="<%=url %>">
        </frameset>
<%    }
    else
    {
        response.getWriter().print("l is null");
    }
%>