Java.lang.IllegalStateException: OutputStream already retrieved

Question

I'm getting this error when I execute the line
response.getWriter().write("yes");why is this the case and how do I correct it?

Answer 1

this is all the code in my JSP, so I'm quite certain I only called it once.
<%
String strUser = request.getParameter("username");
String strPassword = request.getParameter("password");
if(strUser != null && strPassword != null) {
    //System.out.println("before sending response");
     response.setContentType("text/xml");
     response.setHeader("Cache-Control", "no-cache");
    // for plain text response:
     //response.getWriter().write("yes");
    // Or for XML formatted response:
     response.getWriter().write("<message>yes</message>");   
    //System.out.println("after sending response");          
} else {
     //nothing to show
     response.setStatus(HttpServletResponse.SC_NO_CONTENT);
%>is there any other reason for this exception?