How do I extract the query string from a POST request?

You need to build it manually from the ServletRequest.

StringBuilder sb = new StringBuilder("?");
for (Enumeration e = request.getParameterNames();
e.hasMoreElements();)
{
String param = (String) e.nextElement();
sb.append(param)
.append("=")
.append(request.getParameter(param))
.append("&");
}
String queryString = sb.toString().
substring(0, sb.length() - 1);