How can I get the request URL from a Java Filter?

Is this what you're looking for?

if (request instanceof HttpServletRequest) {
 String url = ((HttpServletRequest)request).getRequestURL().toString();
 String queryString = ((HttpServletRequest)request).getQueryString();
}

To Reconstruct:

System.out.println(url + "?" + queryString);

Info on HttpServletRequest.getRequestURL() and HttpServletRequest.getQueryString().

Building on another answer on this page,

public static String getCurrentUrlFromRequest(ServletRequest request)
{
   if (! (request instanceof HttpServletRequest))
       return null;

   return getCurrentUrlFromRequest((HttpServletRequest)request);
}

public static String getCurrentUrlFromRequest(HttpServletRequest request)
{
    StringBuffer requestURL = request.getRequestURL();
    String queryString = request.getQueryString();

    if (queryString == null)
        return requestURL.toString();

    return requestURL.append('?').append(queryString).toString();
}