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().