I'll assume that you are using JSF 2 and Java EE 6 for this answer.
The implementation of the actual mechanism will vary depending on the extent to which you'll need the original URL.
You'll first need to get access to the underlying servlet container (assumed to one, instead of a portlet container) produced HttpServletRequest object. Use the
FacesContext object to access the HttpServletRequest object in the following manner:
HttpServletRequest origRequest = (HttpServletRequest)FacesContext.getCurrentInstance().getExternalContext().getRequest();
HttpServletRequest class provides several utility methods to obtain a near representation of the original request:
getRequestURL(), which provides the original request sans the query string
getQueryStringall of whose outputs can be combined in sequence to obtain the original request. You may have to omit the latter invocations if you want a lesser fragment of the URL.