HttpServletRequest - how to obtain the referring URL?

It's available in the HTTP referer header. You can get it in a servlet as follows:

String referrer = request.getHeader("referer"); // Yes, with the legendary misspelling.

You, however, need to realize that this is a client-controlled value and can thus be spoofed to something entirely different or even removed. Thus, whatever value it returns, you should not use it for any critical business processes in the backend, but only for presentation control (e.g. hiding/showing/changing certain pure layout parts) and/or statistics.

For the interested, background about the misspelling can be found in Wikipedia.

Actually it's: request.getHeader("Referer"), or even better, and to be 100% sure, request.getHeader(HttpHeaders.REFERER), where HttpHeaders is com.google.common.net.HttpHeaders