Not this is Spring related. But what if you wrote JavaScript that when you submit that page, it gets that current URL that the browser is showing and populate a hidden form field with that value, then you can get it in your code, like any other parameter.
This will only work on your own pages, but if the user comes from anywhere outside your site, from say www.google.com you won't be able to get a "previous" url, because you could be coming from anywhere, and to me even if you could that would be a security risk. I don't want you to know that I was coming from www.somepornsite.com