JavaScript varialble in a Servlet

I am developing an JSP application where, i would like to use a JavaScript variable in the Java servlet Code. Please tell me how do i do it?

Well, the way I understand it, you can't do this directly:
the javascript is executed in the client's browser, *after* your jsp page has already done it's thing.
Therefor, the JSP page needs to be executed again.
The way I'd do it is to let let the javascript address the same page with your needed value as a GET or POST parameter.
Hope this solves it.

Thank you for answering my query Mr Thomas.
But could please clear my doubt with an example ??
I just got the idea what should be done..but please give an example.