Jeff is quite correct, you can't just call out to
Java (the term 'JSP function' isn't very meaningful) from JavaScript on the page.
In order to go back to the server, you must submit a form or hit a URL in order to initiate the traditional request/response cycle.
So you have three primary choices:
1) Load up all the possibilities in JavaScript objects when the page is rendered so that you can use DHTML to manipulate the drop-downs.
2) Submit the page so that it is refreshed with the approriate values in the drop-downs.
3) Get really tricky and submit the request to an off-screen frame (usually an iframe of zero size), so that the current page does not refresh. However, JavaScript returned in the invisible frame can manipulate the dropdowns on the displayed page as approriate. Sort of the best of both worlds, but of course, trickier to implement and maintain.
hth,
bear