It's important to realize that JavaScript variables live on the client-side (Browser) and that JSP variables live on the server-side, so the two can't really talk to each other at run time. However, now that AJAX has become popular, it is possible to have JavaScript submit a request to the server and receive a response without refreshing the page.
Here are two possible ways of handling a dependent select box.
1- Use the onchange event of the first select box to submit the form. Then have the action class read the value of the first select box, populate the values for the second box, and redisplay the page. In order for this to work, you will have to set some sort of flag to indicate you're only submiting the form to change the select box, not to signify the data is complete and ready to be processed.
2- In the original JSP, display the second select with no options. Use the onchange event of the first select box to make an AJAX call to the server, which retrieves the set of options for the second select box. You would then use the Document Object Model (DOM) API of JavaScript to add the options to the second select box. If you're not familiar with AJAX, I'd suggest using a framework such as
DWR to handle your AJAX calls. Otherwise, you're going to have to spend some time learning the details of how to make AJAX calls.
If you just want to get your project done on time, I'd suggest using method 1. If you want to explore new territory, method 2 would be a good choice.
