I didn't take a close look at your code -- looking at a bunch of
Struts tags is less than illuminating -- but the premise of the dependent drowdown is fairly easy.
1. When a selection is made from dropdown #1, an Ajax request is made to a server-side resource with the value chosen.
2. Using that value, the server-side resource collects and returns the list of values that are to become the options of dropdown #2. I usually employ JSON for this purpose as it's easy to digest on the client.
3. The client handler for the request converts the JSON to a Javascript construct and uses the information to add the options to dropdown #2.
A cursory look at your code reveals that you are doing all the Ajax stuff directly. That's madness in my opinion. Better to get acquainted with one of the toolkits that vastly simplifies the process by masking browser dependencies and that handle the ready-change protocol. I'm partial to Prototype and jQuery myself.