you can use javascript if you don't mind creating the arrays of javascript objects in the page. the last array can be rather large if you have lots of data (you can find out how to do this by looking for some samples online).
you can also store DropDownItem[] in 1 array for each dropdown, each next DropDownItem[] would be like a data table containing a key to the previous one. store each DropDownItem[] in a session attribute (or you can put all 5 or 6 arrays of objects in 1 wrapper object). your struts Action class then do look ups (possibly using a helper method) against these arrays and return to the request ONLY collections of relevant dropdowns for use with your <html
ptions> tags.
of course, you'll have to have some javascript that does a submit each time the user select a dropdown.
instead of using the action, you can also use JSTL to loop through each dropdown collection in your sessionScope, based on some parameter passing, (say /select.do?d1=1&d2=3&d3=&d4=&d5 that is constructed by javascript on each select) and display (using <c:if> or <logic:match>
only the items for a particular selection. you'll need to add additional check to make sure the correct item is highlighted. complicated? yes, but doable. obviously, you'll need to go back to the server each time, but since the dropdowns are in session, it's not too bad...unless you have 1000's of user at any given time.
