Then, in your JSP, you can
test for the inclusion of a scoped variable that contains the data (a list or map, most likely) for the second dropdown. If it's there you render the dropdown. Otherwise not.
During the first run of the JSP, the variable will not be present and the select can be left empty and disabled. After a value is chose from the first dropdown, the relevant data is obtained and added to the request as the scoped variable and the page is re-rendered. This time, because the data is present, the dropdown will be rendered.