Nothing happens by magic. The JSTL tags will only know about what is explicitly created as scoped variable. I'd assume that's being handled by the page controller.
If I understand what you are asking: no, you cannot affect the functioning of the forEach tags via user actions that happen after the
JSP has been evaluated and the response sent to the browser.
Or are you assuming that the selections have been made in a previous request?
P.S. What's with the import declarations? Superfluous code is dangerous code.