This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
It's not clear what you are asking, as there is no JSTL in your example at all. Did you mean the EL? If so, and you are using a modern JSP container, the EL expression will be evaluated prior to setting it as the value of the custom tag's attribute.