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.
My colleague has an ASPX page that contains server controls (the equivalent of custom tags in a JSP page). In this page, he has a scriptlet (in C#) that looks up one of the server controls on the page by ID, and then calls a method on it. It looks like this:
So, converted to Java / JSP terminology: he has a scriptlet that looks up a custom tag on the page according to its Id attribute, and then calls a method on the tag handler object for that custom tag.
But as far as I know, this isn't possible: the structure of ASP and JSP pages is different; ASP pages are more component-oriented. It'd probably be possible if you were using a component-based page/tag rendering system, but if it's action-based, probably not. The JSP is rendered into a servlet, which is just making function calls to the tag--you'd have to demangle the tag names/instances, and even then, I don't know if it'd work.