We want to make sure that our JSP development efforts are thread-safe. Is it possible that two threads will access one instance of a JSP custom tag handler concurrently?
No it is not possible. There is a defined lifecycle of a custom tag, and during this lifecycle, that instance of the tag will only get called once. Hopefully in newer implementations of JSP engines, the actual tag object will not get reinstantiated each time (as this negatively affects performance), but at least during it's use, it will be thread safe.