Defining custom EL functions

I've been working on porting my sandbox webapp to Spring WebMVC, and in the process I've been learning more about JSP as well. This work led me to ask a question at StackOverflow, a favorite site of mine for a while for finding solutions to problems, though I've only recently begun participating.
In the question I asked about the best way to sort a single List in different ways in JSP, and I didn't get any answers that I really liked, so I kept looking and found an answer that I was able to use to suit my needs. You can see my solution at StackOverflow, but I'll reproduce it here as well.