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.
Your subject line pretty much says the answer. The container checks the timestamp of the jsp page and that of the generated servlet. If the timestamp of jsp page is more recent than the generated servlet, it will recompile the jsp page.