A lot changed in this space in the last decade. I recommend reading something that teaches it from scratch. A few parts might sound familiar, but that is ok.
The Java EE tutorial is good if you want something online. I think you should use something book length whether it is the free tutorial or a real book like Core Servlets. There is too much information to learn it through scattered articles.
Jeremy McNally wrote:That one was perfect and is still up to date. For the most part.
"For the most part". The book covers Servlets 2.5 and JSP 2.1; in other words, JEE 5. Current version of JEE is 7 with Servlets 3.1 and JSP 2.3.
That said, I think the book is certainly up-to-date enough to learn what any novice/intermediate needs to know at this point. JSP hasn't changed very much, and Servlets 3 has yet to enjoy deep penetration.
[Disclaimer: I was a technical reviewer for the book.]