The "Break it Down" Blog has a lengthy post on Which is the Hottest Java Web Framework? Or Maybe Not Java? Comparing Java Web Frameworks is hard because so many people are passionate about the framework they know best. Add a couple more like Flex and Ruby on Rails and its downright difficult. Nevertheless, this post is good in that it contains a lot of pretty trend graphs and it looks like the author has done some good research. It's likely the folks that will scream foul are the ones that did poor in the comparison (Tapestry and Stripes, I'm talking about you).
Surprising among the top Java Web Frameworks is the rise of Struts 2:
Which is much more interesting I think is how Wicket adoption has stayed almost flat while Struts 2 adoption has spiked. Spring MVC/WebFlow seems to be going no where fast and racing JBoss Seam there.
The popularity of Struts 2 really caught me off guard with it being quite a bit different from Struts 1, I figured it got thrown into the "just another web framework" category, but I guess there is something in a name and it's doing quite well.
Regardless of what you think of the post and trends, you have to appreciate the amount of time the author put into it.