Vaadin and Apache Click seem to be equally good, which one should I choose for developing my web application. Or rather, what are the Pros and Cons of each framework.
I'm a committer of Apache Click but hopefully you will find my opinion objective.
I don't know Vaadin at all, just had a cursory look at their website and examples and mission statement.
Apache Click and Vaadin are meant for different problem spaces. Apache Click is targeted at traditional enterprise web applications while Vaadin targets the Rich Internet Application (RIA) space.
Apache Click is a traditional share nothing, stateless framework. Unlike traditional action based web frameworks Click provides a slightly higher level of abstraction by using Pages and Components. At the of the day Click isn't a revolution. Its just one of many approaches of doing web application development. In my opinion its a very good approach, both from a developer and maintainer point of view. (In enterprise environments the developer and maintainer are often not the same individual, so I differentiate between the two roles).
However developers sometimes have requirements that cannot be satisfied by traditional web applications, which is where RIA comes in. They provide a richer user experience (think desktop) but this does not come for free. RIA comes at a cost in terms of complexity, productivity and time-to-market. It makes sense though, if the requirements are for a richer experience, you need to do more in order to deliver that experience.
My advice here is: think carefully about your requirements. Don't simply assume that RIA is better, there is a price to pay, so make sure you get return for your investment.
Lastly, if your requirement is for RIA, then you should compare Vaadin with Flex. If your requirement is for a more traditional web application then compare Click with Struts, Stripes, Wicket, Tapestry etc.
Kind regards
Bob