Vaadin is a solution evolved from a decade of problem-solving approach, provided by a Finnish company named Vaadin, formerly IT Mill.
Therefore, having so many solutions available, one could question the use Vaadin instead of Flex or GWT? Let's first have a look at the state of the market for web application frameworks in Java, then detail what makes Vaadin so unique in this market.
Despite all the cons of the thin-client approach, an important share of applications developed today uses this paradigm, most of the time with a touch of Ajax augmentation.
Unfortunately, there is no clear leader for web applications. Some reasons include the following: