Vaadin (Finnish for doe, a female reindeer) is a Java framework for building web applications. Unlike many other frameworks out there, in Vaadin you develop your front-end from the server side out, or more specifically, you use a server-side Java API to build the front-end layout (using Swing-like layout managers) and behaviour of your application. The demo video on the Vaadin website will make this more clear:

Introduction to Vaadin Development

In my opinion it is an approach that will really appeal to core Java engineers, as they only have to learn the Vaadin Java API and leave the front-end mark-up and JavaScript widgets for Vaadin to generate. For a great example of a back-end Java engineer getting excited about Vaadin, have a read of this blog entry by Christopher Schanck:

For the Love of Vaadin: RIAs Done Right

You can learn more about the framework from the on-line book hosted at the Vaadin website:

Book of Vaadin

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Updated 2022 : note that the above post was originally published in 2011, but is left here for archival purposes. I have fixed some broken links above.