Butterfly Web UI is the latest addition to the Butterfly Component suite - an ultra lightweight collection of components for Java application development.
Butterfly Web UI is a very small, yet very flexible component based web framework. It is designed to use Butterfly DI Container for its configuration, meaning you get all the power of Butterfly DI Container, right in your web applications.
Butterfly Web UI is designed with both support for normal HTML web apps, as well as AJAX and RIA web apps in mind. It is very easy to integrate a Flex front end with a Butterfly Web UI "backend", serving XML instead of HTML to the Flex front end.
Version 1.2.0 of Butterfly Web UI adds a "maintenance mode" servlet filter. This filter makes it possible to switch your web application in and out of "maintenance mode". In maintenance mode, all requests are intercepted and forwarded to the same page. That way you can show your users a "This web app is down for scheduled maintenance" message, while you backup databases, files etc. in peace.
You can find Butterfly Web UI here:
By the way, how to take down a Java web application for maintenance without going offline, is also explained in this article: