FemtoWeb
An extremely minimalistic non-intrusive action based Java web framework. It has been written for projects which can't or don't want to use a full-blown MVC web framework.

Reasons for FemtoWeb might include:

Don't use this framework if you are not limited by those constraints and/or want to develop a large application. It's just more fun that way!

You can find the source code and contribute to the source repository on GitHub.

Scope

Important: Read this section before even considering using FemtoWeb in your project!

The scope of this framework is very narrow. It doesn't care about things that are not directly related to HTTP requests/responses (e.g. persistence, databases, form validation etc.). It just provides a very basic MVC-like abstraction whose sole purpose is to separate the logic (actions, sometimes called controllers) from the presentation (in most cases JSP). You could say that this framework implements the most necessary components of the "VC" portion (view and controller) of MVC, but doesn't impose any more constraints on the developer.

The programmer isn't constrained by any more than the absolutely necessary components.

Documentation