GWT & EXT-GXT

To implement the web tier i will use the GWT library that allow us to crete rich internet application using javascript and XML, all starting from java code.
A good and well explained tutorial can be found at the home page of the Google Web Toolkit.

In this picture we can see how GWT works:

In few word the only thing that we need to implement are the two interfaces:

MyService: containing the methods that we want to implement, and
MyServiceAsync: containing the methods that can be called from the client-side

Obviously we only need to implement the class that implements the MyService interface since the asynchronous one, as we can see in the picture, is automatically generated by the library.
But here you can read more about it.

I will use also the gxt-ext library to implement the user interface part, since it allows to generate a lot of widgets. At this page there are a lot of examples that show the power of this library.