Struts is an open-source project--have you considered just reading the code?
If you were going to implement that kind of functionality, how do you think you might go about doing it? For instance, when I think of a simple example, letting processing a form with a "firstName" element, I'd start by thinking about what's in the request parameters: a parameter named "firstName". If I wanted to initialize an instance property, my first assumption would be that I'd look for a setter named "setFirstName()" that takes a
String.
There are more details than that, but you get the idea. Plus you have access to the source code--use it!