Java developers

I've seen some posts regarding what servlets should and should not do. I'm starting to design a web app using servlets that prompts for data, stores that data in a table, and retrieves the data. Simple. I can right a bunch of servlets to do all this, but I want to do this the right way.
Servlets, databases, and web apps in general are all fairly new to me, so whats the right way to design servlets? What functionality should they handle and should they pass off to other classes?

I have a ton of other specific questions, mainly regarding the GUI, but we'll start here first.

FYI, I'm using OC4J and JDeveloper and can only use servetls, no JSF (no I have no other options).

Thanks

I am not an expert in the use of the Reflection API, but I have a gut feeling that something like this might be possible, though I haven't been able to figure out a simple way of doing it.


Is it possible to discover, at runtime, all the concrete classes that implement a given interface?


I have an interface, lets call it Estimator, that I want other people in the company to implement. These other people know a lot more about estimating than I do, and they all have their own code that estimates the given values. What I want is for them to implement an estimator, that the GUI can use. So, at runtime, I want to show people a list of all the known estimators, along with their results.


I know I could do this by keeping a list somewhere of the known implementors, but that is cludgy. I would rather than every implementation in the classpath just show up "magically" in the list.