There are three types of classes:
A handler handles an incoming command an calls a services. The services does caching and calls other services or a DAO. Services and DAOs are singletons.
Is there a way to create a custom warning if a DAO is used in one of the handlers?
You could try playing with some custom checkstyle rules but... you should rather try to it properly ;)
Create 3 modules (projects in Eclipse). Let Handlers know only about the Services (add a proper dependency to Handlers project) and let Serices know about DAOs (add a DAO project to Services dependencies). This way you will never make a mistake :]
