I would like to know what you used to do the annotations processing and what classes can I find them in the source code?
Axon Framework does its own annotation processing for the message handlers, actually.
So, we do not use a library. It is the library.
There’s a utility class we frequently use to make everything work called AnnotationUtils.
You can find it here, by the way.
If you would clone the main Framework repository, open AnnotationUtils, and find all usages, you should be able to uncover how Axon Framework does all the annotation processing for you. 
By the way, it’s not that I can’t give you a more extensive answer, @zeynab. There is just so much to it, that I recommend you do your own discover first.
If you have follow-up questions on more specific parts of the framework’s annotation processing, be sure to reach out again.