As I undertand, I can maintain relationship between mapped javabeans
either by specifying the relationship the mapping XML,
or I can simply add my own code to the mapped javabeans to maintain relationships.

Which one is easier and mostly used in praactice?

I believe that if you have the link in the relationship in your mapping XML, then you have to have the necessary structure in your objects.
e.g. a Bag relies on a collection of some kind (generally a List), wherehas, if you defined no link in the parent XML mapping, you could simply manually assign the Parent's PK as a FK within each of your child objects (also in the mapping as a normal element) as opposed to having to create a one-to-many link in the child.

I think Hibernate's true ORM power comes from getting those Bag's and one-to-many and many-to-many working properly. The ability to simply say session.save(xyz) and almost nothing more, is really nice. :)

Rather than having to do a whole loop and manually assign FK's, etc.

-G