I am reading James Elloitt's Hibernate: A developer's notebook.
I see exampe where a nullable persistent field is mapped to
int (No Integer) field. How does this field hold a null vaue in Database?

I don't know, shouldn't it be explained in the book?

The sample chapter O'Reilly has online is horrible and full of errors. I can not recommend this book if the other content has the same quality. I'm biased, of course, but I still can see if Hibernate is used correctly or not.