An high abstraction level Model free API

[Stefano]

I'm searching an open/free set of API or a Framework also Free for
persit java object in DBMS or file system.
I found many project for Object/Relational Mapping like Hibernate.

I like to find a framework where, at run time, I can add/remove a
field to an object using only a sub set of java type like String, Date
and Int.
And naturally maintaining the persistance of the values.

Do you know a set oh API or a free project in java that offer this
dynamic behaviour/service?

Thanks in advance.
Bye