Why commit and rollback methods are given in transaction class and not in session class itselt?

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Java

Hibernate

Hello friends,
I just got a question about commit() and rollback() methods used by Transaction class. But in Session class there are save() and update() methods. Why commit() and rollback() method not declared with save() and update() method in Session class and declared in Transaction class.

we write:
Session ses = factory.openSession();
Transaction tx = session.beginTransaction();
then:
try{
. . .
session.save();
tx.commit();
} catch(Exception e){
. . .
tx.rollack();
}

Thanks.

What I have tried:

I tried to find answer using API docs and also tried to googled it.