Short answer: No, you won’t have entity in the database.
Longer answer: hibernate is smart enough not to send insert/updates to the DB until it knows if the transaction is going to be committed or rolled back (although this behavior can be changed by setting a different FlushMode), in your case by calling flush you are forcing the SQL to be sent to the DB but you still have the DB transaction to protect you, when you call rollback the DB transaction will be rolled back removing the changes performed inside itself and hence nothing will be actually saved. Note that depending on your configured transaction isolation level perhaps other transactions will be able to see in some way the EntityA you saved for the short while between the save and the rollback.
Also note that flush is called automatically when you try to read from DB, in 99% of the cases calling it explicitly is not necessary. One exception that comes to mind is when unit testing with auto rolling back tests.