How does an entity get an ID before a transaction is committed in JPA/Play?

Question

See this question.

It turns out that even without committing the transaction manually, before the TX is committed, the person has an ID after calling the save() meth

Answer 1

Yes, the JPA is allowed to communicate with the DB before transaction commit. It can occur i.e. when you explicitly invoke EntityManager#flush().

Moreover, the JPA provider is allowed to do the flush operation whenever it feels it's necessary. However, by the convenience, JPA providers delays DB operations to the time the transaction will be committed.

Some automatic ID generator strategies must hit the database to get the PK value (as far as I remember the IDENTITY strategy works that way).
As a contrary, the TABLE or SEQUENCE generators don't necessary need to hit the DB to get the ID value. They use the allocationSize parameter to ask the DB TABLE or SEQUENCE for a batch of IDs that will be given to new entities without further communication with the database.