In he famous book "Java persistence with Hibernate" we can read the following:
"A persistence context is a cache of persistent entity instances.... Automatic dirty checking is one of the benefits of this caching. Another benefit is repeatable read for entities and the performance advantage of a unit of work-scoped cache... You don’t have to do anything special to enable the persistence context cache. It’s always on and, for the reasons shown, can’t be turned off.
Does this mean that one can never achieve a transaction isolation level "Read uncommitted" with Hibernate?
Does this mean that one can never achieve a transaction isolation level "Read uncommitted" with Hibernate?
No, it doesn't. Hibernate offers application-level repeatable reads for entities. That's different than DB-level repeatable reads which applies to any query.
So, if you want a custom isolation level, like REPEATABLE_READ for all the queries executed by a given transaction, not just for fetching entities, then you can set it like this:
@Transactional(isolation = Isolation.REPEATABLE_READ)
public void orderProduct(Long productId) {
...
}
Now, your question title says:
(How) can we achieve isolation level Read Uncommitted with Hibernate / JPA?
If you are using Oracle and PostgreSQL, you cannot do that since Read Uncommitted is not supported, and you'll get READ_COMMITTED instead.
For SQL Server and MySQL, set it like this:
@Transactional(isolation = Isolation.READ_UNCOMMITTED)