This is the spec:
3.4 Optimistic Locking and Concurrency
This specification assumes the use of "optimistic locking". It assumes that the databases to which persistence units are mapped will be accessed by implementations using read-committed isolation (or a vendor equivalent in which long-term read locks are not held), and that writes to the database typically occur only when the flush method has been invoked
[...]
Applications that prefer the use of pessimistic locking may require that database isolation levels higher than read-committed be in effect. The configuration of the setting of such database isolation levels, however, is outside the scope of this specification.
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