| Lock modes that can be specified by means of the
EntityManager.lock EntityManager.lock() method.
The semantics of requesting locks of type
LockModeType.READ LockModeType.READ and
LockModeType.WRITE LockModeType.WRITE are the following.
If transaction T1 calls lock(entity,
LockModeType.READ LockModeType.READ ) on a versioned object,
the entity manager must ensure that neither of the following
phenomena can occur:
- P1 (Dirty read): Transaction T1 modifies a row.
Another transaction T2 then reads that row and obtains
the modified value, before T1 has committed or rolled back.
Transaction T2 eventually commits successfully; it does not
matter whether T1 commits or rolls back and whether it does
so before or after T2 commits.
-
P2 (Non-repeatable read): Transaction T1 reads a row.
Another transaction T2 then modifies or deletes that row,
before T1 has committed. Both transactions eventually commit
successfully.
Lock modes must always prevent the phenomena P1 and P2.
In addition, calling lock(entity, LockModeType.WRITE) on
a versioned object, will also force an update (increment) to
the entity's version column.
The persistence implementation is not required to support
calling
EntityManager.lock EntityManager.lock() on a
non-versioned object. When it cannot support a such lock call,
it must throw the
PersistenceException .
since:
Java Persistence 1.0 |