Using LockModeType.READ/WRITE

Question

What circumstances make it useful to lock individual entity instances using LockModeType.READ/WRITE? I figured that setting isolation levels would have been enough, but I guess there's a need for further locking?
When you access an entity, isn't it locked automatically unless you're using optimistic locking? I'm pretty lost on this issue...
Could somebody please describe when would you lock individual entities?
Thanks.

Answer 1

Hi this is my understanding although I am not an expert.
When you have READ_COMMITTED mode it means T1 can read a value T2 can modify the same value and then commit. If T1 reads it again it will get a new value or if it commits it will overwrite the previous changes. (Non-repeatable read). You wouldn't get a dirty read although you would get non-repeatable read.
If transaction T1 calls lock(entity, LockModeType.READ) on a versioned object, the entity manager must ensure that neither of the following phenomena can occur:
With LockMode you get extra features.
# 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.