locking model in ODMG

[Jair da Silva Ferreira J�nior]

Hi,
    I am using ojb1.0_rc4, ODMG api with OJB queries and mysql4.
    My questions are: 
        1) What happens when two different ODMG transactions try to write lock the 
same object at the same time? A LockNotGrantedException is thrown in one of the 
transactions or one transaction waits for the other one to finish or release the lock?
        2) What is the difference between Transaction.WRITE and Transaction.UPDATE 
lock modes?

Thanks,
Jair Jr