I have read the article Locking and Concurrency in Java Persistence 2.0, and run the sample application. But i still cant realize the difference between PESSIMISTIC_READ and PESSIMISTIC_WRITE. I tried to modify the code, and where the code using PESSIMISTIC_READ and PESSIMISTIC_WRITE will have the same result that the sql will invoked with "for update".
The difference lies in locking mechanism.
PESSIMISTIC_READ
lock means that dirty reads and non-repeatable reads are impossible when you have such a lock. If data should be changed it's required to obtain PESSIMISTIC_WRITE
lock
PESSIMISTIC_WRITE
lock guarantees that besides dirty and non-repeatable reads are impossible you can update data without obtaining additional locks(and possible deadlocks
while waiting for exclusive lock).
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║ LockModeType ║ PESSIMISTIC_READ ║ PESSIMISTIC_WRITE ║
╠══════════════════════╬══════════════════════════╬══════════════════════════╣
║ type ║ SHARED LOCK ║ EXCLUSIVE LOCK ║
╠══════════════════════╬══════════════════════════╬══════════════════════════╣
║ isReadOnly without ║ ║ ║
║ additional locks ║ YES ║ NO ║
╠══════════════════════╬══════════════════════════╬══════════════════════════╣
║ dirty reads ║ NO ║ NO ║
╠══════════════════════╬══════════════════════════╬══════════════════════════╣
║ non-repeatable reads ║ NO ║ NO ║
╠══════════════════════╬══════════════════════════╬══════════════════════════╣
║ how to update data ║ obtain PESSIMISTIC_WRITE ║ ALLOWED ║
╠══════════════════════╬══════════════════════════╬══════════════════════════╣
║ ║ no one holds ║ no one holds ║
║ how to obtain lock ║ PESSIMISTIC_WRITE ║ PESSIMISTIC_READ or ║
║ ║ ║ PESSIMISTIC_WRITE ║
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║ ║ ║ when there is a high ║
║ ║ you want to ensure no ║ likelihood of deadlock or║
║ when to use ║ dirty or non-repeatable ║ update failure among ║
║ ║ reads are possible ║ concurrent updating ║
║ ║ ║ transactions ║
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