What's the difference between PESSIMISTIC_READ and PESSIMISTIC_WRITE in JPA?

Question

I have read the article Locking and Concurrency in Java Persistence 2.0, and run the sample application. But I still can't 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.

Answer 1

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).

╔══════════════════════╦══════════════════════════╦══════════════════════════╗ ║     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      ║ ╠══════════════════════╬══════════════════════════╬══════════════════════════╣ ║                      ║                          ║   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       ║ ╚══════════════════════╩══════════════════════════╩══════════════════════════╝ 

Resources:

JPA 2.1