Enumerates the kinds of optimistic or pessimistic lock which may be obtained on an entity instance.

A specific lock mode may be requested by passing an explicit LockModeType as an argument to:

• one of the methods of EntityManager which obtains locks (lock(), find(), or refresh()), or
• to Query#setLockMode(LockModeType) or TypedQuery#setLockMode(LockModeType).

Optimistic locks are specified using LockModeType.OPTIMISTIC and LockModeType#OPTIMISTIC_FORCE_INCREMENT. The lock mode types LockModeType#READ and LockModeType#WRITE are synonyms for OPTIMISTIC and OPTIMISTIC_FORCE_INCREMENT respectively. The latter are preferred for new applications.

The semantics of requesting locks of type LockModeType.OPTIMISTIC and LockModeType.OPTIMISTIC_FORCE_INCREMENT are the following.

If transaction T1 calls for a lock of type LockModeType.OPTIMISTIC 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, obtaining a lock of type LockModeType.OPTIMISTIC_FORCE_INCREMENT on a versioned object, will also force an update (increment) to the entity's version column.

The persistence implementation is not required to support the use of optimistic lock modes on non-versioned objects. When it cannot support such a lock request, it must throw the PersistenceException.

The lock modes LockModeType#PESSIMISTIC_READ, LockModeType#PESSIMISTIC_WRITE, and LockModeType#PESSIMISTIC_FORCE_INCREMENT are used to immediately obtain long-term database locks.

The semantics of requesting locks of type LockModeType.PESSIMISTIC_READ, LockModeType.PESSIMISTIC_WRITE, and LockModeType.PESSIMISTIC_FORCE_INCREMENT are the following.

If transaction T1 calls for a lock of type LockModeType.PESSIMISTIC_READ or LockModeType.PESSIMISTIC_WRITE on an 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.
• P2 (Non-repeatable read): Transaction T1 reads a row. Another transaction T2 then modifies or deletes that row, before T1 has committed or rolled back.

A lock with LockModeType.PESSIMISTIC_WRITE can be obtained on an entity instance to force serialization among transactions attempting to update the entity data. A lock with LockModeType.PESSIMISTIC_READ can be used to query data using repeatable-read semantics without the need to reread the data at the end of the transaction to obtain a lock, and without blocking other transactions reading the data. A lock with LockModeType.PESSIMISTIC_WRITE can be used when querying data and there is a high likelihood of deadlock or update failure among concurrent updating transactions.

The persistence implementation must support the use of locks of type LockModeType.PESSIMISTIC_READ and LockModeType.PESSIMISTIC_WRITE with non-versioned entities as well as with versioned entities.

When the lock cannot be obtained, and the database locking failure results in transaction-level rollback, the provider must throw the PessimisticLockException and ensure that the JTA transaction or EntityTransaction has been marked for rollback.

When the lock cannot be obtained, and the database locking failure results in only statement-level rollback, the provider must throw the LockTimeoutException (and must not mark the transaction for rollback).