Lock Modes

The application may increase the level of locking for an entity by specifying the use of lock modes. Lock modes may be specified to increase the level of optimistic locking or to request the use of pessimistic locks.

The use of optimistic lock modes causes the persistence provider to check the version attributes for entities that were read (but not modified) during a transaction as well as for entities that were updated.

The use of pessimistic lock modes specifies that the persistence provider is to immediately acquire long-term read or write locks for the database data corresponding to entity state.

You can set the lock mode for an entity operation by specifying one of the lock modes defined in the javax.persistence.LockModeType enumerated type, listed in Table 45-1.

Table 45-1 Lock Modes for Concurrent Entity Access

Lock Mode

Description

OPTIMISTIC

Obtain an optimistic read lock for all entities with version attributes.

OPTIMISTIC_FORCE_INCREMENT

Obtain an optimistic read lock for all entities with version attributes, and increment the version attribute value.

PESSIMISTIC_READ

Immediately obtain a long-term read lock on the data to prevent the data from being modified or deleted. Other transactions may read the data while the lock is maintained, but may not modify or delete the data.

The persistence provider is permitted to obtain a database write lock when a read lock was requested, but not vice versa.

PESSIMISTIC_WRITE

Immediately obtain a long-term write lock on the data to prevent the data from being read, modified, or deleted.

PESSIMISTIC_FORCE_INCREMENT

Immediately obtain a long-term lock on the data to prevent the data from being modified or deleted, and increment the version attribute of versioned entities.

READ

A synonym for OPTIMISTIC. Use of LockModeType.OPTIMISTIC is to be preferred for new applications.

WRITE

A synonym for OPTIMISTIC_FORCE_INCREMENT. Use of LockModeType.OPTIMISTIC_FORCE_INCREMENT is to be preferred for new applications.

NONE

No additional locking will occur on the data in the database.

Setting the Lock Mode

To specify the lock mode, use one of the following techniques:

Call the EntityManager.lock method, passing in one of the lock modes:

EntityManager em = ...;
Person person = ...;
em.lock(person, LockModeType.OPTIMISTIC);

Call one of the EntityManager.find methods that take the lock mode as a parameter:

EntityManager em = ...;
String personPK = ...;
Person person = em.find(Person.class, personPK,
    LockModeType.PESSIMISTIC_WRITE);

Call one of the EntityManager.refresh methods that take the lock mode as a parameter:

EntityManager em = ...;
String personPK = ...;
Person person = em.find(Person.class, personPK);
...
em.refresh(person, LockModeType.OPTIMISTIC_FORCE_INCREMENT);

Call the Query.setLockMode or TypedQuery.setLockMode method, passing the lock mode as the parameter:
```
Query q = em.createQuery(...);
q.setLockMode(LockModeType.PESSIMISTIC_FORCE_INCREMENT);
```

Add a lockMode element to the @NamedQuery annotation:

@NamedQuery(name="lockPersonQuery",
  query="SELECT p FROM Person p WHERE p.name LIKE :name",
  lockMode=PESSIMISTIC_READ)

Using Pessimistic Locking

Versioned entities, as well as entities that do not have version attributes, can be locked pessimistically.

To lock entities pessimistically, set the lock mode to PESSIMISTIC_READ, PESSIMISTIC_WRITE, or PESSIMISTIC_FORCE_INCREMENT.

If a pessimistic lock cannot be obtained on the database rows, and the failure to lock the data results in a transaction rollback, a PessimisticLockException is thrown. If a pessimistic lock cannot be obtained, but the locking failure doesn’t result in a transaction rollback, a LockTimeoutException is thrown.

Pessimistically locking a versioned entity with PESSIMISTIC_FORCE_INCREMENT results in the version attribute being incremented even if the entity data is unmodified. When pessimistically locking a versioned entity, the persistence provider will perform the version checks that occur during optimistic locking, and if the version check fails, an OptimisticLockException will be thrown. An attempt to lock a non-versioned entity with PESSIMISTIC_FORCE_INCREMENT is not portable and may result in a PersistenceException if the persistence provider does not support optimistic locks for non-versioned entities. Locking a versioned entity with PESSIMISTIC_WRITE results in the version attribute being incremented if the transaction was successfully committed.

Pessimistic Locking Timeouts

Use the javax.persistence.lock.timeout property to specify the length of time in milliseconds the persistence provider should wait to obtain a lock on the database tables. If the time it takes to obtain a lock exceeds the value of this property, a LockTimeoutException will be thrown, but the current transaction will not be marked for rollback. If you set this property to 0, the persistence provider should throw a LockTimeoutException if it cannot immediately obtain a lock.

Note:

Portable applications should not rely on the setting of javax.persistence.lock.timeout, because the locking strategy and underlying database may mean that the timeout value cannot be used. The value of javax.persistence.lock.timeout is a hint, not a contract.

This property may be set programmatically by passing it to the EntityManager methods that allow lock modes to be specified, the Query.setLockMode and TypedQuery.setLockMode methods, the @NamedQuery annotation, and the Persistence.createEntityManagerFactory method. It may also be set as a property in the persistence.xml deployment descriptor.

If javax.persistence.lock.timeout is set in multiple places, the value will be determined in the following order:

The argument to one of the EntityManager or Query methods
The setting in the @NamedQuery annotation
The argument to the Persistence.createEntityManagerFactory method
The value in the persistence.xml deployment descriptor