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Friday, January 13, 2012

JPA: Locking mechanisms in JPA 2

JPA 2 supports both optimistic locking and pessimistic locking. Locking is essential to avoid update collisions resulting from simultaneous updates to the same data by two concurrent users.
Optimistic locking is applied on transaction commit. Any database object that has to be updated or deleted is checked. An exception is thrown if it is found out that an update is being performed on an old version of a database object, for which another update has already been committed by another transaction.
Optimistic locking is enabled by default and fully automatic. Optimistic locking should be the first choice for most applications, since compared to pessimistic locking it is easier to use and more efficient.
In the rare cases in which update collision must be revealed earlier (before transaction commit) pessimistic locking can be used. When using pessimistic locking, database objects are locked during the transaction and lock conflicts, if they happen, are detected earlier.
This post covers the following topics:
  1. Optimistic Locking
  2. Pessimistic Locking
  3. Other Explicit Lock Modes
  4.  Locking during Retrieval
1- Optimistic Locking
JPA maintains a version number for every entity object. The initial version of a new entity object (when it is stored in the database for the first time) is 1. In every transaction in which an entity object is modified its version number is automatically increased by one. Version numbers are managed internally but can be exposed by defining a version field.
During commit (and flush), JPA checks every database object that has to be updated or deleted, and compares the version number of that object in the database to the version number of the in-memory object being updated. The transaction fails and an OptimisticLockException is thrown if the version numbers do not match, indicating that the object has been modified by another user (using another EntityManager) since it was retrieved by the current updater.
Optimistic locking is completely automatic and enabled by default in JPA, regardless if a version field (which is required by some ORM JPA providers) is defined in the entity class or not.
2- Pessimistic Locking
The main supported pessimistic lock modes are:

  • PESSIMISTIC_READ - This represents a shared lock.
  •  PESSIMISTIC_WRITE - This represents an exclusive lock.
Setting a Pessimistic Lock
An entity object can be locked explicitly by the lock method:
  em.lock(employee, LockModeType.PESSIMISTIC_WRITE);
The first argument is an entity object. The second argument is the requested lock mode.
TransactionRequiredException is thrown if there is no active transaction when lock is called because explicit locking requires an active transaction.
LockTimeoutException is thrown if the requested pessimistic lock cannot be granted:

  •  PESSIMISTIC_READ lock request fails if another user (which is represented by another EntityManager instance) currently holds a PESSIMISTIC_WRITE lock on that database object. 
  • PESSIMISTIC_WRITE lock request fails if another user currently holds either a PESSIMISTIC_WRITE lock or a PESSIMISTIC_READ lock on that database object.
- For example, consider the following code fragment:
  em1.lock(e1, lockMode1);
  em2.lock(e2, lockMode2);
em1 and em2 are two EntityManager instances that manage the same Employee database object, which is referenced as e1 by em1 and as e2 by em2 (notice that e1 and e2 are two in-memory entity objects that represent one database object).
If both lockMode1 and lockMode2 are PESSIMISTIC_READ - these lock requests should succeed. Any other combination of pessimistic lock modes, which also includes PESSIMISTIC_WRITE, will cause a LockTimeoutException (on the second lock request).
Pessimistic Lock Timeout
By default, when a pessimistic lock conflict occurs a LockTimeoutException is thrown immediately. The "javax.persistence.lock.timeout" hint can be set to allow waiting for a pessimistic lock for a specified number of milliseconds. The hint can be set in several scopes:
For the entire persistence unit - using a persistence.xml property:
    <properties>
       <property name="javax.persistence.lock.timeout" value="1000"/>
    </properties>
For an EntityManagerFactory - using the createEntityManagerFacotory method:
  Map<String,Object> properties = new HashMap();
  properties.put("javax.persistence.lock.timeout", 2000);
  EntityManagerFactory emf = Persistence.createEntityManagerFactory("pu", properties);
For an EntityManager - using the createEntityManager method:
  Map<String,Object> properties = new HashMap();
  properties.put("javax.persistence.lock.timeout", 3000);
  EntityManager em = emf.createEntityManager(properties);
or using the setProperty method:
  em.setProperty("javax.persistence.lock.timeout", 4000);
In addition, the hint can be set for a specific retrieval operation or query.
Releasing a Pessimistic Lock
Pessimistic locks are automatically released at transaction end (using either commit or rollback).
JPA supports also releasing a lock explicitly while the transaction is active, as so:
  em.lock(employee, LockModeType.NONE);
3- Other Explicit Lock Modes
In addition to the two main pessimistic modes (PESSIMISTIC_WRITE and PESSIMISTIC_READ, which are discussed above), JPA defines additional lock modes that can also be specified as arguments for the lock method to obtain special effects:

  • OPTIMISTIC (formerly READ)
  •  OPTIMISTIC_FORCE_INCREMENT (formerly WRITE)
  • PESSIMISTIC_FORCE_INCREMENT
Since optimistic locking is applied automatically by JPA to every entity object, the OPTIMISTIC lock mode has no effect and, if specified, is silently ignored by JPA.
The OPTIMISTIC_FORCE_INCREMENT mode affects only clean (non dirty) entity objects. Explicit lock at that mode marks the clean entity object as modified (dirty) and increases its version number by 1.
The PESSIMISTIC_FORCE_INCREMENT mode is equivalent to the PESSIMISTIC_WRITE mode with the addition that it marks a clean entity object as dirty and increases its version number by one (i.e. it combines PESSIMISTIC_WRITE with OPTIMISTIC_FORCE_INCREMENT).
4- Locking during Retrieval
JPA 2 provides various methods for locking entity objects when they are retrieved from the database. In addition to improving efficiency (relative to a retrieval followed by a separate lock), these methods perform retrieval and locking as one atomic operation.
For example, the find method has a form that accepts a lock mode:
  Employee employee = em.find(Employee.class, 1, LockModeType.PESSIMISTIC_WRITE);
Similarly, the refresh method can also receive a lock mode:
  em.refresh(employee, LockModeType.PESSIMISTIC_WRITE);
A lock mode can also be set for a query in order to lock all the query result objects.
When a retrieval operation includes pessimistic locking, timeout can be specified as a property. For example:
  Map<String,Object> properties = new HashMap();
  properties.put("javax.persistence.lock.timeout", 2000);

  Employee employee = em.find(Employee.class, 1, LockModeType.PESSIMISTIC_WRITE, properties);
  ...
  em.refresh(employee, LockModeType.PESSIMISTIC_WRITE, properties);

Setting timeout at the operation level overrides setting in higher scopes.

Source: http://www.objectdb.com/java/jpa/persistence/lock