It is not intended that users spend much time worrying about locking strategies. It is usually enough to specify an isolation level for the JDBC connections and then simply let the database do all the work. However, advanced users may wish to obtain exclusive pessimistic locks or re-obtain locks at the start of a new transaction.
Hibernate will always use the locking mechanism of the database; it never lock objects in memory.
LockMode class defines the different lock levels that can be acquired by Hibernate. A lock is obtained by the following mechanisms:
LockMode.WRITE is acquired automatically when Hibernate updates or inserts a row.
LockMode.UPGRADE can be acquired upon explicit user request using
SELECT ... FOR UPDATE on databases which support that syntax.
LockMode.UPGRADE_NOWAIT can be acquired upon explicit user request using a
SELECT ... FOR UPDATE NOWAIT under Oracle.
LockMode.READ is acquired automatically when Hibernate reads data under Repeatable Read or Serializable isolation level. It can be re-acquired by explicit user request.
LockMode.NONE represents the absence of a lock. All objects switch to this lock mode at the end of a
Transaction. Objects associated with the session via a call to
saveOrUpdate() also start out in this lock mode.
The "explicit user request" is expressed in one of the following ways:
A call to
Session.load(), specifying a
A call to
A call to
Session.load() is called with
UPGRADE_NOWAIT, and the requested object was not yet loaded by the session, the object is loaded using
SELECT ... FOR UPDATE. If
load() is called for an object that is already loaded with a less restrictive lock than the one requested, Hibernate calls
lock() for that object.
Session.lock() performs a version number check if the specified lock mode is
UPGRADE_NOWAIT. In the case of
SELECT ... FOR UPDATE is used.
If the requested lock mode is not supported by the database, Hibernate uses an appropriate alternate mode instead of throwing an exception. This ensures that applications are portable.