This covers how locking works with the JDBC Engine, and how you can programmatically control the type of locking you want in various situations.

1. Locking Basics
2. Choosing the Lock

The persistence engine supports the following locking strategies for rows retrieved from the database.

• Optimistic
  
  This is the default locking strategy. The record in the database will be locked, only when it is updated/deleted by the persistence engine. This can happen anytime between a uow.add(aDomainObject) [or uow.update(aDomainObject) or uow.delete(aDomainObject)] and a uow.commit()
  
  
• Cautious
  
  The record in the database will be locked, whenever a field is updated on the domain object. Invocation of the updateXxx method on a domain object, which is retrieved from the database and which is not locked as yet, will lock the corresponding row in the database.
  
  
• Paranoid
  
  The record in the database will be locked, at the instant it is retrieved.
  
  
• ReadOnly
  
  The record in the database will never be modified. Invocation of the updateXxx method on the domain object, will throw ReadOnlyObjectException
  
  

The locking strategy is specified in the Criteria object used for querying the database.

UOW uow = null;
try {
    uow = new UOW();
    // retrieve the domain object(s), providing the appropriate locking strategy
    Criteria c = new Criteria();
    ...
    c.setLocking(Criteria.LOCKING_PARANOID);
    Collection results = uow.query( c );
    for (Iterator itr = results.iterator(); itr.hasNext(); ) {
        ...
        ...
    }
    ...
    ...
} catch (UOWException e) {
    // handle the exception
} finally {
    if (uow != null)
        uow.rollback();
}