Q)  Difference between session.update() and session.lock() in Hibernate ?

A) Both of these methods and saveOrUpdate() method are intended for reattaching a detached object.
The session.lock() method simply reattaches the object to the session without checking or updating the database on the assumption that the database in sync with the detached object.
It is the best practice to use either session.update(..) or session.saveOrUpdate().
Use session.lock() only if you are absolutely sure that the
detached object is in sync with your detached object or if it does not matter because
you will be overwriting all the columns that would have changed later on within the same transaction.

Each interaction with the persistent store occurs in a new Session. However, the same persistent instances are reused for each interaction with the database. The application manipulates the state of detached instances originally loaded in another Session and then “reassociates” them using Session.update() or Session.saveOrUpdate().

// foo is an instance loaded by a previous Session
foo.setProperty(”bar”);
session = factory.openSession();
session.saveOrUpdate(foo);
session.flush();
session.connection().commit();
session.close();
You may also call lock() instead of update() and use LockMode.READ (performing a version check, bypassing all caches) if you are sure that the object has not been modified.