Whats the difference between flush and commit in Hibernate?

Question

Whats the difference between flush and commit in Hibernate?

Answer 1

The process of synchronizing the JDBC connection's state with the state of objects held in memory is called flush.

This occurs at the following points depending on the FlushMode set:

• before some query executions when FlushMode.AUTO (This is the default).
• from org.hibernate.Transaction.commit() when FlushMode.COMMIT
• from Session.flush()

The key difference is that when FlushMode is set to COMMIT, commit() flushes the session and also ends the unit of work and you cannot rollback the transaction where as flush() does just a normal sync of the session.

FlushMode

More info

Answer 2

From Hibernate docs:

Flushing is the process of synchronizing the underlying persistent store with persistable state held in memory.

Well, the above statement is confusing for some folks who may be getting next question (e.g. then what's the difference between flush and commit) in mind after reading the above statement.

Flush is like actually executing the statement but not committing it. For example: you open any SQL execution tool (like Oracle SQL Developer or some other), open a session and fire update statement. Open new SQL session and you won't see updates until or unless you do a commit in the first session. So the query got executed in particular Oracle session but not committed.

When you call Hibernate's save() or update(), it does not mean the underlying query gets executed at the same time. It generally executes when commit is done either explicitly or at the end of transaction. But there are scenarios (like to get an ID assigned to a transient object, to control the size of Hibernate session like in batch updates otherwise you can get out of memory exception) where you would like to execute the query, but not committing it. Flush helps here.