What's the use of session.flush() in Hibernate

Question

When we are updating a record, we can use session.flush() with Hibernate. What\'s the need for flush()?

Answer 1

I would just like to club all the answers given above and also relate Flush() method with Session.save() so as to give more importance

Hibernate save() can be used to save entity to database. We can invoke this method outside a transaction, that’s why I don’t like this method to save data. If we use this without transaction and we have cascading between entities, then only the primary entity gets saved unless we flush the session.

flush(): Forces the session to flush. It is used to synchronize session data with database.

When you call session.flush(), the statements are executed in database but it will not committed. If you don’t call session.flush() and if you call session.commit() , internally commit() method executes the statement and commits.

So commit()= flush+commit. So session.flush() just executes the statements in database (but not commits) and statements are NOT IN MEMORY anymore. It just forces the session to flush.

Few important points:

We should avoid save outside transaction boundary, otherwise mapped entities will not be saved causing data inconsistency. It’s very normal to forget flushing the session because it doesn’t throw any exception or warnings. By default, Hibernate will flush changes automatically for you: before some query executions when a transaction is committed Allowing to explicitly flush the Session gives finer control that may be required in some circumstances (to get an ID assigned, to control the size of the Session)

Answer 2

Calling EntityManager#flush does have side-effects. It is conveniently used for entity types with generated ID values (sequence values): such an ID is available only upon synchronization with underlying persistence layer. If this ID is required before the current transaction ends (for logging purposes for instance), flushing the session is required.

Answer 3

I only know that when we call session.flush() our statements are execute in database but not committed.

Suppose we don't call flush() method on session object and if we call commit method it will internally do the work of executing statements on the database and then committing.

commit=flush+commit (in case of functionality)

Thus, I conclude that when we call method flush() on Session object, then it doesn't get commit but hits the database and executes the query and gets rollback too.

In order to commit we use commit() on Transaction object.