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

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

Posted by: admin November 30, 2017 Leave a comment

Questions:

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

Answers:

Flushing the session forces Hibernate to synchronize the in-memory state of the Session with the database (i.e. to write changes to the database). 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,…).

Questions:
Answers:

As rightly said in above answers, by calling flush() we force hibernate to execute the SQL commands on Database. But do understand that changes are not “committed” yet.
So after doing flush and before doing commit, if you access DB directly (say from SQL prompt)and check the modified rows, you will NOT see the changes.

This is same as opening 2 SQL command sessions. And changes done in 1 session are not visible to others until committed.

Questions:
Answers:

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

Suppose we dont 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

Questions:
Answers:

Flushing the Session gets the data that is currently in the session synchronized with what is in the database.

More on the Hibernate website:

flush() is useful, because there are absolutely no guarantees about when the Session executes the JDBC calls, only the order in which they are executed – except you use flush().

Questions:
Answers:

You might use flush to force validation constraints to be realised and detected in a known place rather than when the transaction is committed. It may be that commit gets called implicitly by some framework logic, through declarative logic, the container, or by a template. In this case, any exception thrown may be difficult to catch and handle (it could be too high in the code).

For example, if you save() a new EmailAddress object, which has a unique constraint on the address, you won’t get an error until you commit.

Calling flush() forces the row to be inserted, throwing an Exception if there is a duplicate.

However, you will have to roll back the session after the exception.

Questions:
Answers:

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)

Questions:
Answers:

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.