Hibernate: Do I ever need to flush()?

Question

I have a bidirectional one-to-many relationship. I'm trying to persist it like in this doc:

http://docs.jboss.org/hibernate/core/3.3/reference/en/html/example-parentchild.html#example-parentchild-bidir

Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
c.setParent(p);
p.getChildren().add(c);
session.save(c);
session.flush();

Is the flush() required there? What exactly does it do? I know what it does, but I discovered it costs me 100 ms and I would really like to avoid it if possible.

When session.load() or even session.refresh() is called and I hadn't flushed, will it include the new Child in the collection?

Answer 1

The call to flush will synchronise the session state with the database. You do not need to explicitly flush at that point. Hibernate is clever enough to know when it needs to update the database with the session e.g. before executing a query.

That 100ms cost will have to be incurred at some point since it's an inevitable consequence of using a database in your application. Hibernate is able to delay this operation and batch up database changes to minimise round trips to the database.

Hibernate will flush the session when the transaction ends (by default) and so if you call load in the same session you will get the same object back from the session. If you call load in a new session Hibernate will have flushed the changes and your child will be in the collection.

If you were to clear the session or evict your object from the session before a flush was performed you would find that your child is not present.

This chapter from the docs should help.