From time to time the Session will execute the SQL statements
needed to synchronize the JDBC connection's state with the state of objects held in
memory. This process, flush, occurs by default at the following
points
before some query executions
from org.hibernate.Transaction.commit()
from Session.flush()
The SQL statements are issued in the following order
all entity insertions, in the same order the corresponding objects
were saved using Session.save()
all entity updates
all collection deletions
all collection element deletions, updates and insertions
all collection insertions
all entity deletions, in the same order the corresponding objects
were deleted using Session.delete()
(An exception is that objects using native ID generation are
inserted when they are saved.)
Except when you explicity flush(), there are absolutely no
guarantees about when the Session executes
the JDBC calls, only the order in which they are executed.
However, Hibernate does guarantee that the Query.list(..)
will never return stale data; nor will they return the wrong data.
It is possible to change the default behavior so that flush occurs less frequently.
The FlushMode class defines three different modes: only flush
at commit time (and only when the Hibernate Transaction API
is used), flush automatically using the explained routine, or never flush unless
flush() is called explicitly. The last mode is useful for long running
units of work, where a Session is kept open and disconnected for
a long time (see Section 11.3.2, “Extended session and automatic versioning”).
sess = sf.openSession();
Transaction tx = sess.beginTransaction();
sess.setFlushMode(FlushMode.COMMIT); // allow queries to return stale state
Cat izi = (Cat) sess.load(Cat.class, id);
izi.setName(iznizi);
// might return stale data
sess.find("from Cat as cat left outer join cat.kittens kitten");
// change to izi is not flushed!
...
tx.commit(); // flush occurs
sess.close();During flush, an exception might occur (e.g. if a DML operation violates a constraint). Since handling exceptions involves some understanding of Hibernate's transactional behavior, we discuss it in Chapter 11, Transactions And Concurrency.