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10.10. Flushing the Session

Sometimes 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, called 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:
  1. all entity insertions in the same order the corresponding objects were saved using Session.save()
  2. all entity updates
  3. all collection deletions
  4. all collection element deletions, updates and insertions
  5. all collection insertions
  6. 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 explicitly 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 or incorrect 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 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.createQuery("from Cat as cat left outer join cat.kittens kitten");

// change to izi is not flushed!
...
tx.commit(); // flush occurs
sess.close();
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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.
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