hi,I am now confused about the relationship between session and sessionfactory,also the relationship between session and jdbc connection.

what does "session.close()" do ? does it close a jdbc connection?
what does "sessionfactory.openSession()" do?

when to close a hibernate session?

any reply would be much appreciated.thx in advance.

>what does "session.close()" do ? does it close a jdbc connection?

No, it does not close jdbc connection, it returns connection to the pool.
Session close() cleans H session cache.

>what does "sessionfactory.openSession()" do?

Creates Hibernate session which is a temporary cache for relevant persistent objects and the session works as operation buffer.

>when to close a hibernate session?
There are multiple strategies. My favorite is session-per-business-transaction
http://www.hibernate.org/42.html
http://www.hibernate.org/43.html

thank you for your reply.these words help me a lot.
according to http://www.hibernate.org/42.html
"
Transactions
Transactions also group data access operations, in fact, every SQL statement, be it queries or DML, has to execute inside a database transaction.
"
but I don't know why queries should be involved in a trasaction?
it has nothing to do with the status of the db.

Well, lets suppose we started to query a table and another process tries to change data in the same table.
Should we see changed data? What if the update transaction is unsuccessful?
To answer these and other questions in a predictable fashion we need transactions with well defined isolation levels: SERIALIZABLE, REPEATABLE_READ, etc.
http://www.postgresql.org/docs/7.4/inte ... n-iso.html

I am using Mysql and proxool,and another problem emerges:
Mysql automatically closes an idle connection every certain minuntes.

consider this situation,
when hibernate closes a session,the db-connection is returned to
proxool pool,then after a certain time Mysql automatically closes
this db-connection.but the proxool seems not aware of this,it return
a closed connection to hibernate the next time I call "sessionFactory.openSession();" then "No operations allowed after connection closed" exceptions occur.

how to handle this?

You may try autoreconnect connection parameter for MySQL JDBC
jdbc:mysql://zzzz:3306/foo?autoReconnect=true

or specify proxool.house-keeping-test-sql parameter for proxool

http://proxool.sourceforge.net/configure.html