I have found the answer from somewhere in internet. Post it here to share with all of us.
1.Why the default auto commit mode is false?
When we are in session bean, the transaction is handled by Application Server. Therefore, all connection created is with manual commit mode.Anyway, you can change is by modifying ejb-jar.xml (<transaction-type>Container</transaction-type>).
2.When i try to commit the connection, i hit this error << java.sql.SQLException: You cannot commit during a managed transaction!>> why this happen?
It's clear that once the connection is handled by Application Server, you are not allowed to commit the connection.Let's application handle this.