"postgresql-9.0-801.jdbc4.jar" always cause "org.postgresql.util.PSQLException: Cannot commit when autoCommit is enabled" Exception

Hello list,

. Postgresql8.3
. mybatis-3.0.5-SNAPSHOT.jar
. mybatis-spring-1.0.1-SNAPSHOT.jar
. spring3.0.5

. postgresql-9.0-801.jdbc4.jar

SqlSession sql_session = sqlSessionFactory.openSession(false);
....
sql_session.commit();

Always got:
===================
### Error committing transaction. Cause:
org.postgresql.util.PSQLException: Cannot commit when autoCommit is enabled.

While for "8.4-702 JDBC 4", the same codes, no error at all.

Is this a bug for "postgresql-9.0-801.jdbc4.jar"?

Thanks a lot!
Emi