Does Hibernate use connection pool by default? If so what is the default value for *connection.pool_size*? Doesn't it conflicts with *hibernate.connection.release_mode*? Isn't the all idea of connection pooling is not closing connections?
The default hibernate connection pool (which shouldn't be used in production) has a default limit of 1, since it is meant to just be used for simple testing. However this is configurable through the hibernate.properties file, so it's worth checking to see if it's defined there in your project.
The property in question is:
hibernate.connection.pool_size
Information on this is largely contained in this link:
http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/session-configuration.html
While this doesn't directly specify the default connection pool size, it does have most of the information you could want on the subject of connection pooling in hibernate.