(I use Proxool 0.9.1.)
Since Hibernate 4.x, ProxoolConnectionProvider had been moved to org.hibernate.service.jdbc.connections.internal packet (in jar: ${hibernate-release-4.x}/lib/optional/proxool/hibernate-proxool-4.x.jar).
When I upgrade to 4.x from 3.x (all configuration did not changed), It always tell me :'Exception: the url can not be null'. so I trace the Hibernate source code, and found that method 'public void configure(Properties props)' in ProxoolConnectionProvider never been called. So, all attributes in ProxoolConnectionProvider is null.

anybody can resove this problem, thx !!