See here: http://hibernate.org/6.html#A6
Run build.xml with Ant under JDK 1.2 or later (JKD 1.4 required"
Should be JDK 1.4 required
However actually "or later" is an error http://opensource.atlassian.com/projects/hibernate/browse/HHH-2446 as JDK 6+ cannot be used to build.
"You need to use JDK 5.0 or later" is said for Hibernate Annotations, but I cannot evaluate the statement by itself nor would it likely be very fun to have H3 compiled into JDK5 classes and H3A in JDK6...
It should be corrected to prevent lots of duplicate JIRA bugs saying "I followed the instructions but H3 doesn't compile!"