I have to explain about my experiences about this matter:
1- I do agree that the problem  may be because of number Threads.in the previous mail I forgot to mention that the number of idle threads should be checked.
2- I can prove that the Session Factory of Hibernate is so weak under high load.the mechanism of pooling of session can not perform well when the requests get high and should be used some extra jar files such as C3P0.
3- It is very simple to distinguish the difference of JDK and JRockIt performance.In equal condition ,just change JVM of Weblogic and check the Figure of memory usage in connection tab.