Unable to locate current JTA transaction?

[freesoul]

when call the portal mbean

UserModule mbean = (UserModule) MBeanProxyExt
 .create(UserModule.class, ObjectNameFactory
 .create("portal:service=Module,type=User"));
 mbean.getUserCount();

the application throws exception as follows,

20:36:21,875 ERROR [HibernateUserModuleImpl] Cannot count users
org.hibernate.HibernateException: Unable to locate current JTA transaction
 at org.hibernate.context.JTASessionContext.currentSession(JTASessionContext.java:61)
 at org.hibernate.impl.SessionFactoryImpl.getCurrentSession(SessionFactoryImpl.java:544)
 at org.jboss.portal.identity.db.HibernateUserModuleImpl.getCurrentSession(HibernateUserModuleImpl.java:298)
 at org.jboss.portal.identity.db.HibernateUserModuleImpl.getUserCount(HibernateUserModuleImpl.java:275)

I search some same problems on the forum but no good answers,
some one said we should put such config to jboss-porlet.xml

<transaction>
 <trans-attribute>Required</trans-attribute>
 </transaction>

but i didn't call it in a portlet so it didn't work.
Anyone can explain the reason of 'Unable to locate current JTA '??

thanks.

[freesoul]

My environment:
jboss portal 2.6.4 + JBOSS AS 4.2.1
MYSQL 5.0
Struts2 + spring + hibernate

when I request an ajax action ,the exception happens.