Nested Transaction not supported in Hibernate

Hi,
I am using hibernate(version-4) for the first time.
i have used
HibernateUtil.beginTransaction();
//some code;
HibernateUtil.commitTransaction();
HibernateUtil.closeSession();
in my action classes.

But, I am getting the following exception repeatedly, due which i have to restart my server.
On restart, the code works properly but the exception occurs again.

I am using jboss5.0.1 as server.

Server log is as follows

ERROR [[action]] Servlet.service() for servlet action threw exception
org.hibernate.TransactionException: nested transactions not supported
at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.begin(AbstractTransactionImpl.java:152)
at org.hibernate.internal.SessionImpl.beginTransaction(SessionImpl.java:1396)
at sun.reflect.GeneratedMethodAccessor336.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.hibernate.context.internal.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:352)
at $Proxy280.beginTransaction(Unknown Source)
at com.aviva.dao.util.HibernateUtil.beginTransaction(HibernateUtil.java:39)
at com.aviva.struts.action.TestAction.execute(TestPageAction.java:73)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:235)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:190)
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:92)
at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.process(SecurityContextEstablishmentValve.java:126)
at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:70)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:330)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:829)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:598)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:662)
10:35:54,936 INFO [STDOUT] org.hibernate.TransactionException: nested transactions not supported
10:35:54,936 INFO [STDOUT] at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.begin(AbstractTransactionImpl.java:152)
10:35:54,936 INFO [STDOUT] at org.hibernate.internal.SessionImpl.beginTransaction(SessionImpl.java:1396)
10:35:54,936 INFO [STDOUT] at sun.reflect.GeneratedMethodAccessor336.invoke(Unknown Source)
10:35:54,936 INFO [STDOUT] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
10:35:54,936 INFO [STDOUT] at java.lang.reflect.Method.invoke(Method.java:597)
10:35:54,936 INFO [STDOUT] at org.hibernate.context.internal.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:352)
10:35:54,936 INFO [STDOUT] at $Proxy280.beginTransaction(Unknown Source)
10:35:54,936 INFO [STDOUT] at com.aviva.dao.util.HibernateUtil.beginTransaction(HibernateUtil.java:39)
10:35:54,936 INFO [STDOUT] at com.aviva.struts.action.TestAction.execute(TestPageAction.java:73)
10:35:54,936 INFO [STDOUT] at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
10:35:54,936 INFO [STDOUT] at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
10:35:54,936 INFO [STDOUT] at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
10:35:54,936 INFO [STDOUT] at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
10:35:54,936 INFO [STDOUT] at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
10:35:54,936 INFO [STDOUT] at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
10:35:54,936 INFO [STDOUT] at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:235)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
10:35:54,936 INFO [STDOUT] at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:190)
10:35:54,936 INFO [STDOUT] at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:92)
10:35:54,936 INFO [STDOUT] at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.process(SecurityContextEstablishmentValve.java:126)
10:35:54,936 INFO [STDOUT] at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:70)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
10:35:54,936 INFO [STDOUT] at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
10:35:54,936 INFO [STDOUT] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:330)
10:35:54,936 INFO [STDOUT] at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:829)
10:35:54,936 INFO [STDOUT] at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:598)
10:35:54,936 INFO [STDOUT] at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
10:35:54,936 INFO [STDOUT] at java.lang.Thread.run(Thread.java:662)