hibernate - Getting javax.resource.ResourceException when execting JPA query -

getting javax.resource.resourceexception: ij000460: error checking transaction,when below code exceuted

query query = entitymanager.createquery("select lower(name) attribute"); list temp= query.getresultlist(); 

exception

17:15:27,838 warn  [org.hibernate.engine.jdbc.spi.sqlexceptionhelper] (http-localhost-127.0.0.1-8080-2) sql error: 0, sqlstate: null 17:15:27,841 error [org.hibernate.engine.jdbc.spi.sqlexceptionhelper]  (http-localhost-127.0.0.1-8080-2) javax.resource.resourceexception: ij000460:  error checking transaction 17:15:27,845 error [com.dao.impl.attributedaoimpl]  (http-localhost-127.0.0.1-8080-2) org.hibernate.exception.genericjdbcexception:  not open connection 

this exception not frequent. not able correct hint.

thanks in advance