ProcessInstanceQuery piq = ProcessInstanceQuery.findForProcess(processId);
piq.where(ROOT_PROCESS_INSTANCE_OID.isEqual(rootPoid));
piq.setPolicy(HistoricalEventPolicy.ALL_EVENTS);
ProcessInstance pi;

try {
     pi = getQueryService(false).findFirstProcessInstance(piq);
} catch (IllegalOperationException e) {
     LOG.error("Unable to execute Process Instance Query for RootProcess ID: " + rootPoid, e);                 
   return null;
}
   List<HistoricalEvent>  dataList = pi.getHistoricalEvents();
Here dataList is always null

ActivityInstanceQuery aiQuery = ActivityInstanceQuery.findAll();
ProcessInstanceFilter processFilter = new ProcessInstanceFilter(rootProcessOid, true);
aiQuery.where(processFilter);
aiQuery.orderBy(ActivityInstanceQuery.START_TIME).and(ActivityInstanceQuery.OID);
aiQuery.setPolicy(HistoricalEventPolicy.ALL_EVENTS);

 
List<HistoricalEvent> getHistoricalEvents()
          Gets a list of requested additional data like notes, delegations, state changes and exceptions.
List<HistoricalState> getHistoricalStates()
          Gets a list of historical states for the activity instance.