IWorkbenchSiteProgressService siteService = (IWorkbenchSiteProgressService)getSite().getAdapter(IWorkbenchSiteProgressService.class); 
			siteService.schedule( new Job()
			{
protected IStatus run(IProgressMonitor monitor) 
		{
			//Get data
                       dataList = GetFileData();
getDisplay().syncExec( new Runnable()
						{
							public void run()
							{
								//Create Nattable here with the data
							}
						} ); 
			monitor.done();
			return Status.OK_STATUS;
		}
}
)

List<DataRow> rows = new ArrayList<>();
Nattable nattable = CreateNattable(rows);
InitializeRows(rows); //just adds to the list.
nattable.refresh();