Well guys, I don't know if It's too late, but I wasted 1 day searching the solutions to this problem, about JasperReport after building an Jar executable. To get your Reports working after building a jar, you just need to write the following lines
String reportUrl = "/reports/billCopyReport.jasper"; //path of your report source.
InputStream reportFile = null;
reportFile = getClass().getResourceAsStream(reportUrl);
Map data = new HashMap(); //In case your report need predefined parameters you'll need to fill this Map
JasperPrint print = JasperFillManager.fillReport(reportFile, data, conection);
JasperViewer Jviewer = new JasperViewer(print, false);
Jviewer.setVisible(true);
/* var conection is a Connection type to let JasperReport connecto to Database, in case you won't use DataBase as DataSource, you should create a EmptyDataSource var*/
