mainreport = JasperCompileManager.compileReport(//path of mainreport.jrxml);
subreport = JasperCompileManager.compileReport(//path of subreport.jrxml);
Map<String, Object> params = new HashMap<String, Object>();
params.put("SUB_REPORT", subreport);
params.put("DATA_SOURCE", empService.getEmpProject());

jprint1 = JasperFillManager.fillReport(mainreport,params,new JRBeanCollectionDataSource(empService.getEmpBean()));
JasperExportManager.exportReportToPdfFile(jprint1, "C://Test.pdf");