Model modelData = repositoryService.getModel(modelId);
BpmnJsonConverter jsonConverter = new BpmnJsonConverter();
JsonNode editorNode = new ObjectMapper().readTree(repositoryService.getModelEditorSource(modelData.getId()));
BpmnModel bpmnModel = jsonConverter.convertToBpmnModel(editorNode);
BpmnXMLConverter xmlConverter = new BpmnXMLConverter();
byte[] bpmnBytes = xmlConverter.convertToXML(bpmnModel);
ByteArrayInputStream in = new ByteArrayInputStream(bpmnBytes);
IOUtils.copy(in, response.getOutputStream());
String filename = bpmnModel.getMainProcess().getId() + ".bpmn20.xml";
response.setHeader("Content-Disposition", "attachment; filename=" + filename);