RepositoryService repositoryService = ProcessEngines.getDefaultProcessEngine().getRepositoryService();

//获取BpmnModel对象
BpmnModel bpmnModel = repositoryService.getBpmnModel(processInstanceId);
//创建转换对象

BpmnXMLConverter converter = new BpmnXMLConverter();
//把bpmnModel对象转换成字符
byte[] bytes = converter.convertToXML(bpmnModel);
String xmlContenxt = new String(bytes);

xmlContenxt即为流程的xml
07-30 07:34