Cannot get all nodes

BpmnModelInstance bpmnModelInstance = repositoryService.getBpmnModelInstance(defId);
Process process = bpmnModelInstance.newInstance(Process.class);
Collection flowElements = process.getFlowElements();

But I flowElements.size () is 0,However, bpmnmodelinstance does exist。How can I get all nodes?
thank you。

请求支援感谢!!