Thanks Tihomir, I hava solved my problem. I think I donn't introduce the detail of my problem. Sorry. My original goal is to get the TaskSummary's name, but I don't know where to set it. Finally, I find it can be set in the *.bpmn file: The TaskName of "User Task" component. If the TaskName had been set, it can be get from the TaskSummary.
Thank you all the same.