SpringMvc小项目代码报错？

Servlet.service() for servlet [springMVC] in context with path [/Test_Shop_CaoZiXin] threw exception [Handler processing failed; nested exception is java.lang.Error: Unresolved compilation problem:
The return type is incompatible with ClientService.login(String, String)
] with root cause
java.lang.Error: Unresolved compilation problem:
The return type is incompatible with ClientService.login(String, String)

这是什么原因造成的呢 导包也没问题 昨天项目还能启动今天就报了这个错 求大佬

https://www.cnblogs.com/wylblogs/p/spring.html