我解决的办法是把.m2仓库所有文件删除，重新maven project就可以了。
但是在做这一步之前，报错如下：
①org.springframework.beans.factory.UnsatisfiedDependencyException
②ContainerBase.addChild: start:
org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/test]]
③Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML. Cause: org.apache.ibatis.builder.BuilderException: Error resolving class. Cause: org.apache.ibatis.type.TypeException:
④org.apache.catalina.LifecycleException
⑤Error creating bean with name 'userService': Unsatisfied dependency expressed through field 'userMapper'
⑥java.lang.IllegalStateException: Failed to load ApplicationContext
⑦Caused by: java.lang.ClassNotFoundException: Cannot find class: com.github.pagehelper.PageInterceptor
大概报了这七个错误，特别恶心。原因可能是我导入其他项目时，maven project污染了.m2仓库中的文件。