SSH框架学习OA项目，类注入错误

error message：Unable to instantiate Action, departmentAction, defined for 'department_list' in namespace '/'Error creating bean with name 'departmentAction': Injection of resource fields failed; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type [cn.itcast.oa.service.PrivilegeService] is defined: Unsatisfied dependency of type [interface cn.itcast.oa.service.PrivilegeService]: expected at least 1 matching bean

问题原因是因为我的PrivilegeServiceImpl没implements PrivilegeService接口，是我粗心忘记写了。

但是不明白为什么PrivilegeServiceImpl没implements PrivilegeService接口，求大牛详细说明下？谢谢