SpringBoot使用@Autowired注入报错 Could not autowire

现象：

@Autowired
private UserService userService ;

SpringBoot 启动报错：

***************************
APPLICATION FAILED TO START
***************************

Description:

Field userService in com.guodx.controller.UserController required a bean of type 'com.guodx.service.UserService' that could not be found.

Action:

Consider defining a bean of type 'com.guodx.service.UserService' in your configuration.

Process finished with exit code 0

可能出现原因：

1. Service实现类 没有添加 @Service 注解
   在 UserServiceImpl类 添加 @Service注解，问题解决 ~
2. Dao实现类 没有添加 @Mapper 注解