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问题描述：

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.demo.dao.UseDao] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)} at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:1301) at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1047) at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:942) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:533) ... 24 more

总是提示无法注入Service或者Dao中的Bean！

后来经研究发现，SpringBoot项目的Bean装配默认规则是根据Application类所在的包位置从上往下扫描！

“Application类”是指SpringBoot项目入口类。这个类的位置很关键：

如果Application类所在的包为：com.demo.module，则只会扫描com.demo.module包及其所有子包，如果service或dao所在包不在com.demo.module及其子包下，则不会被扫描！

所以需在application启动类中加上

@MapperScan("com.*.dao")//加上你项目的dao或service所在文件位置即可

@SpringBootApplication