Spring Boot单元测试报UnsatisfiedDependencyException

用Spring Boot做单元测试的时候，报错：
org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name ‘com.example.maildemo.test.MailTest’: Unsatisfied dependency expressed through field ‘mailService’; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type ‘com.example.maildemo.service.MailService’ available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

一种原因是单元测试的注解未写完整，在测试类上加上@SpringBootTest即可
加注解前：

加注解后：