Add EnableFeignClients to the startup class to enable the feign function. And the annotation imports FeignClientsRegistrar
FeignClientsRegistrar implements three interfaces: ImportBeanDefinitionRegistrar, ResourceLoaderAware, and EnvironmentAware, which are used to obtain environment and resource import bean definitions
The most important thing is that the ImportBeanDefinitionRegistrar interface has two methods for registering bean definitions
FeignClientsRegistrar The core method of this class is registerBeanDefinitions
registerDefaultConfiguration This method contains two methods,
