Spring Cloud封裝Feign，直接使用Spring MVC注解以及HttpMessageConverters來序列化。
Spring Boot有autoconfigure機制，當spring boot中引入了spring-web jar包的時候，HttpMessageConvertersAutoConfiguration便會起作用，另外當引入了ObjectMapper類時候，MappingJackson2HttpMessageConverterConfiguration便會起作用，從而自動加入了HttpMessageConverters中，這樣feign就會直接來序列化json了。
在Declarative REST Client: Feign中有這么一段話
FooConfiguration does not need to be annotated with @Configuration. However, if it is, then take care to exclude it from any @ComponentScan that would otherwise include this configuration as it will become the default source for feign.Decoder, feign.Encoder, feign.Contract, etc., when specified. This can be avoided by putting it in a separate, non-overlapping package from any @ComponentScan or @SpringBootApplication, or it can be explicitly excluded in @ComponentScan.
大致意思是，FeignClient注解上可以指定configuration屬性，但是對於指定的configuration屬性的類不需要使用@Configuration注解，不然這個里面設置的屬性將會覆蓋默認的配置屬性，轉而使用該configuration配置的。如果不使用@Configuration注解，那么只有該FeignClient會使用該configuration。