配置的fallback class也必须在FeignClient Configuration中实例化，否则会报java.lang.IllegalStateException: No fallback instance of type class异常。

@FeignClient(name = "hello", fallback = HystrixClientFallback.class)
    public interface HystrixClient {
        @RequestMapping(method = RequestMethod.GET, value = "/hello")
        Hello iFailSometimes();
    }
    
    public class HystrixClientFallback implements HystrixClient {
        @Override
        public Hello iFailSometimes() {
            return new Hello("fallback");
        }
    }
    
    @Configuration
    public class FooConfiguration {
        @Bean
        @Scope("prototype")
        public Feign.Builder feignBuilder() {
            return Feign.builder();
        }
        
        @Bean
        public HystrixClientFallback fb(){
            return new HystrixClientFallback();
        }
……
    }

feign:
  hystrix:
    enabled: true
# 说明：请务必注意，从Spring Cloud Dalston开始，Feign默认是不开启Hystrix的。否则断路器不会生效。
# 而，Spring Cloud Angel/Brixton/Camden中，Feign默认都是开启Hystrix的。无需设置该属性。