@Configuration
    class PetServiceConfiguration {
      @Bean
      PetDao petDao() {
        return new PetDaoV1();
      }

      @Bean
      PetDao petDao() {
        return new PetDaoV2();
      }

      @Bean
      PetService petService(@PetDaoV1 PetDao petDaoV1, @PetDaoV2 PetDao petDaoV2) {
        if("true".equals(System.getProperty("useV1PetDao"))) {
            return new PetService(petDaoV1);
        } else {
            return new PetService(petDaoV2);
        }
      }
    }

    static class BeanIdentifiers {

        @Retention(RetentionPolicy.RUNTIME)
        @Qualifier
        public @interface PetDaoV1 {

        }

        @Retention(RetentionPolicy.RUNTIME)
        @Qualifier
        public @interface PetDaoV2 {

        }
    }