@Configuration
    class PetServiceConfiguration {
      @Bean(BeanIdentifiers.PET_DAO_V1)
      PetDao petDao() {
        return new PetDaoV1();
      }

      @Bean(BeanIdentifiers.PET_DAO_V2)
      PetDao petDao() {
        return new PetDaoV2();
      }

      @Bean
      PetService petService(@Qualifier(BeanIdentifiers.PET_DAO_V1) PetDao petDaoV1,
                            @Qualifier(BeanIdentifiers.PET_DAO_V1) PetDao petDaoV2) {
        if("true".equals(System.getProperty("useV1PetDao"))) {
            return new PetService(petDaoV1);
        } else {
            return new PetService(petDaoV2);
        }
      }
    }

    static class BeanIdentifiers {
        static final String PET_DAO_V1 = "petDaoV1";
        static final String PET_DAO_V1 = "petDaoV2";
    }