实际上，我通过逐步执行init代码找到了解决方案：

@Configuration
public class Config {

    @Bean
    JavaTimeModule javatimeModule(){
        return new JavaTimeModule();
    }

    @Bean
    Jackson2ObjectMapperBuilderCustomizer jackson2ObjectMapperBuilderCustomizer(){
    return jacksonObjectMapperBuilder -> jacksonObjectMapperBuilder.featuresToEnable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
            .mixIn(MyClass.class, MyClassMixin.class);
    }


    @Bean
    Jackson2JsonEncoder jackson2JsonEncoder(ObjectMapper mapper){
       return new Jackson2JsonEncoder(mapper);
    }

    @Bean
    Jackson2JsonDecoder jackson2JsonDecoder(ObjectMapper mapper){
        return new Jackson2JsonDecoder(mapper);
    }

    @Bean
    WebFluxConfigurer webFluxConfigurer(Jackson2JsonEncoder encoder, Jackson2JsonDecoder decoder){
        return new WebFluxConfigurer() {
            @Override
            public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
                configurer.defaultCodecs().jackson2Encoder(encoder);
                configurer.defaultCodecs().jackson2Decoder(decoder);
            }
        };

    }
}