How do I define a custom HttpMessageConverter for the reactive Spring WebClient (Spring-WebFlux)

Question

For the Spring org.springframework.web.client.RestTemplate, it was relatively easy to define an own HttpMessageConverter:

/**
 * Set the message body converters to use.
 * <p>These converters are used to convert from and to HTTP requests and responses.
 */
public void setMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
    validateConverters(messageConverters);
    // Take getMessageConverters() List as-is when passed in here
    if (this.messageConverters != messageConverters) {
        this.messageConverters.clear();
        this.messageConverters.addAll(messageConverters);
    }
}

When converting my client to an reactive WebClient, I did not find a suitable way to define my own message converter as before with the RestTemplate.

Background: Our spring boot project is based on Scala and we use our own converter (based on com.fasterxml.jackson.module.scala.JacksonModule) to process Scala Case classes.

Answer 1

You can register custom codecs(Encoder, Decoder, HttpMessageReader, HttpMessageWriter) via WebClient.builder() for your WebClient in the reactive world.

  WebClient client = WebClient.builder()
                //see: https://github.com/jetty-project/jetty-reactive-httpclient
                //.clientConnector(new JettyClientHttpConnector())
                .clientConnector(new ReactorClientHttpConnector())
                .codecs(
                        clientCodecConfigurer ->{
                           // .defaultCodecs() set defaultCodecs for you
                           // clientCodecConfigurer.defaultCodecs();

                           //  You can customize an encoder based on the defualt config.
                           //  clientCodecConfigurer.defaultCodecs().jackson2Encoder(...)

                           // Or 
                           // use customCodecs to register Codecs from scratch.
                            clientCodecConfigurer.customCodecs().register(new Jackson2JsonDecoder());
                            clientCodecConfigurer.customCodecs().register(new Jackson2JsonEncoder());
                        }

                )
                .baseUrl("http://localhost:8080")
                .build();

Answer 2

If you're using Spring Boot with WebFlux, you can add a CodecCustomizer bean, which you can use to register your own custom codecs:

@Bean
CodecCustomizer myCustomCodecCustomizer(...) {
    return configurer -> configurer.customCodecs().register(...);
}

By the way, when using Spring Boot with WebMVC, you can just add your HttpMessageConverter implementation as a bean and it will get picked up.