SpringBoot Webflux cannot return application/xml

Question

In my reactive REST API I'm trying to return an XML response. However, I always get a JSON, namely 406 NOT_ACCEPTABLE. Any idea why?

@RestController
@RequestMapping(path = "/xml", produces = APPLICATION_XML_VALUE)
public class RestApi {
    @GetMapping(path = "/get")
    public Publisher<ResponseEntity> get() {
        return Mono.just(ResponseEntity.ok().contentType(APPLICATION_XML).body(new Datta("test")));
    }

    @PostMapping(path = "/post", consumes = APPLICATION_XML_VALUE)
    public Publisher<ResponseEntity<Datta>> post(@RequestBody Datta datus) {
        datus.setTitle(datus.getTitle() + "!");
        return Mono.just(ResponseEntity.ok().contentType(APPLICATION_XML).body(datus));
    }
}

java.lang.AssertionError: Expected :application/xml Actual :application/json;charset=UTF-8

plugins {
    id 'org.springframework.boot' version '2.1.3.RELEASE'
    id "io.spring.dependency-management" version "1.0.7.RELEASE"
}
dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-webflux'
    implementation "com.fasterxml.jackson.dataformat:jackson-dataformat-xml:2.9.8"
    testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

These are the links to my REST controller and a unit test. Thanks!

Answer 1

Apparently, jackson-dataformat-xml does not yet support XML Marshalling in WebFlux. As for now I see two possibilities:

1. Either add org.springframework.boot:spring-boot-starter-web on the classpath (there should be both starter-web and starter-webflux). However, this will only work with Servlet 3.1 runtimes (e.g. Tomcat).
2. Or if you want a fully fledged reactive web server (e.g. Netty) use xml marshalling from JAXB (Jaxb2XmlEncoder and Jaxb2XmlDecoder):

build.gradle:

sourceCompatibility = '11'

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-webflux'
    testImplementation 'org.springframework.boot:spring-boot-starter-test'

    // Java 11 removed these Java EE modules
    implementation "javax.xml.bind:jaxb-api:2.3.1"
    implementation "com.sun.xml.bind:jaxb-core:2.3.0.1"
    implementation "com.sun.xml.bind:jaxb-impl:2.3.2"

    compileOnly "org.projectlombok:lombok"
    annotationProcessor "org.projectlombok:lombok"
}

POJO:

@Data
@AllArgsConstructor
@NoArgsConstructor
@XmlRootElement
public class Datta {
    private String title;
}

Mind the 3 javax.xml.bind dependencies (you don't need these for Java 8) and the @XmlRootElement annotation. This solution works right away, however if you want further customization, implement your own WebFluxConfigurer:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
    @Override
    public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
        configurer.registerDefaults(false);
        configurer.customCodecs().decoder(new Jaxb2XmlDecoder());   // <- here
        configurer.customCodecs().encoder(new Jaxb2XmlEncoder());   // <- here

    }
}

Here is the link to the source code.