Spring Support JAXBElement in Jaxb2RootElementHttpMessageConverter

On the current Spring version, usage of Jaxb2RootElementHttpMessageConverter in RestClient messageConverters don't allow to serialize JAXBElement.

This PR add this feature.

Comment From: snicoll

@deblockt thanks for the PR but it looks a little strange to me. Shouldn't the conversion happen via the content-type rather than via supporting such a type? Where would you be using this exactly?

Comment From: deblockt

Hi @snicoll , This PR is the equivalent of this commit on WebClient https://github.com/mdeinum/spring-framework/commit/9de58d78da431a4a94d08cab964221b62a3c527b.

In my project, I try to serialize XML object using RestClient with this code.

restClient = RestClient.builder()
                .messageConverters((httpMessageConverters -> {
                    httpMessageConverters.add(new Jaxb2RootElementHttpMessageConverter());
                }))
                .build();
restClient
                    .post().uri("/")
                    .contentType(MediaType.APPLICATION_XML)
                    .body(new ObjectFactory().createPojo(pojo))
                    .retrieve()
                    .body(MyPojo.class);

Where new ObjectFactory().createPojo(pojo) return a JAXBElement<MyPojo>.

The ObjectFactory and MyPojo classes are generated using jaxb2-maven-plugin and this plugin don't annotate classes with @XmlRootElement so we should use the JAXBElement to generate a valid XML.

Comment From: snicoll

Thanks for the additional details.

The ObjectFactory and MyPojo classes are generated using jaxb2-maven-plugin and this plugin don't annotate classes with @XmlRootElement so we should use the JAXBElement to generate a valid XML.

Yeah I didn't mean that @XmlRootElement was mandatory but rather that the content type should drive the conversion, not the type to serialize but I can see how that's short sighted. And the fact we've done this for codecs means we should do it for MVC as well.

Comment From: snicoll

Thanks again @deblockt!