JaxbConverterFactory doesn't recognize @XmlJavaTypeAdapter

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The way JAXB deals with classes that doesn't have a no-arg constructor or classes with final fields is by using an XmlAdapter, and to configure an XmlAdapter for some class, we need to annotate this class with XmlJavaTypeAdapter.

The problem happens if the root is an immutable class, in this case, the root java class will be annotated with XmlJavaTypeAdapter, which the JaxbConverterFactory doesn't recognize (it only checks isAnnotationPresent for XmlRootElement), and will return a null value, so retrofit will assume that the request/response cannot be converted using JAXB.

It would be an easy change to also check for XmlJavaTypeAdapter annotation.

Complete tutorial: http://blog.bdoughan.com/2010/12/jaxb-and-immutable-objects.html