spring Jaxb2XmlDecoder fails to decode when using inheritance with #XmlSeeAlso (polymorphism) - spring

Question

While introducing WebClient into my current project I ran into the issue that the call
response.bodyToMono(RealEstate::class.java)
causes the error
org.springframework.core.codec.DecodingException: Could not unmarshal XML to class test.RealEstate; nested exception is javax.xml.bind.UnmarshalException
- with linked exception:
[com.sun.istack.SAXParseException2; lineNumber: 2; columnNumber: 1; Unable to create an instance of test.RealEstate]
at org.springframework.http.codec.xml.Jaxb2XmlDecoder.unmarshal(Jaxb2XmlDecoder.java:242)
Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below:
We have the following simplified class hierarchy:
#XmlAccessorType(XmlAccessType.FIELD)
#XmlType(name = "RealEstate", namespace = "http://anything.you.want", propOrder = {"address"})
#XmlSeeAlso({House.class, Apartment.class})
public abstract class RealEstate {
protected String address;
public String getAddress() { return address; }
public void setAddress(String address) { this.address = address; }
}
#XmlAccessorType(XmlAccessType.FIELD)
#XmlType(name = "Apartment", propOrder = {"livingArea"}, namespace = "http://anything.you.want")
#XmlRootElement(name = "apartment", namespace = "http://anything.you.want")
public class Apartment extends RealEstate {
private String livingArea;
public String getLivingArea() {
return livingArea;
}
public void setLivingArea(String livingArea) {
this.livingArea = livingArea;
}
}
#XmlAccessorType(XmlAccessType.FIELD)
#XmlType(name = "House", propOrder = {"plotArea"}, namespace = "http://anything.you.want")
#XmlRootElement(name = "house", namespace = "http://anything.you.want")
public class House extends RealEstate {
private String plotArea;
public String getPlotArea() {
return plotArea;
}
public void setPlotArea(String plotArea) {
this.plotArea = plotArea;
}
}
One possible response body to be decoded is
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<objects:house xmlns:objects="http://anything.you.want">
<address>Somewhere around the corner</address>
<plotArea>small</plotArea>
</objects:house>
First question would be if there is something wrong with how JAXB is used?
In case there is nothing really wrong with that I go on to my other question.
Looking into the Decoder there is the logic to call the Unmarshaller
Unmarshaller unmarshaller = initUnmarshaller(outputClass);
XMLEventReader eventReader = StaxUtils.createXMLEventReader(events);
if (outputClass.isAnnotationPresent(XmlRootElement.class)) {
return unmarshaller.unmarshal(eventReader);
}
else {
JAXBElement<?> jaxbElement = unmarshaller.unmarshal(eventReader, outputClass);
return jaxbElement.getValue();
}
Since RealEstate is not annotated as XML root element the else block is executed which causes the error. Using the other method without handing over the target class would work.
Now the question would be why is that not working? I assume using XMLSeeAlso should cause the unmarshaller to create the right type of real estate, shouldn't it?
As a side note I also looked into how the RestTemplate solved the task and there it boils down to the MarshallingHttpMessageConverter doing
Object result = this.unmarshaller.unmarshal(source);
if (!clazz.isInstance(result)) {
throw new TypeMismatchException(result, clazz);
}
return result;
It seems that the decoder differentiates for some reason while the message converter is doing the type check after unmarshalling.
And since it might be relevant how the Jaxb2Marshaller is setup
Jaxb2Marshaller().apply {
setPackagesToScan(
RealEstate::class.java.getPackage().name,
)
setCheckForXmlRootElement(false)
setSupportJaxbElementClass(true)
}