Unmarshalling `null` object still leaves the default initialised object in place

[mensinda]

Example:

package it;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;

public class NullObj {
    public static class InnerDto {
        public String someStuff = "FooBar";
    }

    public static class DTO {
        public InnerDto iAmNull = new InnerDto(); // default is not null
        public InnerDto iAmNotNull = new InnerDto();
    }

    public static void main(String ... args) throws JsonProcessingException {
        DTO dto = new DTO();
        dto.iAmNull = null;

        XmlMapper mapper = new XmlMapper();

        final String res = mapper.writeValueAsString(dto);

        // <DTO><iAmNull/><iAmNotNull><someStuff>FooBar</someStuff></iAmNotNull></DTO>
        System.out.println(res);

        DTO reversed = mapper.readValue(res, DTO.class);

        // The following statement should produce a NullPointerException but prints `FooBar`!!!
        System.out.println(reversed.iAmNull.someStuff);
    }
}

jackson version: 2.13.2

[mensinda]

Digging further into this, I managed to get this working by manually changing the generated <iAmNull/> to <iAmNull xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xsi:nil=\"true\"/>.

So, I did some further digging and found that I could get Jackson XML to emit this automatically with XmlMapper.builder().configure(ToXmlGenerator.Feature.WRITE_NULLS_AS_XSI_NIL, true).build();.

I am going to close this, since adding this option solved my issue, but I would still recommend to enable this by default (maybe a bit naive since this could be a breaking change) and / or document this in the README.md since I only found this by digging through the code.