`XmlMapper` not deserializing root-level Enums

[bhkjersten]

I want to deserialize XML using XmlMapper so that it is as mechanically similar to how we deserialize JSON. I'm doing something wrong when it comes to enums, however. Below is my enum definition and my test code.

public enum SimpleEnum {
    MALE,
    FEMALE,
    ;
}

import java.io.IOException;
import org.junit.Test;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;

public class TestEnumSerialization {
        @Test
        public void testJSON() throws IOException {
        SimpleEnum inputEnum = SimpleEnum.MALE;
        ObjectMapper mapper = new ObjectMapper();
        String serializedString;
        SimpleEnum outputEnum;

        //Test that ObjectMapper can serialize json
        serializedString = mapper.writeValueAsString(inputEnum);
        System.out.println("JSON serialized string: " + serializedString);

        //Test that ObjectMapper can deserialize json 
        outputEnum = mapper.readValue(serializedString, SimpleEnum.class);
        System.out.println("JSON deserialized enum: " + outputEnum);
    }

    @Test
    public void testXML() throws IOException {
        SimpleEnum inputEnum = SimpleEnum.MALE;
        ObjectMapper mapper = new XmlMapper();
        String serializedString;
        SimpleEnum outputEnum;

        //Test that ObjectMapper can serialize xml
        serializedString = mapper.writeValueAsString(inputEnum);
        System.out.println("XML serialized string:  " + serializedString);

        //Test that ObjectMapper can deserialize xml 
        outputEnum = mapper.readValue(serializedString, SimpleEnum.class);
        System.out.println("XML deserialized enum:  " + outputEnum);
    }
}

This prints out this:

JSON serialized string: "MALE"
JSON deserialized enum: MALE
XML serialized string: MALE

and it throws a JsonMappingException at the XmlMapper readValue() method, saying it can not deserialize instance of SimpleEnum out of START_OBJECT token.
I do not get this error when write a class that has an enum as a member.

[cowtowncoder]

My first suggestion would actually be to always use a (JSON) Object as the root value, and not try to serialize "bare" Lists, arrays, Maps or Enums as root value. This avoid many of the problems.

But let me see what is the issue here: the only problem with Enums, XML, that I remember was that type id handling was problematic for some inclusion methods. But the non-typed handling should be fine.

[cowtowncoder]

Interesting. I can reproduce the issue, and there was no test for this case.

So upside is that this is not a known problem (which often means "more difficult to fix") but rather "as of yet unknown". I'll add a unit test, see what gives.

Thank you for reporting this!

[cowtowncoder]

Hmmh. Ok, this may be tricky to handle. Problem is with structural impedance between XML, JSON, and that the fix at parser/databinder level relies on POJO properties to allow defining element contents as scalars.

Will add a test, at any rate.

[bhkjersten]

Thanks a lot for your comments yesterday. That cleared up what was going on.

[cowtowncoder]

Ok no problem. We can still try to make that work in future; but for now at least this is documented (I added a note on README)

[cowtowncoder]

Turns out I can finally fix this for 2.12, now that "extract scalar from Object" logic can be plugged in.