Disabling wrapper on list (`defaultUseWrapper = false`) does not serialize/deserialize `null` lists as expected

[vtarchon]

I noticed that when i serialize an object with a null list and disable using a default wrapper around lists, upon deserialization the object returned differs from the object serialized.

Here is a test of the problem

import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlRootElement;
import java.io.IOException;
import java.util.List;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

public class JacksonTest {

    @Test
    public void test() throws IOException {
        XmlMapper mapper = XmlMapper.builder().defaultUseWrapper(false).build();

        Parent a = new Parent();
        String out = mapper.writeValueAsString(a);
        Parent b = mapper.readValue(out, Parent.class);
        Assertions.assertEquals(a.getChildren(), b.getChildren());
    }

    @JacksonXmlRootElement
    static class Parent {
        List<Child> children;

        public Parent() {}

        public List<Child> getChildren() {
            return children;
        }

        public void setChildren(List<Child> children) {
            this.children = children;
        }
    }

    static class Child {
        private String name;
    }
}

class Parent has a list of class Child that is null, when this object is serialized the following xml is

<Parent><children/></Parent>

if you take this xml and deserialize it back into a Parent object what ends up happening is that the children property is a list that has one element, an object of Child with null properties. I was expecting that Parent would have a null list instead.

If you enable wrapper, it works fine and the serialized xml is

<Parent/>

which deserializes into the same object as the original

[vtarchon]

it looks like if the array is initialized to an empty array or if i add @JsonInclude(JsonInclude.Include.NON_EMPTY) to the class, it'll serialized without the empty element when the wrapper is disabled.

[cowtowncoder]

I think there is unfortunately ambiguity wrt XML with default settings, for this case. But enabling ToXmlGenerator.Feature.WRITE_NULLS_AS_XSI_NIL should help preserve null as null -- this settings is disabled by default for backwards-compatibility reasons (added in 2.10).