XML Serialization + JsonAnyGetter as attributes

[asa-git]

Hi there,

I'm having some issues with the serialization of dynamic data as attributes of an XML element.
Note: Tested on both version 2.9.6 and 2.9.7

Given the Object:

@JacksonXmlRootElement(localName = "metadata")
private static class ObjectWithAttributes {

	@JsonAnyGetter
	public Map<String, String> getData() {
		Map<String, String> map = new HashMap<>();
		map.put("a", "some value");
		map.put("b", "and another one");
		return map;
	}
}

I was expecting to get the following XML :

<metadata a="some value" b="and another one" />

but got:

<metadata>
  <a>some value</a>
  <b>and another one</b>
</metadata>

I tried to fiddle around with some of the annotations available be never got it to serialize the values as attributes.
I even tried to write a custom serializer which worked fine when annotating the class with the @JsonSerialize(using =...) but would results in any other attribute fields to not be serialized :(

I couldn't find any documentations regarding the uses of @JsonAnyGetter to get the values as attributes and not sub elements.

Is this use case not supported by Jackson ?

Thanks in advance,
Best regards.

[cowtowncoder]

Currently this is not supported, unfortunately. Part of the problem is that "any getter" induced properties are appended after regular ones, and writer assumes/requires that attributes are output first. Handling of attribute/element detection is quite complicated since this concept only exists in XML output: it basically requires reordering properties internally and keeping track of number of properties that are to be output as attributes. This is done, I think, by custom SerializerModifier.

It would be good to think of a way to allow defining that any-properties are to be serialized as attributes (it would be difficult to allow per-property overrides), although I can not yet think of a simple way to make that happen.

[asa-git]

Thanks for the feedback, I can indeed imaging that this would be a bit of a pain to implement and maintain.

I've ended up writing a Serializer for the whole bean; I just hope it won't grow to much in the future.

For information I did try one other way hoping I could hack it through the use of the @JacksonXmlProperty annotation without specifying a localname, as I assume all the fields annotated with the isAttribute=true would be initially collected to build the root element, but ended up with the exception:

com.fasterxml.jackson.core.JsonGenerationException: Can not write a field name, expecting a value

here is the sample:

public interface MapAsAttributes {
	Map<String, String> getValues();
}

public static class MapSerializer extends StdSerializer<MapAsAttributes> {

	private static final long serialVersionUID = 1L;

	public MapSerializer() {
		super(MapAsAttributes.class, false);
	}

	@Override
	public void serialize(MapAsAttributes obj, JsonGenerator gen, SerializerProvider provider) throws IOException {

		ToXmlGenerator writer = (ToXmlGenerator) gen;

		for (Map.Entry<String, String> entry : obj.getValues().entrySet()) {
			writer.setNextIsAttribute(true);
			writer.writeFieldName(entry.getKey());
			writer.writeRawValue(entry.getValue());
		}
	}
}

@JacksonXmlRootElement(localName = "metadata")
private static class ObjectWithAttributes {

	@JacksonXmlProperty(isAttribute = true)
	public MapAsAttributes getData() {

		Map<String, String> map = new HashMap<>();
		map.put("a", "some value");
		map.put("b", "and another one");
		return () -> map;
	}

	@JacksonXmlProperty(localName = "someelement")
	public String getSomething() {
		return "SomeValue";
	}
}

The serialization was done registering a module with a custom serializer:

final XmlMapper mapper = (XmlMapper) new XmlMapper() //
				.configure(ToXmlGenerator.Feature.WRITE_XML_DECLARATION, true) //
				.enable(SerializationFeature.INDENT_OUTPUT) //
				.registerModule(new SimpleModule().addSerializer(MapAsAttributes.class, new MapSerializer()));

System.out.println(mapper.writeValueAsString(new ObjectWithAttributes()));

[B-R-Bender]

I'm end up with this implementation.
It's for both JSON and XML.
Hope it'll helps someone.

@JsonSerialize(using = Row.Serializer.class)
public class Row {

    private HashMap<String, String> data;

    public HashMap<String, String> getData() {
        return data;
    }

    public void setData(HashMap<String, String> data) {
        this.data = data;
    }

    public static class Serializer extends StdSerializer<Row> {

        public Serializer() {
            this(null);
        }

        public Serializer(Class<Row> t) {
            super(t);
        }

        @Override
        public void serialize(Row row, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
            if (jsonGenerator instanceof ToXmlGenerator) {
                final ToXmlGenerator xmlGenerator = (ToXmlGenerator) jsonGenerator;
                jsonGenerator.writeStartObject();
                for (Map.Entry<String, String> item : row.getData().entrySet()) {
                    if (item.getValue() == null) {
                        continue;
                    }
                    xmlGenerator.setNextIsAttribute(true);

                    jsonGenerator.writeFieldName(item.getKey());
                    jsonGenerator.writeString(item.getValue());
                }
                jsonGenerator.writeEndObject();
            } else {
                jsonGenerator.writeStartObject();
                for (Map.Entry<String, String> item : row.getData().entrySet()) {
                    if (item.getValue() == null) {
                        continue;
                    }
                    jsonGenerator.writeFieldName(item.getKey());
                    jsonGenerator.writeString(item.getValue());
                }
                jsonGenerator.writeEndObject();
            }
        }
    }
}