Jackson：自定义反序列化器：无法使用数组反序列化 XML

Question

I use Jackson to serialize one of my classes as XML. Because of the complex internal structure, I use a custom serializer that generates this data as an array.

The resulting XML looks like this in a simplified example:

<MyDataType>
    <list>
        <entry>
            <order>0</order>
            <value>And then a step to the right</value>
        </entry>
        <entry>
            <order>1</order>
            <value>It's just a jump to the left</value>
        </entry>
        <entry>
            <order>2</order>
            <value>With your hands on your hips</value>
        </entry>
    </list>
</MyDataType>

I only get problems when I deserialize. Then jsonParser.getCodec().readTree(jsonParser) just "sees" the last entry of the array. All other entries are lost.

So the debug output in the deserializer will show:

Deserialize: Incoming data:
{
    "list": {
        "entry": {
            "order": "2",
            "value": "With your hands on your hips"
        }
    }
}

Even more weird: If I serialize/deserialize to JSON, the same code works fine!

Deserializer:

class MyDataTypeDeserialize extends JsonDeserializer<MyDataType> {
    @Override
    public MyDataType deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
        MyDataType result = new MyDataType();

        JsonNode rootNode = jsonParser.getCodec().readTree(jsonParser);

        // Shows only one entry when using XML
        System.out.println("Deserialize: Incoming data: " + rootNode.toString());

        JsonNode valueList = rootNode.get("list");
        JsonNode entryList = valueList.findValue("entry");

        for (Iterator<JsonNode> it = entryList.elements(); it.hasNext(); ) {
            JsonNode element = it.next();

            int order = element.get("order").asInt();
            String value = element.get("value").asText();

            result.addValue(order, value);
        }

        return result;
    }
}

Any hints what I'm doing wrong will be appreciated.

Answer 1

[Edited]

I ran into the same issue a couple of years back. By default Jackson (most specifically the UntypedObjectDeserializer) when sees multiple children with the same name it does not map them into an array or list. What I did is I used this library: https://search.maven.org/classic/#search%7Cgav%7C1%7Cg%3A%22org.json%22%20AND%20a%3A%22json%22 With that you can do:

JSONObject jsonObject = XML.toJSONObject(...);
String jsonText = jsonObject.toString();
// Use Jackson here

Of course this is an additional parsing and then serialization step before you can actually deserialize as a Jackson node. The above worked fine for me, because it wasn't on the critical path of the application.

[Edit]

In your case where you need to use JsonParser in a deserializer you could try something like this:

JsonLocation start = jp.getCurrentLocation();
jsonParser.getCodec().readTree(jsonParser); // The result is ignored
JsonLocation end = jp.getCurrentLocation();

String xmlText = end.getSourceRef().toString().substring(start.getCharOffset() - 1, end.getCharOffset());

JSONObject jsonObject = XML.toJSONObject(xmlText);
String jsonText = jsonObject.toString();

// Parse jsonText into JsonNode

Or an alternative solution: https://github.com/DinoChiesa/deserialize-xml-arrays-jackson/blob/master/src/main/java/com/google/xmldeser/ArrayInferringUntypedObjectDeserializer.java