Deserialising to a map key-value pairs provided as XML attributes (Java)

Question

I would like this XML

<root>

  <firstTag>
    <property key="foo" value="One"/>
    <property key="bar" value="Two"/>
  </firstTag>

  <secondTag>
    <property key="foo" value="1"/>
    <property key="bar" value="2"/>
  </secondTag>

</root>

to be deserialised into a Map<String, String>such that only the keys and values that are provided via attributes inside the <firstTag/> are taken into account. Here is the desired output map:

{foo=One, bar=Two}

And this using XStream, jackson-dataformat-xml, or any popular unmarshalling library.

Answer 1

I needed to implement a Converter for this:

public class MyConverter implements Converter {

    public boolean canConvert(Class clazz) {
        return AbstractMap.class.isAssignableFrom(clazz);
    }

    @Override
    public void marshal(Object arg0, HierarchicalStreamWriter writer, MarshallingContext context) {}

    @Override
    public Object unmarshal(HierarchicalStreamReader reader, UnmarshallingContext context) {

        Map<String, String> map = new HashMap<String, String>();
        reader.moveDown();  // Get down to the firstTag and secondTag level.

        while(reader.hasMoreChildren()) {
            if(reader.getNodeName().equals("firstTag")) {
                while(reader.hasMoreChildren()) {
                    reader.moveDown();
                    String key = reader.getAttribute("key");
                    String value = reader.getAttribute("value");
                    map.put(key, value);
                    reader.moveUp();
                }
            }
        }
        return map;
    }
}

Then use it:

XStream xstream = new XStream();
xstream.alias("root", java.util.Map.class);
xstream.registerConverter(new MyConverter());

String xml = ...
Map<String, String> map = (Map<String, String>) xstream.fromXML(xml);