How to map an Hashmap to key-value-attributes in XML using xstream

Question

I have the following entity:

@XStreamAlias("entity")
public class MapTestEntity {

    @XStreamAsAttribute
    public Map<String, String> myMap = new HashMap<>();

    @XStreamAsAttribute
    public String myText;
}

I use it with xstream like:

MapTestEntity e = new MapTestEntity();
e.myText = "Foo";
e.myMap.put("firstname", "homer");
e.myMap.put("lastname", "simpson");

XStream xstream = new XStream(new PureJavaReflectionProvider());
xstream.processAnnotations(MapTestEntity.class);
System.out.println(xstream.toXML(e));

and get the following xml:

<entity myText="Foo">
  <myMap>
    <entry>
      <string>lastname</string>
      <string>simpson</string>
    </entry>
    <entry>
      <string>firstname</string>
      <string>homer</string>
    </entry>
  </myMap>
</entity>

But I need to map the HashMap to attributes in xml like:

<entity myText="Foo" lastname="simpson" firstname="homer" />

How can I do that with XStream? Can I use a custom converter or mapper or something like that? TIA!!

(Of course my code needs to be ensure that are no duplicates in xml attributes.)

Answer 1

I wrote an own Converter:

public class MapToAttributesConverter implements Converter {

    public MapToAttributesConverter() {
    }

    @Override
    public boolean canConvert(Class type) {
        return Map.class.isAssignableFrom(type);
    }

    @Override
    public void marshal(Object source, HierarchicalStreamWriter writer, MarshallingContext context) {
        Map<String, String> map = (Map<String, String>) source;
        for (Map.Entry<String, String> entry : map.entrySet()) {
            writer.addAttribute(entry.getKey(), entry.getValue().toString());
        }
    }

    @Override
    public Object unmarshal(HierarchicalStreamReader reader, UnmarshallingContext context) {
        Map<String, String> map = new HashMap<String, String>();
        for (int i = 0; i < reader.getAttributeCount(); i++) {
            String key = reader.getAttributeName(i);
            String value = reader.getAttribute(key);
            map.put(key, value);
        }
        return map;
    }
}

Answer 2

The NamedMapConverter can achieve this. Take a look at http://x-stream.github.io/javadoc/com/thoughtworks/xstream/converters/extended/NamedMapConverter.html

The third example shows exactly that, what you want:

    new NamedMapConverter(xstream.getMapper(), "entry", "key", String.class, "value", Integer.class, true, true, xstream.getConverterLookup());

Creates this xml output:

    <map>
        <entry key="keyValue" value="0"/>
    </map>