Generic Map<String, Object> JSON marshalling and unmarshalling with Moxy

Question

Is there any way to marshal Map<String, Object> with Moxy to JSON so that the result uses natural constructs of JSON?
That is, the keys are strings and for all possible values following rules apply (possibly not a complete set):

• Number (e.g., Integer) becomes a JSON number (or a string if it's too big)
• String becomes a JSON string
• Set, Array, Iterable become a JSON array
• and finally for a Map<String, Object>, the same rules are recursively applied
• any other object is marshalled in a natural Moxy way

There is already an example how to marshal Map<String, Integer> and other specific maps by using @XmlVariableNode (see http://blog.bdoughan.com/2013/06/moxys-xmlvariablenode-using-maps-key-as.html?m=1), but I was unable to extend this idea to a point where also subtypes can be inserted as a value.

Note that Moxy should be able to unmarshal the JSON back to the original Map.

Jackson is capable of doing this by default.

Answer 1

I've tried to get something like you want with JAXB RI (XML):

@XmlRootElement
public class Foo {
    @XmlJavaTypeAdapter(MapAdapter.class)
    public Map<String, Object> map;
}

public class MapAdapter extends XmlAdapter<MapEntry[], Map<String, Object>> {
    @Override
    public Map<String, Object> unmarshal(MapEntry[] v) throws Exception {
        Map<String, Object> map = new HashMap<>();
        for (MapEntry me : v)
            map.put(me.key, me.value);
        return map;
    }
    @Override
    public MapEntry[] marshal(Map<String, Object> v) throws Exception {
        MapEntry[] mes = new MapEntry[v.size()];
        int i = 0;
        for (Map.Entry<String, Object> entry : v.entrySet())
            mes[i++] = new MapEntry(entry.getKey(), entry.getValue());
        return mes;
    }
}

public class MapEntry {
    public String key;
    public Object value;

    public MapEntry() {}

    public MapEntry(String key, Object value) {
        this.key = key;
        this.value = value;
    }
}

Unfortunately MOXy has some bug ( https://bugs.eclipse.org/bugs/show_bug.cgi?id=465014 ) and is not able to handle this.

If you need MOXy then you have to use the approach described in Blaise's blog:

@XmlRootElement
public class MoxyFoo {
    @XmlJavaTypeAdapter(MoxyMapAdapter.class)
    public Map<String, Object> map = new HashMap<>();
}

public class MoxyMapAdapter extends XmlAdapter<MoxyMapAdapter.AdaptedMap, Map<String, Object>> {
    @Override
    public AdaptedMap marshal(Map<String, Object> map) throws Exception {
        AdaptedMap adaptedMap = new AdaptedMap();
        for (Entry<String, Object> entry : map.entrySet()) {
            AdaptedEntry adaptedEntry = new AdaptedEntry();
            adaptedEntry.key = entry.getKey();
            adaptedEntry.value = entry.getValue();
            adaptedMap.entries.add(adaptedEntry);
        }
        return adaptedMap;
    }
    @Override
    public Map<String, Object> unmarshal(AdaptedMap adaptedMap) throws Exception {
        List<AdaptedEntry> adaptedEntries = adaptedMap.entries;
        Map<String, Object> map = new HashMap<>(adaptedEntries.size());
        for (AdaptedEntry adaptedEntry : adaptedEntries) {
            map.put(adaptedEntry.key, adaptedEntry.value);
        }
        return map;
    }

    public static class AdaptedMap {
        @XmlVariableNode("key")
        List<AdaptedEntry> entries = new ArrayList<>();
    }

    public static class AdaptedEntry {
        @XmlTransient
        public String key;
        @XmlValue
        public Object value;
    }
}

It's working fine for XML, but not for JSON. For JSON only marshalling is working now. I've filled a bug to fix unmarshalling: https://bugs.eclipse.org/bugs/show_bug.cgi?id=465016 .