How to Marshal a Map into `value` Elements with JAXB?

Marshalling Map into value with JAXB

Problem

Maps are commonly marshaled into XML structures like:

<map>
  <entry>
    <key>KEY</key>
    <value>VALUE</value>
  </entry>
  <entry>
    <key>KEY2</key>
    <value>VALUE2</value>
  </entry>
  ...
</map>

However, sometimes it is desirable to generate XML where the key becomes the element name and the value becomes its content:

<map>
  <KEY>VALUE</KEY>
  <KEY2>VALUE2</KEY2>
 ...
</map>

Solution

Option 1: Dynamically Named XML Elements

Using dynamic attribute names is not recommended, as it violates XML schema and interface contract principles.

Option 2: Enumerated Key Type

To maintain a strict interface contract, use an enumerated key type for the map:

public enum KeyType {
    KEY,
    KEY2;
}

@XmlJavaTypeAdapter(MapAdapter.class)
Map<KeyType, String> mapProperty;

Option 3: Simplified Marshaling

To simplify the default marshaling structure into , use a MapAdapter that converts the Map to an array of MapElements:

class MapElements {
    @XmlAttribute
    public String key;
    @XmlAttribute
    public String value;
    
    // Required by JAXB
    private MapElements() {}
    
    public MapElements(String key, String value) {
        this.key = key;
        this.value = value;
    }
}

class MapAdapter extends XmlAdapter<MapElements[], Map<String, String>> {
    @Override
    public MapElements[] marshal(Map<String, String> arg0) {
        MapElements[] mapElements = new MapElements[arg0.size()];
        int i = 0;
        for (Entry<String, String> entry : arg0.entrySet()) {
            mapElements[i++] = new MapElements(entry.getKey(), entry.getValue());
        }
        return mapElements;
    }
    
    @Override
    public Map<String, String> unmarshal(MapElements[] arg0) {
        Map<String, String> r = new TreeMap<>();
        for (MapElements mapelement : arg0) {
            r.put(mapelement.key, mapelement.value);
        }
        return r;
    }
}

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