Unmarshall XML object List into HashMap with XMLAdapter

I've got the following XML

<demographics>
  <field>
    <value>03290</value>
    <XMLtag>study_identifier</XMLtag>
  </field>
  <field>
    <value>somename</value>
    <XMLtag>name</XMLtag>
  </field>
  <filename>bbRad_BUILD_dem_03290_BUILD_006742.xml</filename>
  <study_cache_dir>somestring</study_cache_dir>
</demographics>

The annotations in Demographics2 class are currently like this:

@XmlRootElement( name = "demographics" )
public class Demographics2 
{
    @XmlElement( name = "study_cache_dir" )
    private String studyCacheDir;

    @XmlElement( name = "filename" )
    private String filename;

    @XmlElement( name = "field" )
    private List<Field2> mutivalueFields;
}

Field2.java ( simple container ):

public class Field2
{
    @XmlElement( name = "XMLtag")
    private String xmlTag;
    @XmlElement( name = "value" )
    private String value;
}

If I unmarshall the XML it works fine, and multivalueFields contains a list of two Field2 objects. What I want is to convert the List< Field2 > to be HashMap< String, Field2 > where the String key will be the xmlTag value from the Field2 object.

So far I've tired changing Demographics2 class and using a XmlAdapter as follows:

@XmlElement( name = "field" )
@XmlJavaTypeAdapter( MultiValueFieldsAdapter.class )
private Map< String, Field2> mutivalueFields;

I thought the adapter should work like this:

public class MultiValueFieldsAdapter extends XmlAdapter< List<Field2>, Map<String, Field2>> {

   @Override
   public List<Field2> marshal( Map<String, Field2> arg0) throws Exception {

       // loop all in Map and just add the Field2 objects to a list then
       // return the list
       return null;
   }

    @Override
    public Map<String, Field2> unmarshal( List<Field2> arg0 ) throws Exception {

         // The problem is here arg0 will be emtpy list, unless we make arg0 a Field2 object
         // it seems to be calling unmarshal for each Field2 object, rather than some list

         // loop each Field2 and add to Map
         Map<String, Field2> newMap = new HashMap<String, Field2>( arg.length() );

         for ( Field2 f : arg0 )
             newMap.put( f.xmlTag, f );
    }
}

My MultiValueFieldsAdapter is wrong, but I also think maybe I have my annotations are incorrect? I can't figure out how it manages to put all the Field2 objects into a List with just @XmlElement( name = "field" ), it must be JAXB that's doing that conversion, but I can't find how to override it and put into map.

Unfortunately the XML is fixed and I have to work with that format of XML. What I'm trying to avoid is that once the XML has been unmarshalled I don't want to have to loop through the multivalueFields and checking each xmlTag of each Field2 for my match. If they are in a Map I can do:

Field2 nameField = multivalueFields.get( "name" );