Solr DateToLongTransfomer: Convert Date to Milliseconds

Scenario
During develop and debug one Solr feature, I constantly need check the value of date field.
In response, the date field is string, but in eclipse debug mode, the value is long(milliseconds from epoch time) - as Solr actually stores the milliseconds in index, so why not write one transformer that add the milliseconds into response.

Here it is.
DateToLongTransfomerFactory

public class DateToLongTransfomerFactory extends TransformerFactory {
  @Override
  public DocTransformer create(String field, SolrParams params,
      SolrQueryRequest req) {
    return new DateToLongTransfomer(field, params);
  }
  
  /**
   * org.apache.solr.search.SolrReturnFields.parseFieldList(String[],
   * SolrQueryRequest) DocTransformers augmenters = new DocTransformers();
   * 
   * DocTransformer is thread safe.
   */
  private static class DateToLongTransfomer extends DocTransformer {
    private String fl;
    private String field;
    
    public DateToLongTransfomer(String field, SolrParams params) {
      // field is the name of transformer [dateToLong]
      this.field = field;
      fl = Preconditions.checkNotNull(params.get("fl"),
          "fl can't be null in transfromer");
    }
    
    @Override
    public void transform(SolrDocument doc, int docid) throws IOException {
      String fieldValue = getFieldValue(doc, fl);
      if (fieldValue != null) {
        doc.addField(field, fieldValue);
      }
    }
    
  public static String getFieldValue(SolrDocument doc, String field) {
    List<String> rst = new ArrayList<String>();
    Object obj = doc.get(field);
    getFieldvalues(doc, rst, obj);
    
    if (rst.isEmpty()) {
      return null;
    }
    return rst.get(0);
  }    

  public static void getFieldvalues(SolrDocument doc, List<String> rst,
      Object obj) {
    if (obj == null) return;
    if (obj instanceof org.apache.lucene.document.Field) {
      org.apache.lucene.document.Field field = (Field) obj;
      String oldValue = field.stringValue();
      if (oldValue != null) {
        rst.add(oldValue);
      }
    } else if (obj instanceof IndexableField) {
      IndexableField field = (IndexableField) obj;
      String oldValue = field.stringValue();
      if (oldValue != null) {
        rst.add(oldValue);
      }
    } else if (obj instanceof Collection) {
      Collection colls = (Collection) obj;
      for (Object newObj : colls) {
        getFieldvalues(doc, rst, newObj);
      }
    } else {
      rst.add(obj.toString());
      // throw new RuntimeException("When this is called? obj.type:"
      // + obj.getClass());
    }
  }    
  }
}