public IndexingFilters(Configuration conf) 
{
     
    /* Get indexingfilter.order property */
    String order = conf.get(INDEXINGFILTER_ORDER);
    ObjectCache objectCache = ObjectCache.get(conf);
    this.indexingFilters = (IndexingFilter[]) objectCache
        .getObject(IndexingFilter.class.getName());
    if (this.indexingFilters == null) {
      /*
       * If ordered filters are required, prepare array of filters based on
       * property
       */
      String[] orderedFilters = null;
      if (order != null && !order.trim().equals("")) {
        orderedFilters = order.split("\\s+");
      }
      try {
        ExtensionPoint point = PluginRepository.get(conf).getExtensionPoint(
            IndexingFilter.X_POINT_ID);
        if (point == null)
          throw new RuntimeException(IndexingFilter.X_POINT_ID + " not found.");
        Extension[] extensions = point.getExtensions();
        HashMap< String, IndexingFilter > filterMap =
          new HashMap< String, IndexingFilter >();
        for (int i = 0; i <  extensions.length; i++) {
          Extension extension = extensions[i];
          IndexingFilter filter = (IndexingFilter) extension
              .getExtensionInstance();
          LOG.info("Adding " + filter.getClass().getName());
          if (!filterMap.containsKey(filter.getClass().getName())) {
            filter.addIndexBackendOptions(conf);
            filterMap.put(filter.getClass().getName(), filter);
          }
        }
        /*
         * If no ordered filters required, just get the filters in an
         * indeterminate order
         */
        if (orderedFilters == null) {
          objectCache.setObject(IndexingFilter.class.getName(),
              filterMap.values().toArray(
                  new IndexingFilter[0]));
          /* Otherwise run the filters in the required order */
        } else {
          ArrayList< IndexingFilter > filters = new ArrayList< IndexingFilter >();
          for (int i = 0; i <  orderedFilters.length; i++) {
            IndexingFilter filter = filterMap
                .get(orderedFilters[i]);
            if (filter != null) {
              filter.addIndexBackendOptions(conf);
              filters.add(filter);
            }
          }
          objectCache.setObject(IndexingFilter.class.getName(), filters
              .toArray(new IndexingFilter[filters.size()]));
        }
      } catch (PluginRuntimeException e) {
        throw new RuntimeException(e);
      }
      this.indexingFilters = (IndexingFilter[]) objectCache
          .getObject(IndexingFilter.class.getName());
    }
}

 public NutchDocument filter(NutchDocument doc,
    Parse parse,
    Text url,
    CrawlDatum datum,
    Inlinks inlinks) throws IndexingException 
{
    for (int i = 0; i <  this.indexingFilters.length; i++) {
      doc = this.indexingFilters[i].filter(doc, parse, url, datum, inlinks);
      // break the loop if an indexing filter discards the doc
      if (doc == null) return null;
    }
    return doc;
}

Run all defined filters.