code: public FilterHolder addFilterServletMapping(String servletName, String filterName, int dispatches)
      {
          FilterHolder holder= (FilterHolder)_filterMap.get(filterName);
          if (holder == null)
              throw new IllegalArgumentException("Unknown filter :" + filterName);
          _servletFilterMap.add(servletName, new FilterMapping(null,holder,dispatches));
          return holder;
      }
docs: Add a servlet filter mapping
  @param servletName The name of the servlet to be filtered.
  @param filterName The name of the filter.
  @param dispatches An integer formed by the logical OR of FilterHolder.__REQUEST,
  FilterHolder.__FORWARD,FilterHolder.__INCLUDE and/or FilterHolder.__ERROR.
  @return The holder of the filter instance.