code: protected Enumeration getContextAttributeNames()
      {
          if (_attributes.size()==0)
              return getHttpContext().getAttributeNames();
          HashSet set=new HashSet(_attributes.keySet());
          Enumeration e=getHttpContext().getAttributeNames();
          while(e.hasMoreElements())
              set.add(e.nextElement());
          return Collections.enumeration(set);
      }
docs: Get context attribute names.
  Combines ServletHandler and HttpContext attributes.