code: public void start()
          throws Exception
      {
          super.start();
          
          if (!javax.servlet.Filter.class
              .isAssignableFrom(_class))
          {
              super.stop();
              throw new IllegalStateException(_class+" is not a javax.servlet.Filter");
          }
  
          _filter=(Filter)newInstance();
          _config=new Config();
          _filter.init(_config);
      }
docs: /* ------------------------------------------------------------