code: public void doAppend(LoggingEvent event)
      {
          if(closed) {
              LogLog.error("Attempted to append to closed appender named [" + name + "].");
              return;
          }
  
          if(!isAsSevereAsThreshold(event.getLevel())) {
              return;
          }
  
          Filter f = this.headFilter;
  
          FILTER_LOOP:
          while(f != null) {
              switch(f.decide(event)) {
              case Filter.DENY: return;
              case Filter.ACCEPT: break FILTER_LOOP;
              case Filter.NEUTRAL: f = f.getNext();
              }
          }
  
          this.append(event);
      }
docs: Unsynchronized version