code: @Override
      public void addAppender(Appender newAppender) {
          if (aai == null) {
              synchronized (this) {
                  if (aai == null) {
                      aai = new NFAppenderAttachableImpl();
                  }
              }
          }
          aai.addAppender(newAppender);
          repository.fireAddAppenderEvent(this, newAppender);
      }
docs: /*
  (non-Javadoc)
  
  @see org.apache.log4j.Category#addAppender(org.apache.log4j.Appender)