code: synchronized public void addAppender(Appender appender) throws IllegalArgumentException {
          if (appender == null) {
              throw new IllegalArgumentException("Appender not allowed to be null");
          }
  
          for (Appender p : appenderList) {
              if (p.getClass() == appender.getClass())
                  return;
          }
          appenderList.add(appender);
      }
docs: Add the specified appender to the output appenders.
  
  @param appender the <code>Appender</code> to add.
  @throws IllegalArgumentException if the <code>appender</code> is <code>null</code>.