code: public static void warn(String msg, Throwable t) {
          if (quietMode)
              return;
  
          System.err.println(WARN_PREFIX + msg);
          if (t != null) {
              t.printStackTrace();
          }
      }
docs: This method is used to output log4j internal warnings. There is
  no way to disable warning statements.  Output goes to
  <code>System.err</code>.