code: public static void warn(Object warnMsg) {
          if (level.ordinal() >= Level.WARN.ordinal())
              if (warnMsg instanceof Throwable) {
                  singleton.warn(null, (Throwable) warnMsg, loggerName());
              } else {
                  singleton.warn(warnMsg, null, loggerName());
              }
      }
docs: print warning level log of the exception message with stacktrace.