public static void warn(String msg) {
        if (quietMode)
            return;

        System.err.println(WARN_PREFIX + msg);
    }
This method is used to output log4j internal warning
       statements. There is no way to disable warning statements.
       Output goes to <code>System.err</code>.
log4j-over-slf4j/src/main/java/org/apache/log4j/helpers/LogLog.java