static private TargetChoice getTargetChoice() {
        String reportStreamStr = System.getProperty(SLF4J_INTERNAL_REPORT_STREAM_KEY);

        if(reportStreamStr == null || reportStreamStr.isEmpty()) {
            return TargetChoice.Stderr;
        }

        for(String s : SYSOUT_KEYS) {
            if(s.equalsIgnoreCase(reportStreamStr))
                return TargetChoice.Stdout;
        }
        return TargetChoice.Stderr;
    }
This system property controls the internal level of chattiness
of SLF4J. Recognized settings are "INFO", "WARN" and "ERROR". The default value is "INFO".
slf4j-api/src/main/java/org/slf4j/helpers/Reporter.java