public Set<String> getKeys() {
        Map<String, String> map = inheritableThreadLocalMap.get();
        if (map != null) {
            return map.keySet();
        } else {
            return null;
        }
    }
Returns the keys in the MDC as a {@link Set} of {@link String}s The
returned value can be null.

@return the keys in the MDC
slf4j-api/src/main/java/org/slf4j/helpers/BasicMDCAdapter.java