@SuppressWarnings("unchecked")
    public void setContextMap(Map contextMap) {
        if (contextMap != null) {
            readWriteThreadLocalMap.set(new HashMap<String, String>(contextMap));
        } else {
            readWriteThreadLocalMap.set(null);
        }
        nullifyReadOnlyThreadLocalMap();
    }
Returns the keys in the MDC as a {@link Set}. The returned value can be
null.
zlt-commons/zlt-log-spring-boot-starter/src/main/java/org/slf4j/TtlMDCAdapter.java