public Map getCopyOfContextMap() {
        Map<String, String> readOnlyMap = getPropertyMap();
        if (readOnlyMap == null) {
            return null;
        } else {
            return new HashMap<>(readOnlyMap);
        }
    }
Return a copy of the current thread's context map. Returned value may be
null.
zlt-commons/zlt-log-spring-boot-starter/src/main/java/org/slf4j/TtlMDCAdapter.java