@SuppressWarnings("unchecked")
    public Map<String, String> getPropertyMap() {
        Map<String, String> readOnlyMap = readOnlyThreadLocalMap.get();
        if (readOnlyMap == null) {
            Map<String, String> current = readWriteThreadLocalMap.get();
            if (current != null) {
                final Map<String, String> tempMap = new HashMap<>(current);
                readOnlyMap = Collections.unmodifiableMap(tempMap);
                readOnlyThreadLocalMap.set(readOnlyMap);
            }
        }
        return readOnlyMap;
    }
<p>Get the current thread's MDC as a map. This method is intended to be used
internally.</p>

The returned map is unmodifiable (since version 1.3.2/1.4.2).
zlt-commons/zlt-log-spring-boot-starter/src/main/java/org/slf4j/TtlMDCAdapter.java