public static MDCAdapter getMDCAdapter() {
        if(MDC_ADAPTER == null) {
            MDC_ADAPTER = getMDCAdapterGivenByProvider();
        }
        return MDC_ADAPTER;
    }
Returns the MDCAdapter instance currently in use.

Since 2.0.17, if the MDCAdapter instance is null, then this method set it to use
the adapter returned by the SLF4JProvider. However, in the vast majority of cases
the MDCAdapter will be set earlier (during initialization) by {@link LoggerFactory}.

@return the MDcAdapter instance currently in use.
@since 1.4.2
slf4j-api/src/main/java/org/slf4j/MDC.java