public static String get(String key) throws IllegalArgumentException {
        if (key == null) {
            throw new IllegalArgumentException("key parameter cannot be null");
        }

        if (getMDCAdapter() == null) {
            throw new IllegalStateException(MDC_APAPTER_CANNOT_BE_NULL_MESSAGE);
        }
        return getMDCAdapter().get(key);
    }
Get the diagnostic context identified by the <code>key</code> parameter. The
<code>key</code> parameter cannot be null.

<p>
This method delegates all work to the MDC of the underlying logging system.

@param key a key
@return the string value identified by the <code>key</code> parameter.
@throws IllegalArgumentException
          in case the "key" parameter is null
slf4j-api/src/main/java/org/slf4j/MDC.java