code: public static String get(String key) throws IllegalArgumentException {
          if (key == null) {
              throw new IllegalArgumentException("key parameter cannot be null");
          }
  
          if (mdcAdapter == null) {
              throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
          }
          return mdcAdapter.get(key);
      }
docs: 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
  @return the string value identified by the <code>key</code> parameter.
  @throws IllegalArgumentException
  in case the "key" parameter is null