code: public void put(String key, String val) {
          org.apache.log4j.MDC.put(key, val);
      }
docs: Put a context value (the <code>val</code> parameter) as identified with
  the <code>key</code> parameter into the current thread's context map. The
  <code>key</code> parameter cannot be null. Log4j does <em>not</em>
  support null for the <code>val</code> parameter.
  
  <p>
  This method delegates all work to log4j's MDC.
  
  @throws IllegalArgumentException
  in case the "key" or <b>"val"</b> parameter is null