code: public static void setProperty(IoSession session, String key, String value) {
          if (key == null) {
              throw new NullPointerException("key should not be null");
          }
          if (value == null) {
              removeProperty(session, key);
          }
          Map<String, String> context = getContext(session);
          context.put(key, value);
          MDC.put(key, value);
      }
docs: Add a property to the context for the given session
  This property will be added to the MDC for all subsequent events
  @param session The session for which you want to set a property
  @param key  The name of the property (should not be null)
  @param value The value of the property