code: public T validationCallback(ValidationCallback callback) {
          if (callback instanceof ApplicationContextAware) {
              ((ApplicationContextAware) callback).setApplicationContext(applicationContext);
          }
  
          getAction().setValidationCallback(callback);
          return self;
      }
docs: Adds validation callback to the receive action for validating
  the received message with Java code.
  @param callback
  @return