code: public MessageBroker getMessageBroker() {
          if (this.messageBroker != null) {
              return this.messageBroker;
          }
          Assert.notNull(FlexContext.getMessageBroker(), "A MessageBroker was not set on the MessageTemplate "
              + "and no thread-local MessageBroker could be found in the FlexContext.");
          return FlexContext.getMessageBroker();
      }
docs: Returns the {@link MessageBroker} for routing messages
  
  @return the message broker