How does MQTT Callback work in openHAB 2?

Setup, Configuration and Use Beginners
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1

Hello, I have just begun using openHAB 2 to develop a binding that uses paho mqtt java library. So far, I can successfully publish messages . However, I am not receiving anything from the broker although the broker is sending messages ( tested with python script ). Obviously, I am missing something. Please guide me through this.

Callback

client.setCallback(new MqttCallback()
{

	@Override
	public void messageArrived(String topic, MqttMessage message) throws Exception
	{
		logger.debug("MQTT message received.");
	}

	@Override
	public void deliveryComplete(IMqttDeliveryToken arg0)
	{
		// TODO Auto-generated method stub
	}

	@Override
	public void connectionLost(Throwable arg0)
	{
		// TODO Auto-generated method stub

	}
});

Connection / Subscription

client.connect();
client.subscribe(topic);
2

What is wrong with the MQTT binding?

At a minimum, you should look at the MQTT binding for some working code.