Subscribe

After the client established the connection with the broker, the client can subscribe its interested topic.

IMessageCallbackHandler

IMessageCallbackHandler is the interface for handling asynchronous events of blockClient.

There are two callback functions the client can override:

• messageArrive: The function for handling arrived messages from the broker. The messages was sent to the client because of the subscription.
• connectionLost: The function for handling connection lost to the broker.

Example implementation of the interface IMessageCallbackHandler:

class MessageCallbackHandler implements IMessageCallbackHandler {

    @Override
    public void messageArrive(ByteBuf payload) {
        logger.info(payload.toString(StandardCharsets.UTF_8));
    }

    @Override
    public void connectionLost(Throwable cause) {
        logger.error("Connection lost: " + cause.getMessage());
        System.exit(1);
    }

}

Set the message callback handler to BlockClient

The message callback handler should be set to the instance of BlockClient.

// Initialization of the blockClient
// Connect to the broker server

// Set the message callback handler to the blockClient
blockClient.setMessageCallbackHandler(new MessageCallbackHandler());

Make a subscription to the broker

Subscrbe to the broker by specifying:

• topic: Interested topic.
• consumer group ID: The consumer group this subscribe client belongs to.

Example: Subscribe to topic "Foo" and specify the consumer group ID to "555".

// Initialization of the blockClient
// Connect to the broker server
// Set the message callback handler.

// Make a subscription.
String topic = "Foo";
int consumerGroup = 555;
blockClient.subscribe(topic, MqttQoS.AT_LEAST_ONCE, 0, consumerGroup);