MQTT Implementation

1. MQTT Message

  

public class MqttMessage{
    private final MqttFixedHeader mqttFixedHeader;
    private final Object variableHeader;
    private final Object payload;
}

public final class MqttFixedHeader {
    private final MqttMessageType messageType;
    private final boolean isDup;
    private final MqttQoS qosLevel;
    private final boolean isRetain;
    private final int remainingLength;
}

public enum MqttMessageType {
    CONNECT(1),
    CONNACK(2),
    PUBLISH(3),
    PUBACK(4),
    PUBREC(5),
    PUBREL(6),
    PUBCOMP(7),
    SUBSCRIBE(8),
    SUBACK(9),
    UNSUBSCRIBE(10),
    UNSUBACK(11),
    PINGREQ(12),
    PINGRESP(13),
    DISCONNECT(14);
}

2. Decode MQTT FixedHeader

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private static MqttFixedHeader decodeFixedHeader(ByteBuf buffer) {
    //read 1 byte
    short b1 = buffer.readUnsignedByte();
    
    //mqtt type 
    MqttMessageType messageType = MqttMessageType.valueOf(b1 >> 4);
    
    //dup flag if PUBLISH TYPE
    boolean dupFlag = (b1 & 0x08) == 0x08;
    
    //QoS if PUBLISH TYPE
    int qosLevel = (b1 & 0x06) >> 1;
    
    //RETAIN if PUBLISH TYPE
    boolean retain = (b1 & 0x01) != 0;

    //read length of payload
    int remainingLength = 0;
    int multiplier = 1;
    short digit;
    int loops = 0;
    do {
        //read 1 byte
        digit = buffer.readUnsignedByte();
        
        // 127 = 0x7F
        remainingLength += (digit & 127) * multiplier;
        
        //128 = 0x80
        multiplier *= 128;
        
        //max bytes is 4 
        loops++;
    } while ((digit & 128) != 0 && loops < 4);

    // MQTT protocol limits Remaining Length to 4 bytes
    if (loops == 4 && (digit & 128) != 0) {
        throw new DecoderException("remaining length exceeds 4 digits (" + messageType + ')');
    }
    
    //construct MqttFixedHeader
    MqttFixedHeader decodedFixedHeader = new MqttFixedHeader(messageType, dupFlag, MqttQoS.valueOf(qosLevel), retain, remainingLength);
    return validateFixedHeader(resetUnusedFields(decodedFixedHeader));
}

3. Decode MQTT VariableHeader

  

private static Result<Integer> decodeMsbLsb(ByteBuf buffer, int min, int max) {
    short msbSize = buffer.readUnsignedByte();
    short lsbSize = buffer.readUnsignedByte();
    final int numberOfBytesConsumed = 2;
    int result = msbSize << 8 | lsbSize;
    if (result < min || result > max) {
        result = -1;
    }
    return new Result<Integer>(result, numberOfBytesConsumed);

private static Result<?> decodeVariableHeader(ByteBuf buffer, MqttFixedHeader mqttFixedHeader) {
    switch (mqttFixedHeader.messageType()) {
        case CONNECT:
            return decodeConnectionVariableHeader(buffer);
        case CONNACK:
            return decodeConnAckVariableHeader(buffer);
        case SUBSCRIBE:
        case UNSUBSCRIBE:
        case SUBACK:
        case UNSUBACK:
        case PUBACK:
        case PUBREC:
        case PUBCOMP:
        case PUBREL:
            return decodeMessageIdVariableHeader(buffer);
        case PUBLISH:
            return decodePublishVariableHeader(buffer, mqttFixedHeader);
        case PINGREQ:
        case PINGRESP:
        case DISCONNECT:
            // Empty variable header
            return new Result<Object>(null, 0);
    }
    //should never reach here
    return new Result<Object>(null, 0); 
}

3.1 If Connect Type

private static Result<Integer> decodeMsbLsb(ByteBuf buffer, int min, int max) {
    short msbSize = buffer.readUnsignedByte();
    short lsbSize = buffer.readUnsignedByte();
    final int numberOfBytesConsumed = 2;
    int result = msbSize << 8 | lsbSize;
    if (result < min || result > max) {
        result = -1;
    }
    return new Result<Integer>(result, numberOfBytesConsumed);
}

private static Result<String> decodeString(ByteBuf buffer, int minBytes, int maxBytes) {
    final Result<Integer> decodedSize = decodeMsbLsb(buffer);
    int size = decodedSize.value;
    int numberOfBytesConsumed = decodedSize.numberOfBytesConsumed;
    if (size < minBytes || size > maxBytes) {
        buffer.skipBytes(size);
        numberOfBytesConsumed += size;
        return new Result<String>(null, numberOfBytesConsumed);
    }
    String s = buffer.toString(buffer.readerIndex(), size, CharsetUtil.UTF_8);
    buffer.skipBytes(size);
    numberOfBytesConsumed += size;
    return new Result<String>(s, numberOfBytesConsumed);
}

 

 

  

private static Result<MqttConnectVariableHeader> decodeConnectionVariableHeader(ByteBuf buffer) {
    final Result<String> protoString = decodeString(buffer);
    int numberOfBytesConsumed = protoString.numberOfBytesConsumed;

    //read level
    final byte protocolLevel = buffer.readByte();
    numberOfBytesConsumed += 1;
    
    //version
    final MqttVersion mqttVersion = MqttVersion.fromProtocolNameAndLevel(protoString.value, protocolLevel);
    
    //read Connect Flags
    final int b1 = buffer.readUnsignedByte();
    numberOfBytesConsumed += 1;

    //read keepAlive
    final Result<Integer> keepAlive = decodeMsbLsb(buffer);
    numberOfBytesConsumed += keepAlive.numberOfBytesConsumed;
    
    final boolean hasUserName = (b1 & 0x80) == 0x80;
    final boolean hasPassword = (b1 & 0x40) == 0x40;
    final boolean willRetain = (b1 & 0x20) == 0x20;
    final int willQos = (b1 & 0x18) >> 3;
    final boolean willFlag = (b1 & 0x04) == 0x04;
    final boolean cleanSession = (b1 & 0x02) == 0x02;
    
    if (mqttVersion == MqttVersion.MQTT_3_1_1) {
        final boolean zeroReservedFlag = (b1 & 0x01) == 0x0;
        if (!zeroReservedFlag) {
            // MQTT v3.1.1: The Server MUST validate that the reserved flag in the CONNECT Control Packet is
            // set to zero and disconnect the Client if it is not zero.
            // See http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc385349230
            throw new DecoderException("non-zero reserved flag");
        }
    }

    final MqttConnectVariableHeader mqttConnectVariableHeader = new MqttConnectVariableHeader(
            mqttVersion.protocolName(),
            mqttVersion.protocolLevel(),
            hasUserName,
            hasPassword,
            willRetain,
            willQos,
            willFlag,
            cleanSession,
            keepAlive.value);
    return new Result<MqttConnectVariableHeader>(mqttConnectVariableHeader, numberOfBytesConsumed);
}

3.2 If Publish Type

 

private static Result<MqttPublishVariableHeader> decodePublishVariableHeader(ByteBuf buffer,MqttFixedHeader mqttFixedHeader) {
    final Result<String> decodedTopic = decodeString(buffer);
    if (!isValidPublishTopicName(decodedTopic.value)) {
        throw new DecoderException("invalid publish topic name: " + decodedTopic.value + " (contains wildcards)");
    }
    int numberOfBytesConsumed = decodedTopic.numberOfBytesConsumed;

    int messageId = -1;
    if (mqttFixedHeader.qosLevel().value() > 0) {
        final Result<Integer> decodedMessageId = decodeMessageId(buffer);
        messageId = decodedMessageId.value;
        numberOfBytesConsumed += decodedMessageId.numberOfBytesConsumed;
    }
    final MqttPublishVariableHeader mqttPublishVariableHeader =    new MqttPublishVariableHeader(decodedTopic.value, messageId);
    return new Result<MqttPublishVariableHeader>(mqttPublishVariableHeader, numberOfBytesConsumed);
}

3.2 If SUBSCRIBE:UNSUBSCRIBE:SUBACK:UNSUBACK:PUBACK:PUBREC:PUBCOMP:PUBREL Type

 

private static Result<MqttMessageIdVariableHeader> decodeMessageIdVariableHeader(ByteBuf buffer) {
    final Result<Integer> messageId = decodeMessageId(buffer);
    return new Result<MqttMessageIdVariableHeader>(MqttMessageIdVariableHeader.from(messageId.value),messageId.numberOfBytesConsumed);
}

3.3 If CONNACK Type

private static Result<MqttConnAckVariableHeader> decodeConnAckVariableHeader(ByteBuf buffer) {
    final boolean sessionPresent = (buffer.readUnsignedByte() & 0x01) == 0x01;
    //ack return code
    byte returnCode = buffer.readByte();
    final int numberOfBytesConsumed = 2;
    final MqttConnAckVariableHeader mqttConnAckVariableHeader =
            new MqttConnAckVariableHeader(MqttConnectReturnCode.valueOf(returnCode), sessionPresent);
    return new Result<MqttConnAckVariableHeader>(mqttConnAckVariableHeader, numberOfBytesConsumed);
}