paho 中 MQTT 协议固定报头实现

概述：

每个 MQTT 控制报文都包含一个固定报头。固定报头由两个字节组成，第一个字节的高4位为报文类型，低4位为报文标志；第二个字节开始为剩余长度，最多4个字节，也就是说剩余长度最少占用一个字节，最多占用4个字节。由此可以看出， MQTT 报文最少就只有两个字节，确实是个很轻量级的协议了。

MQTT 规范：https://github.com/mcxiaoke/mqtt/blob/master/mqtt/02-ControlPacketFormat.md

代码实现：

在 paho 中关于 MQTT 报文的编码解码的实现主要是 MqttWireMessage 类及其子类。MqttWireMessage 是个抽象类，里面存在两个未实现的抽象方法getMessageInfo()和getVariableHeader()，通过跟踪代码我们可以发现getHeader()与createWireMessage(InputStream inputStream)是编解码的核心实现方法。

// MqttWireMessage#getHeader

public byte[] getHeader() throws MqttException {

    try {

        // 这行代码主要是计算出固定报头的第一个字节，

        int first = ((getType() & 0x0f) << 4) ^ (getMessageInfo() & 0x0f);

        // 可变报头通过调用子类实现获取

        byte[] varHeader = getVariableHeader();

        // 剩余长度=可变报头长度+有效荷载的长度

        int remLen = varHeader.length + getPayload().length;

        ByteArrayOutputStream baos = new ByteArrayOutputStream();

        DataOutputStream dos = new DataOutputStream(baos);

        dos.writeByte(first);

        // 这里调用了encodeMBI方法来编码剩余长度

        dos.write(encodeMBI(remLen));

        dos.write(varHeader);

        dos.flush();

        return baos.toByteArray();

    } catch (IOException ioe) {

        throw new MqttException(ioe);

    }

}

// MqttWireMessage#createWireMessage

private static MqttWireMessage createWireMessage(InputStream inputStream) throws MqttException {

    try {

        CountingInputStream counter = new CountingInputStream(inputStream);

        DataInputStream in = new DataInputStream(counter);

        // 读取第一个字节，并解析

        int first = in.readUnsignedByte();

        // 报文类型

        byte type = (byte) (first >> 4);

        // 标记位，不同的报文会由不同的涵义，交给子类来处理

        byte info = (byte) (first &= 0x0f);

        // 这里调用了readMBI方法来解码剩余长度

        long remLen = readMBI(in).getValue();

        long totalToRead = counter.getCounter() + remLen;

        MqttWireMessage result;

        long remainder = totalToRead - counter.getCounter();

        // 剩余有效数据，包括可变报头和有效荷载，交由子类处理

        byte[] data = new byte[0];

        // The remaining bytes must be the payload...

        if (remainder > 0) {

            data = new byte[(int) remainder];

            // 读取剩余有效数据

            in.readFully(data, 0, data.length);

        }

        // 根据类型构造不同的消息

        if (type == MqttWireMessage.MESSAGE_TYPE_CONNECT) {

            result = new MqttConnect(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_PUBLISH) {

            result = new MqttPublish(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_PUBACK) {

            result = new MqttPubAck(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_PUBCOMP) {

            result = new MqttPubComp(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_CONNACK) {

            result = new MqttConnack(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_PINGREQ) {

            result = new MqttPingReq(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_PINGRESP) {

            result = new MqttPingResp(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_SUBSCRIBE) {

            result = new MqttSubscribe(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_SUBACK) {

            result = new MqttSuback(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_UNSUBSCRIBE) {

            result = new MqttUnsubscribe(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_UNSUBACK) {

            result = new MqttUnsubAck(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_PUBREL) {

            result = new MqttPubRel(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_PUBREC) {

            result = new MqttPubRec(info, data);

        } else if (type == MqttWireMessage.MESSAGE_TYPE_DISCONNECT) {

            result = new MqttDisconnect(info, data);

        } else {

            throw ExceptionHelper.createMqttException(MqttException.REASON_CODE_UNEXPECTED_ERROR);

        }

        return result;

    } catch (IOException io) {

        throw new MqttException(io);

    }

}

// MqttWireMessage#encodeMBI

public static byte[] encodeMBI(long number) {

    validateVariableByteInt((int) number);

    int numBytes = 0;

    long no = number;

    ByteArrayOutputStream bos = new ByteArrayOutputStream();

    // Encode the remaining length fields in the four bytes

    do {

        byte digit = (byte) (no % 128);

        no = no / 128;

        if (no > 0) {

            digit |= 0x80;

        }

        bos.write(digit);

        numBytes++;

    } while ((no > 0) && (numBytes < 4));

    return bos.toByteArray();

}

// MqttWireMessage#readMBI

public static MultiByteInteger readMBI(DataInputStream in) throws IOException {

    byte digit;

    int msgLength = 0;

    int multiplier = 1;

    int count = 0;

    do {

        digit = in.readByte();

        count++;

        msgLength += ((digit & 0x7F) * multiplier);

        multiplier *= 128; // multiplier <<= 7;

    } while ((digit & 0x80) != 0);

    if (msgLength < 0 || msgLength > VARIABLE_BYTE_INT_MAX) {

        throw new IOException("This property must be a number between 0 and " + VARIABLE_BYTE_INT_MAX

                              + ". Read value was: " + msgLength);

    }

    return new MultiByteInteger(msgLength, count);

}