MQTT解析

Java

package com.qqbot;

import java.io.*;
import java.nio.charset.StandardCharsets;

public class MqttEntityEncoder {

    public static byte[] encode(MqttEntity entity) throws IOException {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        DataOutputStream dos = new DataOutputStream(baos);

        // 1. mdcSn[20]
        writeFixedLengthString(dos, entity.getMdcSn(), 20);

        // 2. length = 54 + msgLen
        int length = 54 + (entity.getMsgBody() == null ? 0 : entity.getMsgBody().length);
        dos.writeInt(Integer.reverseBytes(length)); // 小端写入

        // 3. timeStamp (uint64_t)
        dos.writeLong(Long.reverseBytes(entity.getTimeStamp()));

        // 4. transId[36]
        writeFixedLengthString(dos, entity.getTransId(), 36);

        // 5. msgType (uint16_t)
        dos.writeShort(Short.reverseBytes(entity.getMsgType()));

        // 6. msgLen (uint32_t)
        int msgLen = entity.getMsgBody() == null ? 0 : entity.getMsgBody().length;
        dos.writeInt(Integer.reverseBytes(msgLen));

        // 7. protVer (uint32_t)
        dos.writeInt(Integer.reverseBytes(entity.getProtVer()));

        // 8. msgBody[msgLen]
        if (entity.getMsgBody() != null) {
            dos.write(entity.getMsgBody());
        }

        dos.flush();
        return baos.toByteArray();
    }

    private static void writeFixedLengthString(DataOutputStream dos, String value, int fixedLength) throws IOException {
        byte[] bytes = value != null ? value.getBytes(StandardCharsets.UTF_8) : new byte[0];
        if (bytes.length > fixedLength) {
            dos.write(bytes, 0, fixedLength); // 截断
        } else {
            dos.write(bytes);                 // 写入实际内容
            for (int i = 0; i < fixedLength - bytes.length; i++) {
                dos.writeByte(0);             // 补0
            }
        }
    }

    public static void main(String[] args) throws IOException {
        MqttEntity entity = new MqttEntity();
        entity.setMdcSn("ABC123EFGGGHDDD");
        entity.setTimeStamp(System.currentTimeMillis());
        entity.setTransId("123e4567-e89b-12d3-a456");
        entity.setMsgType((short) 1);
        entity.setProtVer(1);
        entity.setMsgBody("hello mqtt test ddd".getBytes(StandardCharsets.UTF_8));

        byte[] payload = MqttEntityEncoder.encode(entity);
// payload 可直接通过 MQTT 或 TCP 发送
        System.out.println(payload);

    }
}

C++

#include <iostream>
#include <cstdint>
#include <cstring>

#pragma pack(push, 1)
struct MqttStruct {
    char mdcSn[20];
    uint32_t length;
    uint64_t timeStamp;
    char transId[36];
    uint16_t msgType;
    uint32_t msgLen;
    uint32_t protVer;
    // msgBody follows
};
#pragma pack(pop)

int main() {
    uint8_t data[] = {
        65, 66, 67, 49, 50, 51, 69, 70, 71, 71, 71, 72, 68, 68, 68, 0, 0, 0, 0, 0, // mdcSn
        73, 0, 0, 0,                                                             // length = 73
        33, 227, 216, 122, 152, 1, 0, 0,                                         // timeStamp (小端)
        49, 50, 51, 101, 52, 53, 54, 55, 45, 101, 56, 57, 98, 45, 49, 50, 
        100, 51, 45, 97, 52, 53, 54, 
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                                   // transId 补齐到 36
        1, 0,                                                                    // msgType
        19, 0, 0, 0,                                                             // msgLen = 19
        1, 0, 0, 0,                                                              // protVer
        104, 101, 108, 108, 111, 32, 109, 113, 116, 116, 32, 116, 101, 115, 116, 32, 100, 100, 100
    };

    MqttStruct header{};
    std::memcpy(&header, data, sizeof(MqttStruct));

    std::cout << "mdcSn: " << std::string(header.mdcSn, 20) << std::endl;
    std::cout << "length: " << header.length << std::endl;
    std::cout << "timestamp: " << header.timeStamp << std::endl;
    std::cout << "transId: " << std::string(header.transId, 36) << std::endl;
    std::cout << "msgType: " << header.msgType << std::endl;
    std::cout << "msgLen: " << header.msgLen << std::endl;
    std::cout << "protVer: " << header.protVer << std::endl;

    const char* msgBody = reinterpret_cast<const char*>(data + sizeof(MqttStruct));
    std::cout << "msgBody: " << std::string(msgBody, header.msgLen) << std::endl;

    return 0;
}