1.介绍
Xmodem是一种在串口通信中广泛使用的异步文件传输协议，分为Xmodem（使用128字节的数据块）和1k-Xmodem（使用1024字节即1k字节的数据块）协议两种。
本文实现的是128字节数据块的Xmodem协议，采用CRC16校验，在项目中应用时，发送端和接收端可根据具体情况修改双方的协议。
标准Xmodem协议（使用128字节的数据块）帧格式：
Byte1 | Byte2 | Byte3 | Byte4 ~ byte131 | Byte132 | 控制字符包序号包序号的反码数据校验和
如果你对串口通信还不太了解，可以看下我写的这篇博客使用Java实现串口通信。
2.实现
在和嵌入式同学调试的过程中，发现发送端发送数据过快，导致接收端处理不过来，所以在send方法中开启了一个子线程来处理数据发送逻辑，方便加入延时处理。
接收方法中，发送C是表示以CRC方式校验。
public class Xmodem { private final byte SOH = 0x01; private final byte EOT = 0x04; private final byte ACK = 0x06; private final byte NAK = 0x15; private final byte CAN = 0x18; private final int SECTOR_SIZE = 128; private final int MAX_ERRORS = 10; private InputStream inputStream; private OutputStream outputStream; public Xmodem(InputStream inputStream, OutputStream outputStream) { this.inputStream = inputStream; this.outputStream = outputStream; } /** * 发送数据 * * @param filePath * 文件路径 */ public void send(final String filePath) { new Thread() { public void run() { try { int errorCount; byte blockNumber = 0x01; int checkSum; int nbytes; byte[] sector = new byte[SECTOR_SIZE]; DataInputStream inputStream = new DataInputStream( new FileInputStream(filePath)); while ((nbytes = inputStream.read(sector)) > 0) { if (nbytes < SECTOR_SIZE) { for (int i = nbytes; i < SECTOR_SIZE; i++) { sector[i] = (byte) 0xff; } } errorCount = 0; while (errorCount < MAX_ERRORS) { putData(SOH); putData(blockNumber); putData(~blockNumber); checkSum = CRC16.calc(sector) & 0x00ffff; putChar(sector, (short) checkSum); outputStream.flush(); byte data = getData(); if (data == ACK) { break; } else { ++errorCount; } } blockNumber = (byte) ((++blockNumber) % 256); } boolean isAck = false; while (!isAck) { putData(EOT); isAck = getData() == ACK; } } catch (Exception e) { e.printStackTrace(); } }; }.start(); } /** * 接收数据 * * @param filePath * 文件路径 * @return 是否接收完成 * @throws IOException * 异常 */ public boolean receive(String filePath) throws Exception { int errorCount = 0; byte blocknumber = 0x01; byte data; int checkSum; byte[] sector = new byte[SECTOR_SIZE]; DataOutputStream outputStream = new DataOutputStream( new FileOutputStream(filePath)); putData((byte) 0x43); while (true) { if (errorCount > MAX_ERRORS) { outputStream.close(); return false; } data = getData(); if (data != EOT) { try { if (data != SOH) { errorCount++; continue; } data = getData(); if (data != blocknumber) { errorCount++; continue; } byte _blocknumber = (byte) ~getData(); if (data != _blocknumber) { errorCount++; continue; } for (int i = 0; i < SECTOR_SIZE; i++) { sector[i] = getData(); } checkSum = (getData() & 0xff) << 8; checkSum |= (getData() & 0xff); int crc = CRC16.calc(sector); if (crc != checkSum) { errorCount++; continue; } putData(ACK); blocknumber++; outputStream.write(sector); errorCount = 0; } catch (Exception e) { e.printStackTrace(); } finally { if (errorCount != 0) { putData(NAK); } } } else { break; } } outputStream.close(); putData(ACK); return true; } /** * 获取数据 * * @return 数据 * @throws IOException * 异常 */ private byte getData() throws IOException { return (byte) inputStream.read(); } /** * 发送数据 * * @param data * 数据 * @throws IOException * 异常 */ private void putData(int data) throws IOException { outputStream.write((byte) data); } /** * 发送数据 * * @param data * 数据 * @param checkSum * 校验和 * @throws IOException * 异常 */ private void putChar(byte[] data, short checkSum) throws IOException { ByteBuffer bb = ByteBuffer.allocate(data.length + 2).order( ByteOrder.BIG_ENDIAN); bb.put(data); bb.putShort(checkSum); outputStream.write(bb.array()); }}
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CRC16校验算法，采用的是查表法。
public class CRC16 { private static final char crctable[] = { 0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7, 0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef, 0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6, 0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de, 0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485, 0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d, 0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4, 0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc, 0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823, 0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b, 0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12, 0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a, 0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41, 0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49, 0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70, 0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78, 0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f, 0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067, 0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e, 0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256, 0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d, 0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405, 0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c, 0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634, 0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab, 0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3, 0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a, 0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92, 0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9, 0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1, 0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8, 0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0 }; public static char calc(byte[] bytes) { char crc = 0x0000; for (byte b : bytes) { crc = (char) ((crc << 8) ^ crctable[((crc >> 8) ^ b) & 0x00ff]); } return (char) (crc); }}
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3.使用
Xmodem xmodem = new Xmodem(serialPort.getInputStream(),serialPort.getOutputStream());xmodem.send(filePath);
4.写在最后
完整的代码下载
