思路

一个16进制数占4位，一个字节8位，意味着一个字节存储两个16进制数。所以我们要将16进制转成一个字节就需要两个16进制数。

1. 从左往右，两两一组将字符分开。
2. 两个字符一组当成一个字节，分别判断两个字符在a-z、A-Z、0-9哪个区间，然后用字符减去a或者A或者0的字符，得到差，如果是A-Z或者a-z，那么需要加10。
3. 将第一个字符当成16进制的高位，并左移4个位。
4. 将3步骤左移4个位之后的字节|(或)第二个字符的字节数，得到这一组的16进制最终的字节。

如果是将字节转成十六进制串，则是上面的逆操作。

java代码

public class HexCode {
    private static final char[] UPPER_HEX_CHARS =
            {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};

    private static final char[] LOWER_HEX_CHARS =
            {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};

    public static char[] encode2char(byte[] bytes, boolean lowercase) {
        char[] chars = new char[bytes.length*2];
        for (int i = 0; i < chars.length; i = i + 2) {
            byte b = bytes[i / 2];

            char[] HEX_CHARS = LOWER_HEX_CHARS;
            if (!lowercase) {
                HEX_CHARS = UPPER_HEX_CHARS;
            }

            chars[i] = HEX_CHARS[(b >> 0x4) & 0xf];
            chars[i + 1] = HEX_CHARS[b & 0xf];
        }
        return chars;
    }

    public static String encode(byte[] bytes, boolean lowercase) {
        char[] endata = encode2char(bytes, lowercase);
        return new String(endata);
    }

    public static byte[] decode(String data) throws Exception {
        int len = (data.length()+1)/2;
        byte[] bytes = new byte[len];
        int index = 0;
        for (int i = 1; i< data.length(); i+=2) {
            char h = data.charAt(i-1);
            char l = data.charAt(i);

            bytes[index] = decodeByte(h,l);
            index++;
        }

        return bytes;
    }

    public static byte decodeByte(char hight, char low) throws Exception {
        byte data = 0;
        if (hight>='A' && hight <= 'F') {
            int value = hight-'A'+10;
            data = (byte)(value<<4);
        } else if (hight>='a' && hight <= 'f') {
            int value = hight-'a'+10;
            data = (byte)(value<<4);
        } else if (hight>='0' && hight <= '9') {
            int value = hight-'0';
            data = (byte)(value<<4);
        } else {
            throw new Exception("decode hexadecimal failed");
        }

        if (low>='A' && low <= 'F') {
            int value = low-'A'+10;
            data |= (byte)value;
        } else if (low>='a' && low <= 'f') {
            int value = low-'a'+10;
            data |= (byte)value;
        } else if (low>='0' && low <= '9') {
            int value = low-'0';
            data |= (byte)value;
        } else {
            throw new Exception("decode hexadecimal failed");
        }

        return data;
    }

    public static void main(String[] args) throws Exception {
        String hexStr = "C2B109";
        byte[] bs = decode(hexStr);
        for (byte b : bs) {
            System.out.printf("%08d%n",Integer.valueOf(Integer.toBinaryString(Byte.toUnsignedInt(b))));
        }
        System.out.println(encode(bs,false));
    }
}