code: public static byte[] convert(int data) {
          long len = Integer.toUnsignedLong(data);
          byte[] b = new byte[Datagram.LEN_LIMIT];
          for (int i = 0; i < b.length; i++) {
              b[i] = (byte) (len >> ((b.length - i - 1) * 8));
          }
          return b;
      }
docs: 将一个int类型转换为四个字节的byte数组
  
  @param data int类型的参数
  @return byte类型的数组