UUID格式:8-4-4-4-12 -为什么？

Question

为什么UUID的格式是“8-4-4-4-12”（数字）？我环顾四周，但找不到需要它的决定。

格式化为十六进制字符串的UUID示例:58d 5 e 212-165 b-4ca 0-909 b-c 86 b 9 ce 0111

Answer 1

“8-4-4-4-12”格式只是供人类阅读。UUID实际上是一个128位的数字。

考虑到字符串格式在存储或存储在内存中时需要比128位数多一倍的字节。我建议在内部使用数字，当需要在UI上显示或在文件中导出时，使用字符串格式。

Answer 2

格式在第3节的IETF RFC4122中定义。输出格式在它说“UUID=…”的地方定义

3.-名称空间注册模板

名称空间ID: UUID注册信息:注册日期:2003-10-01

名称空间的声明注册人：JTC 1/SC6（ASN.1报告组）

句法结构声明：UUID 是相对于所有 UUID 的空间而言，在空间和时间上都是唯一的标识符。由于 UUID 是固定大小并包含时间字段，因此值可能会滚动更新（公元 3400 年左右，具体取决于所使用的特定算法）。UUID 可用于多种用途，从标记生命周期极短的对象到可靠地识别网络中非常持久的对象。

  The internal representation of a UUID is a specific sequence of
  bits in memory, as described in Section 4.  To accurately
  represent a UUID as a URN, it is necessary to convert the bit
  sequence to a string representation.

  Each field is treated as an integer and has its value printed as a
  zero-filled hexadecimal digit string with the most significant
  digit first.  The hexadecimal values "a" through "f" are output as
  lower case characters and are case insensitive on input.

  The formal definition of the UUID string representation is
  provided by the following ABNF [7]:

  UUID                   = time-low "-" time-mid "-"
                           time-high-and-version "-"
                           clock-seq-and-reserved
                           clock-seq-low "-" node
  time-low               = 4hexOctet
  time-mid               = 2hexOctet
  time-high-and-version  = 2hexOctet
  clock-seq-and-reserved = hexOctet
  clock-seq-low          = hexOctet
  node                   = 6hexOctet
  hexOctet               = hexDigit hexDigit
  hexDigit =
        "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" /
        "a" / "b" / "c" / "d" / "e" / "f" /
        "A" / "B" / "C" / "D" / "E" / "F"

Answer 3

它由时间、版本、clock_seq_hi、clock_seq_lo、节点分隔，如以下rfc所示。

根据IETF RFC4122：

4.1.2.  Layout and Byte Order

   To minimize confusion about bit assignments within octets, the UUID
   record definition is defined only in terms of fields that are
   integral numbers of octets.  The fields are presented with the most
   significant one first.

   Field                  Data Type     Octet  Note
                                        #

   time_low               unsigned 32   0-3    The low field of the
                          bit integer          timestamp

   time_mid               unsigned 16   4-5    The middle field of the
                          bit integer          timestamp

   time_hi_and_version    unsigned 16   6-7    The high field of the
                          bit integer          timestamp multiplexed
                                               with the version number  

   clock_seq_hi_and_rese  unsigned 8    8      The high field of the
   rved                   bit integer          clock sequence
                                               multiplexed with the
                                               variant

   clock_seq_low          unsigned 8    9      The low field of the
                          bit integer          clock sequence

   node                   unsigned 48   10-15  The spatially unique
                          bit integer          node identifier

   In the absence of explicit application or presentation protocol
   specification to the contrary, a UUID is encoded as a 128-bit object,
   as follows:

   The fields are encoded as 16 octets, with the sizes and order of the
   fields defined above, and with each field encoded with the Most
   Significant Byte first (known as network byte order).  Note that the
   field names, particularly for multiplexed fields, follow historical
   practice.

   0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          time_low                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       time_mid                |         time_hi_and_version   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |clk_seq_hi_res |  clk_seq_low  |         node (0-1)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         node (2-5)                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+