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Date: Sun, 20 Jan 2013 12:00:04 -0800 (PST)
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Subject: Re: single instance
From: Lew <lewbloch@gmail.com>
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Xref: csiph.com comp.lang.java.programmer:21590

Roedy Green wrote:
> Knute Johnson wrote, quoted or indirectly quoted someone who said :
>> Below is what I ended up with.  I'm curious about the UUID.  How do you=
=20
>> create a time based UUID?  And does it have less granularity that=20
>> currentTimeMillis?
>=20
> there are specified formulas for creating UUIDs. I think one way of
> rapidly generating them would be to use System.currentTimeMillis for
> the high 64 bits and System.nanotime for the lower 64 bits, but I
> suspect that would not be considered kosher.
>=20
> Another would be to use SecureRandom to generate 16 bytes, though that
> probably is not as unique as you might hope.
>=20
> a UUID is not just 128 bits. It has embedded fields that tell you how
> it was generated.

http://docs.oracle.com/javase/7/docs/api/java/util/UUID.html
=ABThe layout of a variant 2 (Leach-Salz) UUID is as follows: The most sign=
ificant long consists of the following unsigned fields:

 0xFFFFFFFF00000000 time_low
 0x00000000FFFF0000 time_mid
 0x000000000000F000 version
 0x0000000000000FFF time_hi
...

=ABFor more information including algorithms used to create UUIDs, see RFC =
4122: A Universally Unique IDentifier (UUID) URN Namespace,=20
http://www.ietf.org/rfc/rfc4122.txt
section 4.2 "Algorithms for Creating a Time-Based UUID".=BB

--=20
Lew