On Apr 8, 2014, at 9:15 PM, Mike Duigou <[email protected]> wrote:
>> For the case of incorrect signed comparison is it sticking around because
>> there is code dependent on the current broken behaviour?
>
> Probably even if the dependence is implicit such as expecting a particular
> iteration order
>
>> or do you think it would be possible to change that?
>
> I requested fixing the compareTo long ago (back when I was a mere user of the
> JDK) and was told that it could not be changed.
Any more details on that?
>
>> i have my suspicious that code dependent compareTo may not break if the
>> total ordering changes, since many cases are likely to be for randomly
>> generated UUIDs.
>
> I agree that most code would probably be fine. I worry though that there will
> be cases that either explicitly or implicitly depend upon current ordering.
> Surprisingly there a lot of cases where UUIDs are created using the
> UUID(long, long) constructor with non-random data such as incrementing
> numbers:
>
> UUID myUUID = new UUID(SOME_CONSTANT, atomicSerial.incrementAndGet());
>
That seems a terribly broken usage of UUID for 128 bit numbers or a pair of
signed 64 bit numbers :-)
Part of me thinks we should not be supporting such broken usage. Might be worth
getting some usage data from grepcode or maven central.
> This usage exists, I believe, because we haven't provided direct support for
> MAC version UUIDs. Unfortunately the UUID(long,long) constructor does not
> check that the UUID constructed is even valid and usually they are not. This
> usage, while egregious, is common. These people would probably be impacted by
> changes in ordering.
>
For correct usage the lsb long will for practical purposes always be -ve, since
the msb is 1, as per the variant [1].
For time-based (MAC) UUIDs the comparison is totally broken as the time_low
field bits will cycle often (~ every 3.5 mins by my calculation), so i doubt
anyone would be using the comparator for such UUIDs to derive any meaningful
order, for example, try this:
UUID a = UUID.fromString("a938c470-bfc1-11e3-8a33-0800200c9a66");
UUID b = UUID.fromString("14857ca0-bfc2-11e3-8a33-0800200c9a66");
UUID c = UUID.fromString("d7a34910-bfc2-11e3-8a33-0800200c9a66");
System.out.println(a.timestamp() < b.timestamp());
System.out.println(b.timestamp() < c.timestamp());
System.out.println(a.timestamp() < c.timestamp());
System.out.println(a.compareTo(b));
System.out.println(b.compareTo(c));
System.out.println(a.compareTo(c));
>>> We could provide static methods to return appropriate comparators for
>>> version 1 and version 2 UUIDs--I've actually written them before for other
>>> projects.
>>>
>>
>> It would be nice to just have one compareTo that does the right thing based
>> of the UUID types being compared.
>
> If it were up to me only the time and DCE UUIDs would be comparable, there's
> no ordering relationship for other versions.
I think it is fine for random UUIDs to be comparable with each other.
> The comparators I've considered adding would only allow comparisons within
> the same version.
>
Yes, although for a general comparator the primary sort key could be the
version value.
>>> I also note that UUID does not currently support version 5, SHA-1, which it
>>> should.
>>>
>>> I am hoping to do other performance work on UUID within the scope of Java
>>> 9. Adding additional Comparators would fit right in with that.
>>>
>>
>> OK, although it might be nice to bash this on the head sooner? as it might
>> be possible to get into an 8u release depending on what we conclude about
>> backwards compatibility.
>
> Any change to the ordering of the compareTo would have to happen in a major
> version if at all. I am very reluctant to change this behaviour when
> providing alternative comparators might just be a better choice.
>
Here is an alternative, deprecate UUID and create a new UUID class that is not
broken. If we cannot fix the existing class lets just recognize it for what it
is, a defective implementation for 128 bit numbers.
Paul.
[1] https://tools.ietf.org/html/rfc4122#section-4.1.1
