On 4 December 2016 at 14:56, Stephen Connolly
<[email protected]> wrote:
> I'm currently trying to figure out how to make mixins possible in POM 5.
This is wonderful news!
> Mixins basically bring a form of multiple inheritance to the POM... which
> leads to the problems of how to solve conflicts.
Why allow both inheritance and mixins? Why not simply allow only one
or the other? Or rather, allow inheritance only until mixins enter the
picture. So you could have a grandparent POM (without mixins), a
parent POM (without mixins), and then a child POM (potentially with
mixins). I don't really see the value in inheritance if mixins are
available. (But I can see why it would be nice to try and re-use an
existing company POM or similar.)
That still leaves the problem of including contradicting mixins. I
would simply disallow any contradictions. Keep it as simple as
possible, you can always make it more complex later. It is already
going to be quite difficult to clearly explain any encountered
problems to the user without any additional complexity.
Some more thoughts:
1. Mixin A says <element>A</element> but Mixin B says
<element>B</element>. Maven should disallow that. The solution for
this would be to reference a property. Then both A and B could simply
state <element>${element.value}</element>. Obviously, this may clash
(there may have been a very good reason for requiring "A" or "B") but
it is now up to the user to set ${element.value} appropriately or
change the combination of mixins. I don't see how Maven would be able
to resolve this.
2. Mixin A says <elements><a/><b/></elements> but Mixin B says
<elements><b/><a/></elements>. Again, this should not be allowed.
Maven can't possibly tell whether the order is relevant.
I have been thinking of a tool to do exactly this. It would use mixins
to generate the entire POM. With the above restrictions it seems
possible to achieve a real "define it once" type of build. Obviously,
it would be much nicer if Maven simply supported this natively. :-)
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