Raise the minimum supported version of Gradle to 4.x

[wilkinsona]

Gradle 4.0 has been released. Given that 2.0 doesn't GA until November, I think we should seriously consider requiring Gradle 4.x in 2.0. Doing it now is, perhaps, a little too aggressive. Waiting a month or two for things to settle down feels about right to me. That would also give the IDEs a chance to catch up if necessary.

[wilkinsona]

One advantage of requiring 4.0 is that we can drop our use of convention mapping (here's one example), which requires the use of internalish API, in favour of the new PropertyState support.

[michael-barker]

Is it safe to use 4.x with Spring Boot 1.5.x as well as Spring Cloud?

[wilkinsona]

@michael-barker It may work (I haven't tried), but it isn't supported.

[michael-barker]

@wilkinsona I tried it out 4.x with several of our projects and didn't run into any major issues. There were a couple dependencies that resolved differently but really this was probably a problem that we had to begin with that didn't manifest itself in 3.x. I was able to resolve any problems I encountered.

[wilkinsona]

@michael-barker That's really useful info. Thank you.

[wilkinsona]

There's a bug in 4.0 that can prevent exclusions from working properly. If we raise the minimum to 4.x, we should probably require 4.1 or later.

[eriwen]

Hi @wilkinsona, I have suggested that we publish Gradle 4.0.2 with a fix for the referenced bug.

My hope is that your compatibility convention can be "latest patch release of Gradle 4.x".

[wilkinsona]

@eriwen Thanks, that would be appreciated if you can do it. However, if the command line @Option enhancement lands in 4.x, we may end up requiring that anyway.

[eriwen]

@wilkinsona note that Gradle 4.1 is locked so earliest possible for @Option is 4.2 shipping in September.

[wilkinsona]

I've opened a separate issue to track replacing convention mappings with the use of the new property state API. I have some reservations about doing so (see the issue for details) and think we should consider it separately to a Gradle version bump.