Provides a solution for caching build dependencies for maven gradle

[malston]

#68 Removed the m2 input from all the tasks and added caches for .m2 and .gradle directories.
Generate settings no longer copies contents from m2 resource and uses the Concourse cache feature instead, therefore, we could remove the m2 resource entirely along with the param to the m2 folder. This PR also caches the wrapper by setting the MAVEN_USER_HOME variable to the location of the m2 cache folder.

[marcingrzejszczak]

@nebhale - do you have anything to add to this? I remember that you mentioned something about doing symbolic links to .m2...

[nebhale]

I’d say that the work done around moving $M2_HOME around is a bit of overkill. @philwebb and I have more or less settled on the following pattern. A maven or gradle cache and a conditional symbolic link. The conditional link is useful for builds that run on non-containerized platforms (macOS) so that they don’t screw up the shared file system.

I found that moving $M2_HOME around was problematic specifically with regards to the Maven Wrapper (as you’ve seen) while the symbolic link was reliable in all cases.

[philwebb]

@nebhale @marcingrzejszczak I started putting together some generally useful scripts here. The setup_symlinks function will do what you want. You can see it being used here.

[marcingrzejszczak]

Nice! I'll definitely check this out :) Thanks @philwebb

[malston]

Got it... very straightforward approach. Thanks @philwebb @nebhale .

[nebhale]

There’s no real reason that these need to be hidden directories. In fact, it’s probably easier that they aren’t, so that when you fly hijack it’s obvious that they’re there and being used. I’d just call them gradle and maven and then link them to their hidden locations.

[malston]

Yep, makes sense.

[nebhale]

From an inspection-only perspective (I didn’t run it, so don’t know if it actually works 😉) this looks good to me.