Title clarification: OpenJDK specifically, there are many name-branded JDKs in circulation.
It's not a trivial undertaking to compile OpenJDK from sources.
There are not very many JDK implementations that don't have OpenJDK sources at their core.
OpenJDK is also treated as the reference implementation, and is where JEP's and things are worked out. Effectively, OpenJDK is "upstream" Java.
uh? assuming you already have a working C toolchain and JDK installed, you just hg^Wgit clone; bash configure; make.
edit: you may have more trouble compiling it on windows, but compiling anything on windows is a pain unless it uses the entire proprietary non-portable microsoft-specific project management tools.
Apple is honestly giving Microsoft a run for their money here. Many FOSS apps seem to have given up on OSX and few ever tried with iOS.
I don't have a macbook. I can legally run a trial copy of windows in virtualbox to test out cross-compiling stuff, or spin up a windows EC2 instance for a few hours, or even use circleci or other CI system's windows workers to build stuff.
For macOS, I can't legally run a VM on my hardware, ec2 doesn't support it so I can't spin up a VM, and most CI platforms don't support it (probably because many of them just resell fleets of ec2 instances).
Many FOSS developers are just hacking away on 3 year old laptops or desktops for fun, and can't afford a $2k investment just to support one more operating system for their little project.
I did that regularly some years ago for the company I work for, because we needed OpenJDK Windows binaries, and there wasn't such a convenient thing as AdoptOpenJDK. So in order to not get caught in stupid licensing debacles with Oracle it came down to just compile the Windows binaries by yourself.
Got some links to these GPL licensed OpenJDK forks?
OpenJ9 uses a whole different JVM even though they use the OpenJDK standard library.
Well yeah, especially not open source, as you can't get a TCK for non-OpenJDK open source implementations of the standard library. Oracle forbids it.
As in GPL violations? (I've never had the pleasure(?) to use java properly, so I'm not familiar)
For example Amazon Corretto is on GitHub, and Amazon gives a pretty good breakdown of the patches that make it different from the equivalent OpenJDK version (funnily enough, most of the changes are actually just backporting bug fixes from OpenJDK11 into Amazon's equivalent of OpenJDK8)
There's the Oracle JDK which has some onerous licensing, but IIRC the actual JDK is the same, they just include some extra binaries in the distribution