This is the whole point of the "classpath exception". You can run, build and distribute Java programs under any license you choose.
> But this also raises the question: what differs between the various JDK builds?
Very little typically. Some like RedHat or Amazon enable extra options, like the Shenandoah GC. The big difference is who you contact for support.
> what is the legal status of "OpenJDK" particularly WRT Oracle?
I'm not sure what this question even means, but frankly given Microsoft, IBM and Amazon are all building and distributing their own builds of OpenJDK I don't think there are any significant legal issues, these are all companies with teams of lawyers on retainer.
Meanwhile other vendors have "OpenJDK" builds: Amazon (corretto), Red Hat, etc. This means that they offer binaries that implement the OpenJDK specs.
My mental model is currecntly: So there's a spec and an implementation, OpenJDK can refer to both, and a vendor can have multiple implementations of OpenJDK (spec), and some of impls can be closed/commercial, as in the case of Oracle (and maybe Red Hat?).
OpenJDK is Oracle's reference implementation of Java, and provides a GPL+Classpath exception licensed build on their website. Additionally, Oracle provides a build a commercially supported build from their website. The commercial builds from Oracle have included other additions, but over the years the amount of closed sourced add-ons has decreased.
Other vendors provide builds of the OpenJDK, most include additional code, although for the most part the changes are minimal. All of these builds are based on the OpenJDK, and adhere to the GPL+Classpath exception.
There is also Azul Platform Prime/Zing. Which is there own JVM with an LLVM-based JIT, I'm not sure if they're using any OpenJDK code, but if so I assume they have a commercial agreement with Oracle.
Please educate yourself before spreading misinformation on this forum.
OpenJDK is not a "clone", a "fork", or some other re-implementation of the Oracle JDK. It's the other way around. OpenJDK IS the source code for the Java Virtual Machine and standard library. All binary distributions are based on this source.
Some binary distributions are freely available. Others include additional proprietary tools, or commercial support or whatever, and require paid licensing. But these all derive from the some common upstream.
You are painting a picture that is equivalent to saying that the Linux source code is a "clone" of RedHat Enterprise Linux. That is not accurate whatsoever.
If you'd like an example of how to correct someone's misunderstanding in a constructive manner, see sibling comments, which were posted prior to yours by an hour.
It is an open source project, comprising a codebase, a set of contributors, and various mailing lists, processes, and so on, just like any other open source project.
The codebase is a continuation of the original Sun (later Oracle) JDK, now licensed under GPL 2 [1], with a linking exception that makes it fairly similar to the LGPL [2].
The contributors are mostly employees of big companies, with Oracle having by far the largest share.
The OpenJDK project releases source code, but does not itself distribute binary builds. In this respect, it is like numerous other open source projects, such as GNU coreutils.
Binary builds are made and distributed by operating system distributors (Red Hat, Debian, FreeBSD, etc), and also by various independent groups. As usual, each distributor may or may not add its own patches before building, and may or may not contribute these upstream. Eclipse Adoptium (formerly known as AdoptOpenJDK) [1] does not apply significant proprietary patches (just branding stuff), so it's a good place to get a plain vanilla OpenJDK build. Amazon's Corretto [4] in principle could contain whatever patches Amazon thinks are cool, but in practice usually just has a few backported bugfixes [5]. Azul's Core (formerly known as Zulu) has a bunch of patches (i couldn't find a list), and an option of commercial support) [6]. Oracle have a couple of builds, but you can ignore them. There are also builds from SAP, Bellsoft, and maybe others, but i have never really looked into them. Most people i know just use the AdoptOpenJDK / Adoptium builds, or whatever their distro gives them.
EDIT: I got the Adoptium thing slightly wrong. AdoptOpenJDK made builds; those builds are now called Eclipse Temurin. Eclipse Adoptium is some sort of 'marketplace' where different builds are validated and made available. To me, this sounds like one of those Apache grand ideas that won't go anywhere, but we'll see.
[1] https://github.com/openjdk/jdk/blob/master/LICENSE
[2] https://opensource.stackexchange.com/questions/1410/what-is-...
[3] https://adoptium.net/releases.html
[4] https://aws.amazon.com/corretto/
[5] https://docs.aws.amazon.com/corretto/latest/corretto-11-ug/p...
But seriously, you don't see how this might be confusing? And what you've described is really only the tip of the iceberg. Who defines the specs? Who writes the compliance tests with the specs? Who runs those and certifies that implementations are compatible? What limits are there on entities that make builds, either on the patches they can apply (and still call it "Java") or the licenses they can adopt? What does "GPL with classpath exception" mean in the real world, that JDK-itself changes must be upstreamed, but linked applications need not be? What are the limits there?
The point is, getting an open source JDK binary is not easy, especially if you want to understand what you're getting. "OpenJDK" may exist as an OSS project, but I wouldn't call it typical. Not because it doesn't release binaries (although I just discovered it does, provided by Oracle for Windows/Linux only), but because of the license, the owner, and the owner's relationship to binary builders. The binaries you can get in practice have unknown additions from the vendor, and possible adjustments to the license (or is only Oracle allowed to make alternative licenses for an OpenJDK build?)
Compare java distribution to, say, node or python. Is it really fair to accuse someone who is confused of being an idiot?
The exception is very broad and fairly simple.
Azul has a good breakdown of it https://www.azul.com/blog/why-the-classpath-exception-is-so-...
The short answer though is if you link to a GPL+Classpath exception library you may distribute the results under any license you choose.
