A fork can be many things.

I've maintained in house forks of major code bases with a handful of minor code changes while we've tried to convince upstream to accept them.

You don’t need to be big, you just need to handle lots of money.

fair point, I guess this was a high frequency trading firm or similar, who could justify paying a few engineers to maintain a jvm fork

Off-hand I think that there were at least 7 forks/implementations out there. Oracle/Sun, Oracle's Graal, IBM, OpenJDK, Amazon Coretto, Twitter's fork, and Azul. There's also a GNU effort but I don't think that really went very far. I think there's probably a few others that aren't publicly well known for trading firms, etc.

> there were at least 7 forks/implementations out there

There are 17 listed on SDKMAN's website: https://sdkman.io/jdks

> OpenJDK

As far as I know, all other distributions of the JDK are forks of OpenJDK. Would love to know if there's any that isn't.

Commercial embedded vendors with RTOS capabilities,

https://www.ptc.com/en/products/developer-tools/perc

https://www.aicas.com/products-services/jamaicavm/

JikesRVM (https://www.jikesrvm.org) no longer active, but it was its own thing.

Then you have Azul Zing and OpenJ9, which use parts of OpenJDK for the standard library part, but the foundations are their own thing.

https://www.azul.com/products/prime/

https://developer.ibm.com/components/open-j9/

Then you have that mobile OS, which isn't really JVM compliant implementation, but it is close enough for many workloads and libraries to work on top of it.

JetBrains also has a fork that they use to run some (all?) of their IDEs, such as CLion (C/C++) and IntelliJ (Java). As I understand, it has many fixes for HiDPI environments when painting GUI widgets (Swing, etc.).

SAP has also a fork: https://github.com/SAP/SapMachine.