GPL is a copyleft license, so if you would take linux kernel (same license), added some patches to it and you would distribute the resulting software, you would have to make sure your sources are available.

This would make Java as a runtime a very bad target for any company - they don’t want to share their application code - hence the classpath exception, giving you basically a boundary over which the copyleft doesn’t propagate over.

But the JVM itself still benefits from any updates made by any company.