Preview Features: A Look Back, and A Look Ahead

[Alex Buckley]

It's been over four years since we introduced the idea of preview 
features in JEP 12 (https://openjdk.org/jeps/12), so it's a good time to 
review how they've worked out, and see if anything needs tweaking as we 
move forward.

Rather than post a lengthy write-up here on jdk-dev, I've created an 
informational JEP so it's easy to find in future:

   https://openjdk.org/jeps/8300604

We welcome comments on the revised expectations for preview APIs that 
are laid out near the end. Subsequently, I'll adjust JEP 12 as necessary 
to ensure there's a single source of truth about how previewing works.

We also welcome feedback on your experiences with preview features, 
especially those in JDK 19 and 20.

Alex