JEP 442: Foreign Function & Memory API => why is it again preview API?

[Maurizio Cimadamore]

On 16/05/2023 16:38, Uwe Schindler wrote:
> It exists now since Java 14, where it first appeared as incubation. I 
> know theres still work on the foreign linker, but why aren't the 
> stable classes like MemorySegment, ValueLayout not public now?
Hi Uwe,

while I understand the frustration (and I'm grateful for the feedback 
you and others provided along the way), I think I disagree on this 
assessment. While it's true that layouts and segments have not changed 
much, the story about how to reason about the lifetime of a memory 
segment has. So, yes, MemorySegment per se has not changed, but the way 
in which you obtain segments has changed quite a bit.

Now, we could probably have finalized a subset of FFM which did *not* 
support linking of foreign function **and** deterministic deallocation. 
But would that have been useful? IIRC, Lucene has two main reasons to 
use FFM:

* deterministically unmap memory-mapped memory segments w/o using Unsafe
* mapping files that are bigger than 2GB

While a MemorySegment-only version of FFM would have allowed the latter, 
it would not have allowed for the former - meaning that there would be 
still part used by Lucene that would be under "preview".

So, while we have considered such a splitting move many times (believe 
me!), we always came up with the same conclusion: an FFM API w/o a story 
on lifetimes would not be very useful compared to what's already 
provided in the Java SE API.

Hopefully the lifetime-centric heavy-lifting is behind us. But I still 
think it would have been too risky to finalize the FFM API with a 
new-ish lifetime API that was not validated in the wild. As much as we 
try to be responsible with API changes, and do code analysis on the code 
using FFM code we know about, there's always a chance to miss something. 
For this reasons, I'm a strong believer that, when in doubt, the answer 
should always be "wait one more release" - at the end of the day that's 
why we have 6-month releases to begin with.

Cheers
Maurizio