Preview APIs in the Java Platform

[Chris Hegarty]

>>> ...
>>> On 03/03/2020 21:15, Alex Buckley wrote:
>>>> I intend to update JEP 12 to incorporate preview APIs in the near
>>>> future, hopefully in time for 15 so that projects such as Panama can
>>>> benefit from them.


When working on the HTTP Client ( incubated in JDK 9, re-incubated in
JDK 10, and standardized in Java 11 ), it would have been cosmetically
nice to have been able to locate the API in java.net.http from the
beginning, but in reality having it in its own incubator module/package
didn't burden the implementation all that much. As was said previously,
this is because the HTTP Client is largely a "user-level" library, which
has very little ( if any ) coupling with java.base.

When working on JEP 11 ( Incubator Modules ), we knew that features
requiring tight coupling with java.base ( or the VM itself ) would
likely emerge, and that squeezing such features into the incubation
mechanism would require some "implementation pain". This seemed like an
acceptable trade-off at the time, to enable the opportunity to better
solicit feedback for such features. Remember, all this predates JEP 12.

Fast forward to where we are now, and given the issues that the Foreign
Memory API has had to deal with to squeeze into incubation, maybe there
is a better way of achieving the goal - to gather feedback before
standardizing. With the move to already have Preview Feature related
API elements in java.*, then it seems ( to me ) a natural progression
to expand this to, what are effectively, Preview APIs.

Would HTTP Client have previewed ( rather than incubated )? Impossible
to say for sure, but as one of the developers working on the feature, it
is my opinion that we would not have previewed HTTP Client in 9. The
reason being that I do not think the feature would meet the higher
completeness level required by Preview. If Preview was an option back
in the 9 time frame, then, again this is just my opinion, I think I
would have held off incubating in 9 and iterated a little longer so that
it could preview in 10. ( yes, while I said it would have been
"cosmetically nice" to have used java.net.http from the beginning,
avoiding the temporary incubation name would likely have avoided some
developer confusion, which for me, on balance, would have been worth
doing )

---

If there is a specific issue with recognising if preview features are
enabled or not, then maybe that issue can be addressed by adding a
property, API, or some such, that will reveal the enable-preview status
at run-time.

-Chris.