Preview APIs in the Java Platform

[Alex Buckley]

On 3/5/2020 12:31 PM, Remi Forax wrote:
> I've a code that run on Java 13 using Unsafe to clean a ByteBuffer,
> the jdk 14 introduces the incubator module jdk.incubator.foreign, so
> i've written a code that uses a segment instead of unsafe if I detect
> that jdk.incubator.foreign is available, this code is compiled with
> the jdk 14 and works with the jdk 14 and jdk 15 (at least for now). 

I hope this code isn't something you're shipping to other people. 
Incubating APIs are meant to be used for personal exploration, not to 
build libraries that check the user's runtime and dive into incubating 
APIs if available.

> Now, let suppose that the memory access API is implemented using the
> proposed preview API, if i want to be able to use it for jdk 14 and
> 15, i need to first detect if --enable-preview is set (there is no
> API for that), and i need the same class to be compiled once per jdk,
> so one for jdk14 with preview enabled and one for jdk15 with preview
> enabled. But there is no version of javac able to compile those two
> classes at the same time.

So, run javac twice? And use an MRJAR to ship both class files? Wait, 
why am I saying this -- you know it perfectly well. You have a scenario 
involving support for multiple JDKs in one codebase that will get harder 
for you if we ship preview APIs instead of incubating APIs. That's 
unfortunate, but your scenario isn't one we wish to support.

> If i'm me, i will use bytecode patching but i'm pretty sure nobody
> sane will do that, so in practice very few people will test your API
> in real life.

OK, got it.

Alex