From pm at goldmensch.dev  Tue Jul  8 03:30:35 2025
From: pm at goldmensch.dev (Goldmensch)
Date: Tue, 08 Jul 2025 03:30:35 +0000
Subject: Locally enabled preview features
Message-ID: <ohJVdvDRSGUsAVKd-j0115yxU1UaHpzCPMotVUKOiYujdFAzbU7hj6fama2Q_ZhLIIq28xKU8SZ6bpATIvy5lDs50mTWf3OaBGL4RjsZr54=@goldmensch.dev>

Hi folks!
I?m wondering if the idea of ?locally? enabled preview features came up yet. It would make it possible to enable certain preview features for a class, package or module by annotating it for example. Doing so would allow to use preview features much easier in production to test them, while keeping the usage ?scope? small so that changes can be easily integrated without risking the integrity of the codebase. Of course some feature like virtual threads (by the time they were in preview) could not be enabled in such way due to the consequences to ?non preview enabled code? but flexible constructor bodies, module import declarations, perhaps even scoped values and much more could be.

Was this idea ever considered yet and if yes, what are the points against it?

Best regards,
Nick Hensel
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From forax at univ-mlv.fr  Tue Jul  8 09:05:55 2025
From: forax at univ-mlv.fr (Remi Forax)
Date: Tue, 8 Jul 2025 11:05:55 +0200 (CEST)
Subject: Locally enabled preview features
In-Reply-To: <ohJVdvDRSGUsAVKd-j0115yxU1UaHpzCPMotVUKOiYujdFAzbU7hj6fama2Q_ZhLIIq28xKU8SZ6bpATIvy5lDs50mTWf3OaBGL4RjsZr54=@goldmensch.dev>
References: <ohJVdvDRSGUsAVKd-j0115yxU1UaHpzCPMotVUKOiYujdFAzbU7hj6fama2Q_ZhLIIq28xKU8SZ6bpATIvy5lDs50mTWf3OaBGL4RjsZr54=@goldmensch.dev>
Message-ID: <1353378114.37870818.1751965555884.JavaMail.zimbra@univ-eiffel.fr>

> From: "Goldmensch" <pm at goldmensch.dev>
> To: "amber-dev" <amber-dev at openjdk.org>
> Sent: Tuesday, July 8, 2025 5:30:35 AM
> Subject: Locally enabled preview features

> Hi folks!
> I?m wondering if the idea of ?locally? enabled preview features came up yet. It
> would make it possible to enable certain preview features for a class, package
> or module by annotating it for example. Doing so would allow to use preview
> features much easier in production to test them, while keeping the usage
> ?scope? small so that changes can be easily integrated without risking the
> integrity of the codebase. Of course some feature like virtual threads (by the
> time they were in preview) could not be enabled in such way due to the
> consequences to ?non preview enabled code? but flexible constructor bodies,
> module import declarations, perhaps even scoped values and much more could be.

> Was this idea ever considered yet and if yes, what are the points against it?

It was considered and rejected. 

The preview mechanism has to be rough / raw, so it conveys the fact that there is no guarantee of backward compatibility from a release to the next one. 

Also, the implementation of a feature may change from one version to the next, a feature can be implemented as only a javac change in one version and as a bytecode change in the next version, so the notion of "scope" of a feature does not really exist. 

> Best regards,
> Nick Hensel

regards, 
R?mi 
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From davidalayachew at gmail.com  Tue Jul  8 11:47:51 2025
From: davidalayachew at gmail.com (David Alayachew)
Date: Tue, 8 Jul 2025 07:47:51 -0400
Subject: Locally enabled preview features
In-Reply-To: <1353378114.37870818.1751965555884.JavaMail.zimbra@univ-eiffel.fr>
References: <ohJVdvDRSGUsAVKd-j0115yxU1UaHpzCPMotVUKOiYujdFAzbU7hj6fama2Q_ZhLIIq28xKU8SZ6bpATIvy5lDs50mTWf3OaBGL4RjsZr54=@goldmensch.dev>
 <1353378114.37870818.1751965555884.JavaMail.zimbra@univ-eiffel.fr>
Message-ID: <CAA9v-_Os8BOx1PfENPkndUAL_ecHJbb7-4GHsVUSwN1_4N62sQ@mail.gmail.com>

To add onto that, having the option as a commandline makes it 100%
unavoidable for the person running the code to know that they are running
preview code. That's the entire point of making it a CLI option vs an
annotation -- so that the person running the code can be made aware of the
risk.

On Tue, Jul 8, 2025, 5:06?AM Remi Forax <forax at univ-mlv.fr> wrote:

>
>
> ------------------------------
>
> *From: *"Goldmensch" <pm at goldmensch.dev>
> *To: *"amber-dev" <amber-dev at openjdk.org>
> *Sent: *Tuesday, July 8, 2025 5:30:35 AM
> *Subject: *Locally enabled preview features
>
> Hi folks!
> I?m wondering if the idea of ?locally? enabled preview features came up
> yet. It would make it possible to enable certain preview features for a
> class, package or module by annotating it for example. Doing so would allow
> to use preview features much easier in production to test them, while
> keeping the usage ?scope? small so that changes can be easily integrated
> without risking the integrity of the codebase. Of course some feature like
> virtual threads (by the time they were in preview) could not be enabled in
> such way due to the consequences to ?non preview enabled code? but flexible
> constructor bodies, module import declarations, perhaps even scoped values
> and much more could be.
>
> Was this idea ever considered yet and if yes, what are the points against
> it?
>
>
> It was considered and rejected.
>
> The preview mechanism has to be rough / raw, so it conveys the fact that
> there is no guarantee of backward compatibility from a release to the next
> one.
>
> Also, the implementation of a feature may change from one version to the
> next, a feature can be implemented as only a javac change in one version
> and as a bytecode change in the next version, so the notion of "scope" of a
> feature does not really exist.
>
>
> Best regards,
> Nick Hensel
>
>
> regards,
> R?mi
>
>
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From chen.l.liang at oracle.com  Tue Jul  8 14:03:58 2025
From: chen.l.liang at oracle.com (Chen Liang)
Date: Tue, 8 Jul 2025 14:03:58 +0000
Subject: Locally enabled preview features
In-Reply-To: <ohJVdvDRSGUsAVKd-j0115yxU1UaHpzCPMotVUKOiYujdFAzbU7hj6fama2Q_ZhLIIq28xKU8SZ6bpATIvy5lDs50mTWf3OaBGL4RjsZr54=@goldmensch.dev>
References: <ohJVdvDRSGUsAVKd-j0115yxU1UaHpzCPMotVUKOiYujdFAzbU7hj6fama2Q_ZhLIIq28xKU8SZ6bpATIvy5lDs50mTWf3OaBGL4RjsZr54=@goldmensch.dev>
Message-ID: <SJ2PR10MB76695FCE771137AA5529B945A24EA@SJ2PR10MB7669.namprd10.prod.outlook.com>

I think there are something related to your problem - the multi-release jars do attempt to increase integration with different feature sets, and in fact, JLS requires class files that is compiled with --enable-preview but did not make use of any preview features to have their minor version set to 0. So to control the scope, I think either using multiple compilation (multiple source sets) or checking the minor version in class files to determine the scope works.

As to your proposal - you see others have already listed a few arguments against it, and it is not the only way to address your problem. I hope the two existing solutions I listed above can somewhat alleviate the problem you face.

Regards,
Chen
________________________________
From: amber-dev <amber-dev-retn at openjdk.org> on behalf of Goldmensch <pm at goldmensch.dev>
Sent: Monday, July 7, 2025 10:30 PM
To: amber-dev at openjdk.org <amber-dev at openjdk.org>
Subject: Locally enabled preview features

Hi folks!
I?m wondering if the idea of ?locally? enabled preview features came up yet. It would make it possible to enable certain preview features for a class, package or module by annotating it for example. Doing so would allow to use preview features much easier in production to test them, while keeping the usage ?scope? small so that changes can be easily integrated without risking the integrity of the codebase. Of course some feature like virtual threads (by the time they were in preview) could not be enabled in such way due to the consequences to ?non preview enabled code? but flexible constructor bodies, module import declarations, perhaps even scoped values and much more could be.

Was this idea ever considered yet and if yes, what are the points against it?

Best regards,
Nick Hensel


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