RFR: JDK-8266670: Better modeling of access flags in core reflection [v20]

[Mandy Chung]

On Wed, 25 May 2022 00:35:24 GMT, Joe Darcy <darcy at openjdk.org> wrote:

>> This is an early review of changes to better model JVM access flags, that is "modifiers" like public, protected, etc. but explicitly at a VM level.
>> 
>> Language level modifiers and JVM level access flags are closely related, but distinct. There are concepts that overlap in the two domains (public, private, etc.), others that only have a language-level modifier (sealed), and still others that only have an access flag (synthetic).
>> 
>> The existing java.lang.reflect.Modifier class is inadequate to model these subtleties. For example, the bit positions used by access flags on different kinds of elements overlap (such as "volatile" for fields and "bridge" for methods. Just having a raw integer does not provide sufficient context to decode the corresponding language-level string. Methods like Modifier.methodModifiers() were introduced to cope with this situation.
>> 
>> With additional modifiers and flags on the horizon with projects like Valhalla, addressing the existent modeling deficiency now ahead of time is reasonable before further strain is introduced.
>> 
>> This PR in its current form is meant to give the overall shape of the API. It is missing implementations to map from, say, method modifiers to access flags, taking into account overlaps in bit positions.
>> 
>> The CSR https://bugs.openjdk.java.net/browse/JDK-8281660 will be filled in once the API is further along.
>
> Joe Darcy has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 32 additional commits since the last revision:
> 
>  - Target JDK 20 rather than 19.
>  - Merge branch 'master' into JDK-8266670
>  - Add mask values to constants' javadoc.
>  - Implement review feedback from mlchung.
>  - Fix type in @throws tag.
>  - Merge branch 'master' into JDK-8266670
>  - Respond to review feedback.
>  - Merge branch 'master' into JDK-8266670
>  - Make workding changes suggested in review feedback.
>  - Merge branch 'master' into JDK-8266670
>  - ... and 22 more: https://git.openjdk.java.net/jdk/compare/43363358...05cf2d8b

With the `AccessFlag` API, what is the role of the `Modifier` API going forward?   [Value Objects JEP](https://openjdk.java.net/jeps/8277163) defines the new `identity` and `value` modifiers.  [PR #698](https://github.com/openjdk/valhalla/pull/698) proposes to add `Modifier.IDENTITY` and `Modifier.VALUE` constants as  the `identity` and `value` modifiers are encoded in a class file using the `ACC_IDENTITY` and `ACC_VALUE` flags.  However, with the new improved `AccessFlag` API, the new flags will be defined in the `AccessFlag` API.  I think we should avoid adding the new flags in `Modifier` and leave it for the existing usage.  Use `AccessFlag` for new features.

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PR: https://git.openjdk.java.net/jdk/pull/7445