RFR: JDK-8266670: Better modeling of access flags in core reflection [v6]

[Joseph D. Darcy]

On 2/17/2022 8:34 AM, Roger Riggs wrote:
> On Thu, 17 Feb 2022 01:38:42 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 ten additional commits since the last revision:
>>
>>   - Update JVMS references.
>>   - Merge branch 'master' into JDK-8266670
>>   - Reorder constants by mask value per review feedback.
>>   - Merge branch 'master' into JDK-8266670
>>   - Respond to more review feedback.
>>   - Respond to review feedback explicitly stating returned sets are immutable.
>>   - Fix typo from review feedback.
>>   - Merge branch 'master' into JDK-8266670
>>   - JDK-8266670: Better modeling of modifiers in core reflection
> The jvms also has `access_flags` for parameters.
> [4.7.24. The MethodParameters Attribute](https://docs.oracle.com/javase/specs/jvms/se17/html/jvms-4.html#jvms-4.7.24)

I'll augment/edit the spec describing where the flags come from to 
mention method parameters.

>
> Even if java.lang.reflect.Parameter is not a "Member", it would be useful for Parameter to have an `accessFlags()` method and loosen up the spec for AccessFlag to allow its use in Parameter.
> Its access flags have the same underlying bit patterns and definitions.

An accessFlags method on Parameter has been part of this proposal since 
the first push:

https://openjdk.github.io/cr/?repo=jdk&pr=7445&range=00#udiff-5-src/java.base/share/classes/java/lang/reflect/Parameter.java

-Joe