RFR: 8306697: Add method to obtain String for CONSTANT_Class_info in ClassDesc [v2]

[Chen Liang]

On Sat, 22 Apr 2023 18:40:45 GMT, Chen Liang <liach at openjdk.org> wrote:

>> Add a method `internalName` to `ClassDesc`, and unifies handling of string representation of a class constant in CONSTANT_Class_info via `ofInternalName` and `internalName` APIs, documented in `ClassDesc` itself. In particular, `ofInternalName` now accepts arrays.
>> 
>> The motivation of this API is that avoiding frequent String creations via caching (enabled by this new API, will be in a separate patch) would speed up Classfile API's [writing of simple class files](https://github.com/openjdk/jdk/blob/master/test/micro/org/openjdk/bench/jdk/classfile/Write.java) by 1/3. See https://mail.openjdk.org/pipermail/classfile-api-dev/2023-April/000296.html for more context.
>> 
>> This API is futureproof: for Valhalla's Q-types, it will return their string representation in CONSTANT_Class_info, which is most likely their full descriptor string.
>> 
>> Javadoc: https://cr.openjdk.org/~liach/8306697/java.base/java/lang/constant/ClassDesc.html
>
> Chen Liang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Unify ofInternalName and internalName, document about CONSTANT_Class_info, remove misleading JVMS 4.4.1 links

For API design, I have 3 choices for the accessor method:
1. Return Optional<String> only for internal names for classes or interfaces; Optional.empty() for primitives and arrays
2. Return Optional<String> suitable for CONSTANT_Class_info; Optional.empty() for primitives
3. Return String suitable for CONSTANT_Class_info; throw for primitives

Which route should I take? I personally reject 1 as internal names are meaningless if they are used alone without being part of CONSTANT_Class_info. For 2 and 3, the main problem is the overhead of Optional vs whether there are better ways to detect if a class cannot be encoded in CONSTANT_Class_info (for now it's only primitives, but valhalla might change that if non-null types are loadable but cannot be in CONSTANT_Class_info)

On second thought, I think I will go option 2; it will be in line with `Class.describeConstable()`; though the current check is based on `Class.isHidden()`, criteria may change when Valhalla is out, and `describeConstable()` is better suited for future API evolutions.

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PR Comment: https://git.openjdk.org/jdk/pull/13598#issuecomment-1526055365