RFR: 8199149: Improve the exception message thrown by VarHandle of unsupported operation [v4]

[Jorn Vernee]

On Thu, 20 Jul 2023 01:04:08 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> `VarForm::getMemberName` currently throws UOE with no information if the requested access mode is unsupported.   To provide the var handle information, move the access mode check to `VarHandle` so that the exception message can include the var handle information.  Changes include:
>> 
>> 1. change `VarHandle::checkAccessModeThenIsDirect` to check if the access mode is unsupported.  This check is only needed for direct var handle.
>> 2. change `VarHandle::getMethodHandleUncached` to call `getMemberNameOrNull` and throw UOE with an informative exception message if the access mode is unsupported
>> 
>> The error message looks like:
>> 
>> java.lang.UnsupportedOperationException: compareAndSet is not supported for VarHandle[varType=java.lang.String, coord=[class Foo$Goo]]
>
> Mandy Chung has updated the pull request incrementally with one additional commit since the last revision:
> 
>   update copyright header

src/java.base/share/classes/java/lang/invoke/VarHandle.java line 2010:

> 2008:         static AccessMode valueFromOrdinal(int mode) {
> 2009:             return VALUES[mode];
> 2010:         }

Also, I'll throw this out there, though I'm not sure how big of an issue it is: this array creation might have an effect on startup. But, since it is only used on a slow path right before we throw an exception (at least, at the moment), calling `values()` every time inside the `valueFromOrdinal` method, and avoiding the array creation on startup, might be 'better'.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14928#discussion_r1269616973