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

[Chen Liang]

On Wed, 19 Jul 2023 00:12:53 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]]

src/java.base/share/classes/java/lang/invoke/IndirectVarHandle.java line 93:

> 91:     @ForceInline
> 92:     boolean checkAccessModeThenIsDirect(VarHandle.AccessDescriptor ad) {
> 93:         if (exact && accessModeType(ad.type) != ad.symbolicMethodTypeExact) {

Can we add a comment that the detailed UOE is thrown via `directTarget.getMethodHandleUncached`?

src/java.base/share/classes/java/lang/invoke/VarHandle.java line 2020:

> 2018: 
> 2019:         static AccessMode valueFromOrdinal(int mode) {
> 2020:             return modeToAccessMode.get(mode);

Can't this be simplified to `AccessMode.values()[mode]` since this is only rarely used in the exception logic?

If we do need a cache, I would recommend a stable array like

private static final @Stable AccessMode[] VALUES = values();

and users call this accessor instead.

The code in `getMethodHandleUncached` can benefit from this caching mechanism.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14928#discussion_r1267424106
PR Review Comment: https://git.openjdk.org/jdk/pull/14928#discussion_r1267422748