On Fri, 23 Jan 2026 20:16:12 GMT, Serguei Spitsyn <[email protected]> wrote:
>> src/jdk.jdi/share/classes/com/sun/tools/jdi/VirtualMachineImpl.java line 838:
>>
>>> 836: }
>>> 837:
>>> 838: public boolean supportsValueClasses() {
>>
>> What happens if not running in preview mode? Value classes are not supported
>> in that case, but as implemented this API will say they are supported,
>> leading to isValueClass() looking at modifiers in a way that is not
>> compatible when value classes are not supported.
>>
>> JDI didn't really run into this type of problem with virtual thread support.
>> It did modify a couple of new public APIs with:
>>
>> @PreviewFeature(feature = PreviewFeature.Feature.VIRTUAL_THREADS)
>>
>> However, I don't see how that helps us here. It seems we need to query the
>> debug agent to find out if value classes are supported. There's no good way
>> to do that. Is there any way for JDI to somewhat detect that value class
>> support is supported (preview mode is enabled).
>
>> What happens if not running in preview mode? Value classes are not supported
>> in that case, but as implemented this API will say they are supported,
>> leading to isValueClass() looking at modifiers in a way that is not
>> compatible when value classes are not supported.
>
> It should work correctly. The IDENTITY modifier bit is not used for other
> purposes in JDK 27 (Alex, please, fix me if it is wrong). So, the check for
> version >= 27 is needed. If preview is not enabled in target VM, so it does
> not support value classes then the IDENTITY modifier bit will be always set
> which results in `isValueClass()` to always return `false`.
So in JDK 27, with or without enable preview, the IDENTITY modifier bit is
accurate and can be looked at, but in previous JDK versions it might be used
for something else?
-------------
PR Review Comment:
https://git.openjdk.org/valhalla/pull/1834#discussion_r2722726810
