From: MRasmussen@perforce.com:

Retransforming or redefining a sealed class can lead to a JVMTI_ERROR_FAILS_VERIFICATION, with Exception <a 'java/lang/IncompatibleClassChangeError'{0x00000000feb820a0}: class autotest.TransparentRectangle cannot inherit from sealed class autotest.Rectangle> (0x00000000feb820a0), even though the class actually does that.

I've tried to investigate the reason, and from what I can tell, it boils down to the check in InstanceKlass::has_as_permitted_subclass.

In jdk-16+27, this method always seems to take the Symbol branch (_constants->tag_at(cp_index).is_klass() is false).
But for jdk-16+28, it takes the Klass branch after the classes are loaded, and from what I can tell the (k2 == k) comparison can then fail during the redefinition since the Klass stored in the _permitted_subclasses is not the same as the one being verified, as a new InstanceKlass is allocated for the stream parser.

Unfortunately, I haven't been able to produce a small self-contained test example, but I can reliably reproduce it while developing JRebel (which utilizes class retransformation), but hopefully the above is at least helpful in finding the issue and fixing it.

/Michael