As discussed during the EG meeting, I've pulled together some examples [1] that change the classfile version number during retransformation without changing anything else in the file. The results were a little surprising to me in how different OpenJ9 and Hotspot behave in these cases as I recall this being a common "solution" used by bytecode agents in the Java 7 time frame to allow their classes to load despite faulty StackMapTables by falling back to the flow verifier.
There are two examples: * one that back-dates a Java 8 classfile to Java 6 or 7 and this fails with a ClassFormatError on both OpenJ9 and Hotspot. * one that back-dates a Java 16 classfile that uses NestMates to Java 8 and here the behaviour between OpenJ9 and Hotspot diverge. Hotspot appears to parse the attributes only if they apply to a given classfile version and thus the redefinition fails as it sees the Nest* attributes as being removed after the version change. OpenJ9 on the other hand continues to parse the attribute and allows the redefinition to succeed. The interesting thing - at least to me - is that both behaviours appear to be valid in the current spec. There's an interesting bit of unspecified behaviour here which treats classfile attributes as both a property of the VM version being run and of the classfile version being parsed. The spec isn't clear on whether the VM must take both into account when determining if a class is valid or not. This hole appears to have shrunk a bit over time as the spec has spent more ink on when attributes were added but would only close with the proposed update. --Dan [1] https://github.com/DanHeidinga/version_change On Fri, Jun 4, 2021 at 12:42 PM Dan Smith <daniel.sm...@oracle.com> wrote: > > Posted a new JEP draft, here: > > http://openjdk.java.net/jeps/8267650 > > This is some preparatory work to allow us to more cleanly introduce the > variety of new JVM features coming with primitive objects (JEP 401). > > Most of the spec work is already done; you can browse the proposed spec > changes here: > http://cr.openjdk.java.net/~dlsmith/8267650/8267650-20210603/specs/ >
