Draft JVMS changes for Nestmates

Remi Forax forax at univ-mlv.fr
Tue Apr 18 20:12:16 UTC 2017

----- Mail original -----
> De: "Dan Smith" <daniel.smith at oracle.com>
> À: valhalla-dev at openjdk.java.net
> Envoyé: Mardi 18 Avril 2017 20:44:11
> Objet: Fwd: Draft JVMS changes for Nestmates

> For those not subscribed to (or not paying attention to) valhalla-spec-experts,
> I just posted a link to an updated draft of JVMS changes.
>> Begin forwarded message:
>> From: Dan Smith <daniel.smith at oracle.com>
>> Subject: Draft JVMS changes for Nestmates
>> Date: April 18, 2017 at 12:42:16 PM MDT
>> To: valhalla-spec-experts at openjdk.java.net
>> Hi, all.
>> I've uploaded a draft of JVMS changes for JEP 181 "Align JVM Checks with Java
>> Language Rules for Nested Classes" to:
>> http://cr.openjdk.java.net/~dlsmith/private-access.html
>> Some comments below on my thinking in drafting the spec text, and on the JEP
>> generally.
>> JEP name
>> I don't know how permanent JEP names are supposed to be, but I'd prefer a
>> different name at this point. Something like: "Expanded JVM Access to Private
>> Members"—shorter, focused on the feature itself rather than its relationship to
>> the Java language. Or maybe "Class Nests for Access to Private Members".
>> Terminology
>> The term "nest" is nice because it's short and mostly unspoiled by overloading
>> in this context (I think?); it's not great because it's informal and doesn't
>> mean anything the first time you hear it. I thought about something more
>> clinical like "access control context", but I'm not convinced that's an
>> improvement. How do others feel?
>> The JEP uses "nest top" to describe the class that nest members reference; I
>> prefer "host class", which better describes the class's role and isn't tied to
>> the Java "top level class" concept. I know we use "host class" internally in
>> Hotspot, perhaps when working with anonymous classes (of the JVM flavor), but I
>> think in that context it will ultimately mean the same thing? Are we
>> comfortable repurposing the term in this way?

The semantics of an anonynmous host class and a nest host class are different.
an anonymous host class is used for every access check while the nest host class is used 

Anyway, nest host class works for me.

>> I follow Brian's model when it comes to nest membership (5.4.4): every class
>> belongs to a nest, possibly (in the absence of MemberOfNest) the nest hosted by
>> itself. Many nests are singletons without any associated explicit attributes.
>> Verification of MemberOfNest
>> I include a discussion block about different options of validating MemberOfNest.
>> I think the consensus, and my preference, is to do it during verification of
>> the member class. (NestMembers, on the other hand, is never validated, except
>> as a side-effect of checking MemberOfNest.)

It means that a NestMembers can contains Class that are not yet/never defined.
It's not a problem for me.

>> Verification of invokespecial
>> Allowing invokespecial to refer to classes other than the current class and its
>> supers is a significant change. I noticed and relied heavily on the parallel
>> with invokevirtual making protected method calls. So I tried to make the two as
>> similar as possible. In a few places, the treatment of protected methods
>> doesn't seem ideal, and rather than trying to mirror that with non-private
>> invokespecial, I modified the protected method treatment.
>> The "protected check" of verification, in particular, was a mess before, and I
>> think I've made it a lot more manageable, and compatible with a parallel rule
>> for invokespecial. I could use some feedback on exactly what Hotspot's verifier
>> has been doing here, though, since I'm pretty sure it didn't match the old
>> specified rules.
>> MethodHandle resolution
>> The spec ( is vague about what errors can occur during MethodHandle
>> resolution. I assume any linkage error specified for the instruction can also
>> occur via MethodHandle resolution, and that will include failures due to
>> invokespecial improperly referencing a class.

method handle behavior is the same as their corresponding bytecodes behavior, errors are converted to exceptions.  

>> Dynamic checking of invokespecial receivers
>> When invokespecial involves interface types, the verifier can't guarantee that
>> receiver objects actually implement that interface (JDK-8134358). It's an open
>> question whether this is really a problem, but I included a tentative fix in
>> the invokespecial runtime rules.

I'm not sure it's related to nestmate ?

>> Compiler changes
>> The JEP text can't seem to decide if compiler changes are part of it, or a
>> follow-up exercise. I think we're best off explicitly including the compiler
>> changes, which will provide opportunities for design validation and testing.
>> API changes
>> I haven't tried to address changes that need to be made to APIs. Somebody will
>> need to. For example, Lookup.findSpecial probably needs to make adjustments to
>> account for private members in nestmates, and to parallel the new
>> verification/linkage rules (e.g., follow Lookup.findVirtual in restricting the
>> receiver type).
>> Security risk
>> The JEP text should acknowledge that, while this does allow compilers to grant
>> finer-grained access to members shared by nestmates, it also pushes compilers
>> to grant broader access to members that were previously kept private. It's a
>> trade-off, and presumably a good one because nestmates are completely trusted,
>> while package-mates might sometimes be suspect.
>> (I guess this argument really ought to be made from the top: declaring things
>> with package access is worse than inventing a new level of access because
>> ________.)
> > —Dan


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