CONSTANT_Class_info and L-envelopes

Brian Goetz brian.goetz at
Fri May 8 14:44:29 UTC 2020

It is sad, but reasonable, that we can't remove this bit of spec tech 
debt all at once.

I would step back and say: what is the goal of having this flag?  Is it 
to experiment with tools ability to deal with a mixed world while we try 
and move towards a unified one?  If so, then your 
inferred-mode-per-classfile is good; it enforces that file writers do 
one or the other, while allowing different tools to migrate at their own 

On 5/8/2020 9:15 AM, Frederic Parain wrote:
> Greetings,
> After the discussions of the past weeks about checkcast and the use of
> envelopes in CONSTANT_Class_info entries, a consensus has been reached
> for the case of Q-types. However, the case of L-types (should they use
> an L-envelope or not) is still subject to discussion. In one hand,
> forcing L-type to use envelopes would be a nice unifying move, in the
> other hand, it would have a bigger impact on tools, requiring a flag
> day approach instead of a smooth transition.
> The current approach is to continue not to use L-envelopes in
> CONSTANT_Class_info entries, but because some people might want to
> experiment with L-envelopes, it has been suggested to add a VM flag
> to support them.
> Having a global flag to require that all class files use the same
> format (either L-envelopes everywhere or L-envelopes nowhere) seems
> impractical because it would require to recompile all classes to
> switch from one mode to the other.
> Being more lenient and supporting both modes causes a new issue:
> the double syntax would allow a symbolic reference to a class to
> exist in two forms inside a single class file: one form with the
> L-envelope, one form without the L-envelope, creating two distinct
> constant pool entries. The problem with having two entries representing
> a symbolic reference to the same class is that there’s no guarantee
> that the resolution of these two entries will produce the same result.
> One solution to prevent this to happen would be to verify that a given
> class is never referenced through two different entries, but this would
> add a lot of checks during class file parsing.
> Another solution would be to have a “mode” per class file, either
> always using L-envelopes, or never using them for CONSTANT_Class_info.
> The class file doesn’t have to declare which mode it uses, this will
> automatically detected by the JVM while processing the first
> CONSTANT_Class_info entry, and then enforced for the remaining of the
> processing of the class file. A global VM flag controls if the VM
> must support the “L-envelope required” mode or reject it (default
> is to reject it).
> This solution has been prototyped here:
> It has been tested with manually modified class files, but has no unit
> tests provided with the changeset.
> Before submitting a pull-request, I’d like to have some feedback about this
> approach. If you think this approach makes sense, I can push it, otherwise
> I’ll drop it.
> Thank you,
> Fred

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