Record attribute binary format

Maurizio Cimadamore maurizio.cimadamore at
Fri Oct 11 22:50:12 UTC 2019

On 11/10/2019 22:19, forax at wrote:
> ----- Mail original -----
>> De: "Brian Goetz" <brian.goetz at>
>> À: "Remi Forax" <forax at>, "amber-spec-experts" <amber-spec-experts at>
>> Envoyé: Vendredi 11 Octobre 2019 22:57:23
>> Objet: Re: Record attribute binary format
>> No need.  Varargs-ness is reified with the (mandated) canonical ctor.
> hum, this looks like a hack, it means you can not just read the Record attribute, you have to crawle all the methods to find the primary constructor.

It _looks_ like an hack, but I think that, after some more thinking it's 
actually more correct than what you are proposing. A record contains a 
bunch of components, the components have a name and a type. Vararg-ness  
is immaterial here; both

record Foo(Object[] arr) { ... }


record Bar(Object... arr) { ... }

Denote the same underlying state (e.g. an Object array). The only 
difference between Foo and Bar is that their _construction protocol_ is 
different (which might also mean, in the future, that Bar will get more 
liberal _deconstruction_ protocols too!); but this is cleanly captured 
in the canonical constructor method_info.

Pedagolical test: would an 'isVarargs()' predicate on the core 
reflection record component be acceptable? Ugh, I don't think so. 
Varargs is a method concept; record components are a bit ephemeral, yes, 
but they are definitively _not_ methods.


> and if we take a look to the past, we have added access flags to methods, fields and classes, so there is a non negligible chance that we will want to do the same to record components in the future.
> Rémi
>> On 10/11/2019 4:43 PM, Remi Forax wrote:
>>> It seems we have to add an access_flag for each record component because we have
>>> to mark if the record component is a varargs or not.
>>> So the format should be
>>> Record_attribute {
>>>     u2 attribute_name_index;
>>>     u4 attribute_length;
>>>     u2 number_of_record_components;
>>>     {   u2 component_access_flags;
>>>         u2 component_name_index;
>>>         u2 component_descriptor_index;
>>>         u2 attribute_count;
>>>         attribute_info attributes[attributes_count];
>>>     }
>>> }
>>> with the component_access_flags allowing ACC_VARARGS.
>>> Rémi

More information about the amber-spec-experts mailing list