Fields and methods of a record are marked MANDATED

Joe Darcy joe.darcy at
Fri Oct 11 17:16:03 UTC 2019


Some modifiers are implementation properties, like strictfp, native, and 
synchronized  -- they not part of the "interface" of the method per se, 
but still useful to apply and query in some circumstances. The 
"mandated" property can have a similar character as partially a function 
of the particulars of how the record was declared.

The mandated information about records is used by the printing 
processor, javac -Xprint, to provide a more source-faithful 
representation of a record by eliding the mandated elements. The 
regression tests


also looks at mandated vs explicit for record elements. At present, the 
test is only run against the source version of a record as the the 
information isn't (yet) available to the same degree from class file.



PS Lack of consistent mandated information around method parameters has 
made it difficult to address several long-standing issue in the core 
reflection API.

On 10/10/2019 10:41 AM, Alex Buckley wrote:
> Enum types are specified such that a `values` method is always 
> implicitly declared. (If you declare one explicitly, you have two 
> method declarations with override-equivalent signatures, which is an 
> error per JLS 8.4.) Accordingly, the corresponding method in the class 
> file should always be marked as mandated. Sadly we don't have room for 
> an ACC_MANDATED flag in `method_info`, but morally the method is 
> mandated and Core Reflection should expose that fact.
> Record types are specified more sensitively: a component's accessor is 
> implicitly declared if and only if it isn't explicitly declared. (Same 
> deal as the default constructor of a class.) If a component's accessor 
> is explicitly declared, then there's nothing more to say; if it's 
> implicitly declared, then it should be marked as mandated.
> I wouldn't characterize this as member descriptor v. member 
> implementation, because that sounds like "the signature" v. "the 
> body". Fundamentally, the topic at hand is component accessors, which 
> are non-abstract methods of non-abstract classes; for such methods of 
> such classes, EITHER you declare both the signature and the body (in 
> which case there's an explicit declaration of both signature and body) 
> OR you declare neither (in which case there's an implicit declaration 
> of both signature and body). Ordinary consumers of the record type are 
> happy because they can be assured that component accessors are always 
> declared (i.e., the compiler will always find the signature, the VM 
> will always link the descriptor, and the subsequent execution of the 
> linked method will always do something useful), while reflective 
> consumers of the record type are happy because they can tell whether a 
> component accessor (the signature and body as one undivided entity) 
> was declared explicitly or implicitly. You say "random", I say 
> "accurate".
> Alex
> On 10/10/2019 9:09 AM, Brian Goetz wrote:
>> Under that interpretation, that leaves record members in a funny 
>> place, since a given mandated member (e.g., an accessor for a 
>> component) _might_ have been explicit in the source, or might not 
>> have been.  Should ACC_MANDATED describe the member descriptor (“spec 
>> mandates a member with this descriptor”) or only the implementation 
>> (“the source didn’t have it, but its here in the byte code”)?  In the 
>> latter interpretation, the presence of ACC_MANDATED on a mandated 
>> member would basically be random, based on implementation-of-the-day, 
>> which seems wrong.
>>> On Oct 10, 2019, at 12:06 PM, Joe Darcy <joe.darcy at> wrote:
>>> A mandated construct is one that is mandated by the specification, 
>>> but not explicitly declared. Constructs of that sort have been in 
>>> the platform since the beginning, such as default constructors. 
>>> ACC_MANDATED was added to the platform only more recently and has 
>>> some exposure through javax.lang.model.
>>> I recommend going forward ACC_MANDATED to be used more widely, on 
>>> all the mandated structures, including the values methods on enum 
>>> types, etc.
>>> Cheers,
>>> -Joe
>>> On 10/10/2019 8:50 AM, Brian Goetz wrote:
>>>> We should match the behavior of methods like `Enum::values`.
>>>>> On Oct 10, 2019, at 10:15 AM, Remi Forax <forax at> wrote:
>>>>> Hi all,
>>>>> fields and methods of a record are marked ACC_MANDATED which 
>>>>> contradict JLS 13.1.12 that explains that you can not use 
>>>>> ACC_MANDATED on field and method.
>>>>> regards,
>>>>> Rémi

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