Spec: ACC_MANDATED

[Jonathan Gibbons]

On 11/22/2019 12:04 PM, Alex Buckley wrote:
> (This question was asked internally, and it wasn't clear if it was 
> about JEP 359 (Records) specifically or ACC_MANDATED more broadly. 
> Since the question is being driven by JEP 359 changes, and since JEP 
> 359 is in any case the next feature which will change the JVMS, let's 
> discuss on amber-spec-experts, NOT on compiler-dev. I have REMOVED 
> compiler-dev from the header.)
>
> On 11/22/2019 11:22 AM, Leonid Kuskov wrote:
>> So specification allows RI  to mark final fields associated with 
>> components and some methods by ACC_MANDATED flag (0x8000). The latest 
>> JVMS specification 
>> (https://docs.oracle.com/javase/specs/jvms/se13/html/index.html) 
>> permits this flag only for 2 attributes: MethodParameters_attribute, 
>> Module_attribute . The flag is not mentioned in both tables: Table 
>> 4.5-A. "Field access and property flags", Table 4.6-A. "Method access 
>> and property flags".
> ...
>> Does it make sense to add a definition of ACC_MANDATED to the tables?
>
> Good question. The mask 0x8000 is presently defined as ACC_MODULE in 
> ClassFile.access_flags, but it would be legitimate to define the same 
> mask as ACC_MANDATED in {field_info,method_info}.access_flags. This 
> would mirror the situation for the mask 0x0020, which is defined as 
> ACC_SUPER in ClassFile.access_flags but defined as ACC_SYNCHRONIZED in 
> method_info.access_flags. I'm sure we have discussed ACC_MANDATED 
> before so I'm not sure why the JEP 359 JVMS draft is silent on the 
> matter.
>> And:
>>
>> Earlier the spec stated that the Enum.values() and Enum.valueOf() are 
>> exceptions to the requirement that a class member that does not 
>> appear in the source code must be marked using a Synthetic attribute, 
>> or have its ACC_SYNTHETIC flag set.
>> Now names of methods are removed and the more loose statement 
>> "mandated members of enums and records" is used. Does it mean that 
>> spec won't enumerate "mandated" methods anymore?
>
> JVMS 4.7.8 should cross-ref to all JLS sections which define mandated 
> members. JVMS 4.7.8 should not list them explicitly. (There are 
> numerous "JLS §..." cross-refs in JVMS ch.4, they are quite legitimate.)
>
>> And the setting of this flag will be implementation-specific? 
>
> No, the presence of mandated members is JLS-defined. Search for 
> "mandated" in JLS 13.1 to dispel any notion that mandated members are 
> implementation-specific.
>
>> Should the JCK signature test take into account the ACC_MANDATED flag?
>
> If ACC_MANDATED is added to {field_info,method_info}.access_flags, 
> then yes, its presence on certain members of enum and record types is 
> required.
>
> Alex

Could someone also specify definitively the behavior when a user chooses 
to explicitly define a method, such as `equals` or `hashCode` for a 
record.   In other words, just because a method may be mandated in JLS, 
I'm expecting that this does not imply the use of ACC_MANDATED in those 
situations where the user explicitly defines the method.

-- jon