Spec: ACC_MANDATED

[Alex Buckley]

(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