Queries and patch for JDK-8034854: outer_class_info_index of synthetic class is not zero

[Jan Lahoda]

Hi Alex,

Thanks for the comments.

I was briefly considering filling some inner_name for the synthetic 
classes, but using zeroing outer_class_info_index seemed somewhat 
cleaner, safer (no risk of name clashes or misinterpretation of the 
name) and simpler. But if generating an inner_name for the synthetic 
classes would be (strongly) preferred, I can investigate it.

Jan

On 02/19/2014 08:13 PM, Alex Buckley wrote:
> Hi Jan,
>
> The requirement that outer_class_info_index must agree with
> inner_name_index w.r.t. an anonymous class was added in JVMS7 because we
> saw class files where they disagreed and it simply made no sense. The
> requirement was conditioned on 51.0 class files because we didn't want
> to break pre-7 class files with insensible InnerClasses.
>
> The auxiliary classes generated by javac appear to have a meaningful
> "owner" - your word - so it would seem appropriate to have a non-zero
> outer_class_info_index. Just generate a random name for
> inner_name_index. (The 4.7.6 text assumes the "original simple name" can
> be derived from source code, but that's not applicable for synthetic
> classes.) This change could reasonably affect all target levels, since
> no-one should be relying on the value of inner_name_index for these
> auxiliary classes.
>
> OTOH, your proposal to represent the auxiliary classes as true anonymous
> classes in InnerClasses is attractive because it exposes even less
> information than at present. This change could reasonably affect all
> target levels too, since no-one should be relying on the value of
> outer_class_info_index for these auxiliary classes.
>
> Alex
>
> On 2/19/2014 4:34 AM, Jan Lahoda wrote:
>> Hello,
>>
>> I have a few questions about JDK-8034854 and a possible patch/fix for
>> it. The bug URL:
>> https://bugs.openjdk.java.net/browse/JDK-8034854
>>
>> The problem is that while JVMS 7, 4.7.6. (The InnerClasses Attribute)
>> mandates that:
>>   If a class file has a version number that is greater than or equal to
>>   51.0, and has an InnerClasses attribute in its attributes table, then
>>   for all entries in the classes array of the InnerClasses attribute,
>>   the value of the outer_class_info_index item must be zero if the value
>>   of the inner_name_index item is zero.
>> javac in some cases produces non-zero "outer_class_info_index" even if
>> "inner_name_index" is zero. This happens for synthetically generated
>> auxiliary classes. These classes are generated for a number of reasons,
>> for example to be used as tags when accessing private constructors. The
>> synthetic classes internally have an empty name, so the generated
>> "inner_name_index" is zero, but their owner is a class, so they get the
>> non-zero "outer_class_info_index".
>>
>> I've sketched out a simple fix for this problem, which ensures that
>> "outer_class_info_index" is zero for classes that have empty name:
>> http://cr.openjdk.java.net/~jlahoda/8034854/webrev.00/
>>
>> After this change, the generated synthetic classes look a lot like
>> anonymous classes defined in an initializer of the given class (based on
>> the InnerClasses attribute and the EnclosingMethod attribute). That
>> seems reasonable to me.
>>
>> My questions are:
>> -does the fix above make sense?
>> -the change affects all target levels. It seems to me that the new
>> behavior makes sense even for pre-7 classfiles, but I'll gladly limit
>> the new behavior to only some minimal target level if desired.
>>
>> Any comments welcome.
>>
>> Thanks,
>>     Jan