java.lang.classfile model enumeration order

[Brian Goetz]

So, it sounds easy to do that when you are reading from the classfile, 
since all the field_info structures are colocated.  But as soon as you 
start transforming, you may inject methods between the fields, or vice 
versa.  So in practice the utility of such guarantees is low, because it 
will come with caveats like "as long as you are the first guy in the 
transform pipeline."  So better to just tell people "you'll get all the 
elements, but with no ordering guarantee" -- they'll write better code 
as a result.


On 11/22/2024 1:50 PM, Chen Liang wrote:
> I think the interest in the order is more among the same kind of 
> elements instead of among different kinds of elements.
>
> ASM currently allows you to mix-and-match fields and methods in class 
> visiting; it just has weird requirements such as visitSource -> 
> visitModule -> visitNestHost -> visit other attributes.  Such 
> requirements are extremely error-prone and make little sense, and do 
> not exist in the ClassFile API.
>
> However, there might be value for the order for the same kind of 
> element in class file streaming.  For example, if you stream f1, m1, 
> m2, f2, you might want f1 before f2 and m1 before m2.  This order is 
> currently a complementary gift from the implementation, but nothing 
> guarantees that stay the way in the long run. The order of fields and 
> methods are not important in class file interpretation, after all.
>
> Regards,
> Chen Liang
> ------------------------------------------------------------------------
> *From:* Brian Goetz <brian.goetz at oracle.com>
> *Sent:* Friday, November 22, 2024 12:09 PM
> *To:* Mark Roberts <markro at cs.washington.edu>; Adam Sotona 
> <adam.sotona at oracle.com>; Chen Liang <chen.l.liang at oracle.com>; 
> classfile-api-dev at openjdk.org <classfile-api-dev at openjdk.org>
> *Subject:* Re: java.lang.classfile model enumeration order
> Correct.  ASM does guarantee an order of events, but our experience is 
> that such constraints do not help the user very much, are error-prone 
> to rely on, and can limit future flexibility for the implementation.
>
>
> On 11/22/2024 1:07 PM, Mark Roberts wrote:
>>
>> So to summarize: it’s true today, but don’t rely on it.
>>
>> Mark
>>
>> *From:*Brian Goetz <brian.goetz at oracle.com 
>> <mailto:brian.goetz at oracle.com>>
>> *Sent:* Friday, November 22, 2024 8:02 AM
>> *To:* Adam Sotona <adam.sotona at oracle.com 
>> <mailto:adam.sotona at oracle.com>>; Mark Roberts 
>> <markro at cs.washington.edu <mailto:markro at cs.washington.edu>>; Chen 
>> Liang <chen.l.liang at oracle.com <mailto:chen.l.liang at oracle.com>>; 
>> classfile-api-dev at openjdk.org <mailto:classfile-api-dev at openjdk.org>
>> *Subject:* Re: java.lang.classfile model enumeration order
>>
>> This is a property of the implementation, but not something that the 
>> spec guarantees, though.  Also, in the middle of a transformation 
>> pipeline, there is no longer a "classfile order" to fall back on.  So 
>> it is best to not assume anything about ordering, unless the ordering 
>> is part of the semantics (like the bytecodes in a method body.)
>>
>>
>>
>> On 11/22/2024 5:42 AM, Adam Sotona wrote:
>>
>>     Hi Mark,
>>
>>     Fields and methods are included in the ClassElement stream or
>>     iterator in the same order, and this order corresponds to their
>>     order in the class file.
>>
>>     This principle is common for the whole API, except for some
>>     PseudoInstructions identifying their start and stop positions  in
>>     the stream by Labels. These PseudoInstructions are always
>>     streamed before respective LabelTargets.
>>
>>     Adam
>>
>>     *From: *classfile-api-dev <classfile-api-dev-retn at openjdk.org>
>>     <mailto:classfile-api-dev-retn at openjdk.org> on behalf of Mark
>>     Roberts <markro at cs.washington.edu> <mailto:markro at cs.washington.edu>
>>     *Date: *Friday, 22 November 2024 at 1:12
>>     *To: *Chen Liang <chen.l.liang at oracle.com>
>>     <mailto:chen.l.liang at oracle.com>, classfile-api-dev at openjdk.org
>>     <mailto:classfile-api-dev at openjdk.org>
>>     <classfile-api-dev at openjdk.org>
>>     <mailto:classfile-api-dev at openjdk.org>
>>     *Subject: *java.lang.classfile model enumeration order
>>
>>     Are there any rules/guarantees that the order of items in
>>     ClassModel::methods() will match the order in ‘for (ClassElement
>>     ce : cm)’  for the ce elements that are MethodModel?  And also
>>     for similar items down the class hierarchy?
>>
>>     Thank you, Mark
>>
>
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