module annotations

Peter Levart peter.levart at
Wed Oct 10 07:33:47 UTC 2018

Hi Michał,

On 10/09/2018 09:11 PM, Alan Bateman wrote:
> On 09/10/2018 17:27, Michał Zegan wrote:
>> Hello,
>> I know that modules/module declarations can be annotated. Contrary to
>> other elements from packages downwards, java has a way to read module
>> descriptions before creating modules themselves (in fact it is a
>> required step). I am wondering, why doesn't ModuleDescriptor
>> contain/parse module annotations from class file? That way the
>> module-info.class has to be read twice, once for module descriptor, once
>> for annotations. In addition, if I would like to load annotations of a
>> module x before it is instantiated (like introspection), I would have to
>> still use a library like asm or similar.
> This is to avoid bloating the API and also to avoid overlapping with 
> the javax.lang.model API. So yes, if you are scanning observable 
> modules, maybe to select which modules to resolve based on 
> annotations, then you will need ASM or something to help you parse the 
> class files. It's the same thing for tools that scan classes or 
> elements for annotations.
> -Alan

In addition to what Alan says, I may add that I have tried to prototype 
an API that would allow reading of annotations at the ModuleDescriptor 
time, before Module is instantiated. I had the following problem: the 
API can't be the same as implemented in existing AnnotatedElement 
classes (Class, Method, Field, Constructor, Module, ...). Why? Because 
of chicken-egg problem. The annotation types used in a particular may be declared in the module itself. For constructing 
annotation objects (which is a necessity for AnnotatedElement API), the 
annotation types have to be loaded 1st and if they are declared in a 
module, the Module has to be instantiated first.

So this would technically be possible only with an entirely different 
API than what we have today in java.lang[.relfect]. Something like the 
javax.lang.model API.

That said, I wonder what is the status of JEP 119 today. If it was 
updated to support module(s) then I guess it uses the instantiated 
Module(s) to get the job done.

OTOH there's java compiler API that implements javax.lang.model too. I 
haven't investigated it but it might be possible to employ it to parse 
class files and provide you with javax.lang.model objects including 
annotations. Something to try...

Regards, Peter

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