Why the annotation processing API ?

Stéphane NICOLAS steff.nicolas at gmail.com
Thu May 26 16:02:13 UTC 2016

Thx Jonathan for pointing this. You are right, I was talking about the
language model API, not the annotation processing api itself.

So yeah, here I was suggesting that the language model should be replaced,
or there should be an abstraction layer that would make it look like the
reflection API. Developers would know that the Classes, Methods, Fields etc
exposed during annotation processing time by this new API are not the same
as reflection objects exposed during runtime. But still it would so much
easier to manipulate this reflection like API at the annotation processing

That's a simple suggestion, though it would imply a big change. Thx for
taking the time to answer to it.


2016-05-26 8:30 GMT-07:00 Jonathan Gibbons <jonathan.gibbons at oracle.com>:

> On 05/25/2016 09:32 PM, Stéphane NICOLAS wrote:
>> Hi folks,
>> I know a simple mail like this generally has very minimal value. But
>> still, it's been around 2-3 years now that I work on Android, doing a lot
>> of work around reflection and annotation processing.
>> I don't like the annotation processing API. I still find every concept
>> hard to remember, and it is always taking time to get immersed into it for
>> a new project.
>> Thus, I don't understand, very humbly and very honestly, why this API
>> even exists. Why can't we simply use reflection-like APIs ? It would not be
>> that complex to expose an API that looks like reflection (read-only of
>> course, no new instance of classes, to method calls or setting/getting
>> field values, just the discovery part).
>> I could see over the past years that the newer versions of the JDK are
>> aligning more and more the capacities of the annotation processing API with
>> reflection APIs. Some subtle changes but always in this direction. All you
>> can do with reflection is now possible with annotation processing
>> (read-only still, as stated above).
>> Does it sound completely stupid to ask that javac annotation processor
>> API for JDK 10 would actually match reflection API ? Reflection API is so
>> simple and it describes OO code in a natural way for all java coders.
>> Methods, classes, fields... much simpler than an element type with a kind
>> that you can cast into I don't know what ! Annotation processing API is not
>> clear. It is obviously overworked, complex, abstract, very scholar and hard
>> to use.
>> Is there anything that can be done with respect to this problem ?
>> I hope I am not saying something completely dumb here, but I have been
>> thinking about this for a very long time now.
>> S.
> You are probably conflating two concepts.  There is the "Annotation
> Processing API", javax.annotation.processing, and there is the "Language
> Model API", javax.lang.model.
> You can think of the Language Model API as "compile-time reflection".   It
> allows you to reflect over declarations without the constraints of the
> classes having to be loaded into class loaders, and it can also work on
> classes that only exist in source form. As such, it can even give access to
> documentation comments contained in the source code.   One notable feature
> is that you can use the Language Model API to reflect over different
> versions of classes than those in the current runtime.  For example, you
> can run javac on JDK 9 and use the Language Model API to examine the
> classes for JDK 8, and vice versa.
> The Annotation Processing API can be thought of as an API for compiler
> "plugins". It allows you to extend the capabilities of the compiler to
> examine the code being compiled, using the Language Model API, and (within
> some well defined limitations) it allows you to generate additional classes
> to be included in the compilation.
> All three APIs, the Annotation Processing API, the Language Model API, and
> the runtime-reflection API (java.lang.reflect) all have their place in the
> Java ecosystem.
> -- Jon
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