Annotating lambda expressions

[Alex Buckley]

On 7/10/2015 2:31 AM, Maurizio Cimadamore wrote:
> On 10/07/15 09:43, Ron Pressler wrote:
>> (Related to
>> http://mail.openjdk.java.net/pipermail/compiler-dev/2015-January/009220.html)
>>
>> It is useful to add an annotation to a lambda expression -- as in
>> foo(@Special () -> { ...}) -- which will be applied to the static
>> method implementing the lambda (not to the interface the lambda
>> implements). This would allow instrumentation tools operating on
>> method implementations to handle lambdas. So far, AFAICT, this is not
>> possible (or am I wrong?).
> This is not possible, currently - as Remi said; you have annotations on
> declarations (i.e. variable, classes, methods) - you have annotation on
> types (i.e. type of a cast, type argument, etc.) - but you don't have
> annotations on _expressions_.
> I take it that, under the current translation scheme, there seems to be
> a plausible description for what should happen when the lambda is
> annotated; but what if the lambda is turned into a method reference (by
> some IDE refactoring, or human intervention) ? In that case it is not
> guaranteed that there would still be a physical place where to put the
> annotation (unless we tell the compiler to 'expand' all annotated method
> references).

Yes, the problem with annotations-on-expressions is not the language 
grammar but the storage of the annotations in the class file. With no 
standard way of translating source expressions to bytecode, there can be 
no standard API for retrieving annotations-on-expressions from bytecode. 
More broadly, there has never been a standard reflective API capable of 
inspecting method bodies.

This explains why JSR 308 stayed away from annotations-on-expressions -- 
we were not in a position to let them be read back. It was already bad 
enough that some annotations-on-types are stored in the class file at 
the level of individual bytecodes (e.g. an annotation on the type of a 
cast) so are not retrievable by any standard reflective API.

Alex