What can we improve in JSR292 for Java 9?

[Vladimir Ivanov]

>> * try/finally as a core atom of MethodHandles API.
>>
>> Libraries like invokebinder provide a shortcut API To generating the
>> large tree of handles needed for try/finally, but the JVM may not be
>> able to optimize that tree as well as a purpose-built adapter.
>
> I agree there.  We should put this in.
>
>     MHs.tryFinally(target, cleanup)(*a)
>       => { try { return target(*a); } finally { cleanup(*a); } }
>
> (Even here there are non-universalities; what if the cleanup
> wants to see the return value and/or the thrown exception?
> Should it take those as one or two leading arguments?)
Probably, it's too much. Result-agnostic cleanup logic looks like the 
cleanest solution from API perspective.

There are catchException and foldArguments/filterReturnValue to combine 
with. Or am I missing something?

>> * Implementation-specific inspection API.
>>
>> I know there are different ways to express a MH tree on different JVMs
>> (e.g. J9) but it would still be a big help for me if there were a good
>> way to get some debug-time structural information about a handle I'm
>> using. Hidden API would be ok if it's not too hidden :-)
>
> Idea of the day:  An ASM-like library for method handles.
> Make a MethodHandleReader which can run a visitor over the MH.
> The ops of the visitor would be a selection of public MH operations
> like filter, collect, spread, lookup, etc.
> Also ASM-like, the library would have a MethodHandleWriter
> would could be hooked up with the reader to make filters.
Not sure how useful it would be for an API user. I find it hard to 
recover MH transformation chains from MethodHandle trees, since the 
representation we use is quite low-level. They end up as trees of 
BoundMethodHandles and you can guess the intention only by carefully 
inspecting LambdaForms.

You can try it yourself by turning on 
-Djava.lang.invoke.MethodHandle.DEBUG_NAMES=true and calling 
MethodHandle.toString() on a method handle of your choice.

Best regards,
Vladimir Ivanov