Converting MethodHandle into reflective Method instance

[MacGregor, Duncan (GE Energy Management)]

On 11/01/2013 14:16, "Eric Bodden" <eric.bodden at ec-spride.de> wrote:

>Thanks Duncan, for the fast response.
>
>To clarify, when you say...
>> The call site created by a bootstrap method will normally
>> change its own target during execution into a a guardWithTest chain
>>which
>> would dispatch to several targets, and when it gets too big for that may
>> start fetching and directly invoking other method handles or passing
>>them
>> back to an exact invoker.
>... do you mean that this is implemented with in the java.lang.invoke
>API or do you mean that client implementations using that API will
>normally generate such a guardWithTest chain?

Yes. :-) MutableCallSites (which are specified in the java.lang.invoke
APIs) support setTarget() to set the callsite's target, and there are a
set of methods on java.lang.invoke.MethodHandles for combining method
handles in various ways. I would expect almost all language
implementations using method handles would make quite heavy use of those
APIs, and most will bootstrap to a call site with some sort of fallback
target and change that target on the first invocation to a guardWithTest.

>> I think your only real option is intercept the calls that get the method
>> handles in the first place, so you'd need to transform most calls to
>> Lookup objects and unreflect(). That shouldn't be too hard a bit of code
>> transformation to do.
>
>That's a good idea which we can try. Unfortunately it will require
>modifications to the JDK. That's something I was trying to avoid.

Could you not do this by transforming the code outside the JDK that calls
those methods? I assume you have to do something similar to existing code
which uses reflection to avoid it bypassing your advice-execution code.

Oh, and I've just realised you'll also have to do something about any
MethodHandles that have been compiled into a classes constant pool as
well. Some of those will be the bootstrap methods for invokeDynamic
instructions, but other things could have been stuffed in their.

Regards, Duncan.