The curious case of MHS.Lookup.unreflect on MethodHandle.invoke/invokeExact

[Paul Sandoz]

On Mar 17, 2015, at 8:24 PM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:

> Paul,
>> A call to the following:
>> 
>>   Object o = rmh.invokeExact((MethodHandle) null, new Object[]{});
>> 
>> Will result in a:
>> 
>>   java.lang.UnsupportedOperationException: cannot reflectively invoke MethodHandle
>> 
>> However, the stack trace corresponds to the stack where the call to "unreflect" was performed and not where the invocation occurs.
> The reason is that the preconstructed exception is thrown and not created on every invocation:
>  mh = mh.bindTo(new UnsupportedOperationException("cannot reflectively invoke MethodHandle"));
> 

Yes.


>> Further it does "mh.withInternalMemberName(method, false)", that i cannot explain. Why do we need to re-associate the MH throwing the USO with the member name corresponding to the MH.invokeExact/invoke method?
> I think the main reason is to keep direct method handle cracking API (MethodHandles.revealDirect()) working for MethodHandle.invoke*.
> 
> Actual method handle structure in this case is more complex than a simple DMH, so additional trick with WrappedMember is needed to preserve an illusion an ordinary direct method handle is returned.
> 

Ah, i see, i had my suspicions in might be something to do with that.


>> For such edge cases perhaps caching is not required.
> Agree, caching shouldn't be important for such cases.
> 

Perhaps such method handles were originally cached to avoid an explosion (deliberate or otherwise) of class generation of LFs, but now there is more sophisticated LF caching in place this is not necessary.?

Thanks,
Paul.


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