bootstrap method constant pool entry type question

[Rémi Forax]

On 01/30/2011 09:08 AM, John Rose wrote:
> On Jan 29, 2011, at 11:40 PM, Kirill Shirokov wrote:
>
>> The invokedynamic instruction requires a MethodHandle constant pool
>> entry for a bootstrap method.
>>
>> This MH CP entry can be one of 9 REF_xxx types.
>>
>> Questions:
>>
>> Q1. Which types are valid in the case of bootstrap method?
>
> Any CONSTANT_MethodHandle for which the resulting MH can accept an 
> invokeGeneric call on the specified methods, which returns a CallSite.
>
>> Q2. What happens if I try to use invalid types?
>
> The MH will be materialized for the BSM, and the invokeGeneric call 
> will fail, either with WMTE or CCE.  This is an implication of the 
> specification.  Some of the details are spelled out here in the javadoc:
>
> After resolution, the linkage process may fail in a variety of ways. 
> All failures are reported by an BootstrapMethodError, which is thrown 
> as the abnormal termination of the dynamic call site execution. 
> The following circumstances will cause this:
>
> • the index to the bootstrap method specifier is out of range
> • the bootstrap method cannot be resolved
> • the MethodType to pass to the bootstrap method cannot be resolved
> • a static argument to the bootstrap method cannot be resolved (i.e., 
> a CONSTANT_Class, CONSTANT_MethodType, orCONSTANT_MethodHandle argument cannot 
> be linked)
> • the bootstrap method has the wrong arity, causing invokeGeneric to 
> throw WrongMethodTypeException
> • the bootstrap method has a wrong argument or return type
> • the bootstrap method invocation completes abnormally
> • the result from the bootstrap invocation is not a reference to an 
> object of type CallSite
> • the target of the CallSite does not have a target of the 
> expected MethodType
>
>> I could not find answers in the current specification.
>
> Since we're editing it furiously, maybe you looked at an earlier spec. 
> which didn't have the language above.
>
>> I guess that the correct answers would be:
>>
>> A1. The valid types are: REF_getStatic, REF_invokeStatic (? and maybe
>> REF_newInvokeSpecial). All others are invalid for bootstraping.
>
> Not getstatic.  That is always a zero-arity method, which will not 
> accept the 3 or more arguments passed to the BSM.
>
> Yes, invokestatic and newinvokespecial are the key use cases.
>
> Since the Lookup object comes first in the list, if the Lookup class 
> had a method which could accept a name and type and then return a call 
> site, that would work also, as a REF_invokevirtual.  But there is no 
> such method.

And Lookup is final.

>
>> A2. If the type is invalid, JVM throws an exception (and doesn't try to
>> call bootstrap MH taking arguments from Java stack)
>
> It tries to call the MH, and immediately fails.  The failure is 
> wrapped in a bootstrap method error, which is thrown.

If the failure is an error (a subtype of Error), should it wrapped too ?
The backport doesn't wrap errors.

>
> Good questions.  Thanks.
>
> -- John

Rémi

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