RFR 8145964: NoClassDefFound error in transforming lambdas

[David Holmes]

Hi Dan,

This is going somewhat OT but should probably be discussed somewhere:

 > When an agent acquires can_redefine_any_class or
 > can_retransform_any_class, it is telling the VM that it plans to use
 > redefine classes or class transformation on as many classes as possible
 > so it would be a good idea to disable some optimizations like sharing.

That is very interesting but there is absolutely nothing in the JVM TI 
specification that I can see that even alludes to such a meaning. That 
is a flaw in the spec. I also note that the spec is very clear that 
implementations of JVM TI can impose their own restrictions on how 
capabilities can be applied, yet there seems to be absolutely no 
documentation of what those restrictions may be in our implementation. 
There is no way for anyone writing an agent to know a-priori whether 
they need the "any" capability or not!

There's also an issue with the description in "Class File Load Hook" but 
I'll take that up in response to Coleen's actual changes.

Thanks,
David

On 16/08/2016 10:38 PM, Daniel D. Daugherty wrote:
> Hi David,
>
> Need to chime in on a couple of parts from earlier in the thread.
>
>
> On 8/14/16 10:05 PM, David Holmes wrote:
>> Hi Dan,
>>
>> On 15/08/2016 12:20 PM, Daniel D. Daugherty wrote:
>
> <snip>
>
>>
>>> There are various places that refer to can_redefine_any_class. Those
>>> places
>>> also need updating. Here's the one from RedefineClasses:
>>>
>>>> can_redefine_any_class    Can modify (retransform or redefine) any
>>> non-primitive
>>>>                           non-array class. See IsModifiableClass.
>>>
>>> Should probably be changed to something like:
>>>
>>> can_redefine_any_class    Can modify (retransform or redefine) any class
>>> except for
>>>                           a few specific classes. See IsModifiableClass
>>> for the classes
>>>                           that cannot be modified.
>>
>> <sigh> This is so frustrating. Why even bother with the notion of
>> IsModifiableClass (which suggests any class might be non-modifiable
>> under some circumstances) when it is then completely undermined by the
>> can_redefine_any_class capability? If the intent was that only
>> primitives and arrays are ever non-modifiable then that should have
>> simply been listed in the spec for RedefineClasses and
>> restransformClasses, instead of introducing IsModifiableClass.
>> Otherwise can_redefine_any_class should simply refer to
>> IsModifiableClass.
>
> Acquiring a capability is how an agent tells the VM what it plans to do.
> When an agent acquires can_redefine_classes or can_retransform_classes,
> it is telling the VM that it plans to use redefine classes or class
> tranformation. When an agent acquires can_redefine_any_class or
> can_retransform_any_class, it is telling the VM that it plans to use
> redefine classes or class transformation on as many classes as possible
> so it would be a good idea to disable some optimizations like sharing.
>
> A properly written agent will check possible target classes with
> IsModifiableClass() before attempting redefine classes regardless of
> whether the agent has acquired just can_redefine_classes or both
> can_redefine_classes and can_redefine_any_class. This is similarly
> true for class transformation and the can_retransform_classes and
> can_retransform_any_class capabilities.
>
> We should probably update the wording for all four capabilities to
> refer to IsModifiableClass() so that we can avoid having the same
> wording in multiple places.
>
>
> <snip>
>
>> The only thing that needs fixing in my opinion is the definition of
>> can_redefine_any_class to not refer to primitives or arrays but simply
>> to any class for which IsModifiableClass returns true!
>
> Yup. That's where we should go, but we should do it for all four
> capabilities.
>
>
>>
>> can_redefine_any_class is really puzzling if you consider
>> can_redefine_classes. It is far from clear to me what set of classes
>> can_redefine_any_class gives access to that can_redefine_classes does
>> not ??? What purpose do two capabilities serve here?
>
> can_redefine_classes and can_retransform_classes are needed if you
> are going to redefine classes or transform classes at all. The "any"
> capabilities are provided as a way to indicate that your agent
> intends to redefine or transform as many classes as possible
> including those that might normally have other optimizations applied
> to them, e.g., sharing.
>
> Dan
>