Re: subclass and superclass transformation question

Hi all,

thanks for the advice from all of you. This discussion has cleared
up a view questions I had for some time:

In my current rewriter that is directly linked to the class loader,
I load all classes by getResourceAsStream. I have managed to do load,
analyse, and define all classes in the right order, which was as I
must admit quite tricky ... problems with cyclic class loading etc ...
but however, in the end it worked.

Making it work in parallel will be a challenge, as I have to protect
the analysis information on class level in the right way. In addition
I have to block multiple requests on the same class until the rewrit-
ten and defined class is available - right?

The advantage here is, I can test this with the conventional class
loader pattern.

Now, when I switch to the agent concept, I have to wait for each
class until the bytecode is provided to the transformer before I
returning the rewritten bytecode. Anyhow, I'm allowed to load all
resources needed for analysis from the class loader using
getResourceAsStream().

The big questions remaining are:

(1) How am I supposed to introduce new classes on the fly in the
agent concept - may be be defineClass() of the class loader?

(2) When it is allowed to use defineClass() of the class loader,
what is the difference against loading a class using getResource-
AsStream() and defining it in the class loader?

(3) Wouldn't the getResourceAsStream() concept fail, if another
transformer modifies a class before it is provided to my transformer?

Any suggestions how this is best handled?

CU Tronje

Marcel Patzlaff wrote:
> Just load the parent classes/interfaces as resources and analyse them 
> with ASM. If you have to perform modifications, you should do what 
> Eugene suggested: Prepare the transformations virtually and apply them 
> if the classloader eventually loads the parent classes/interfaces.
> 
> Regards,
> Marcel
> 
> Tronje Krop schrieb:
>> I'm currently not using the java agent, but I like to do son in
>> future.
>>
>> My problem concerning the order of class loading is, that I need
>> at least to analyse parent classes and interfaces before trans-
>> forming the subclasses.
>>
>> But how can I do this without forcing the class loader to load
>> interfaces and parent classes. Any suggestion?
>>
>> CU Tronje
>>
>> Eugene Kuleshov wrote:
>>   
>>>   I wouldn't advise loading classes from the agent. Instead you should
>>> be able to prepare required transformations "virtually" when
>>> analyzing/loading subclasses, save some state and do the actual
>>> transformation when parent is actually loaded by JVM.
>>>
>>>   regards,
>>>   Eugene
>>>
>>>
>>> On Tue, Feb 23, 2010 at 10:01 AM, Rémi Forax <[email protected]> wrote:
>>>     
>>>> Hi Peter,
>>>>
>>>> Le 23/02/2010 14:49, Peter Veentjer a écrit :
>>>>       
>>>>> I have a question about the details of superclass/subclass
>>>>> transformations using a Java agent.
>>>>>
>>>>> If there is a class Fruit and a subclass Apple, and if a new Apple is
>>>>> created before the Fruit class is used in the JVM.
>>>>> What is the exact order of classes loaded in the Java agent?
>>>>>
>>>>> My experience is that the Apple class is transformed before the Fruit
>>>>> class is transformed. Is this correct?
>>>>>
>>>>>         
>>>> Yes, the VM read the transformed class, find the super class and load it,
>>>> so superclass are loaded after class if there aren't loaded before.
>>>>
>>>> But you can load the superclass in the agent :)
>>>> you have the classloader and you can find the superclass in the classfile.
>>>> Just be careful about synchronisation, jdk7 classloading is done in
>>>> parallele.
>>>>
>>>>
>>>>       
>>>>> And is there any way to force to load the super class (Fruit) is
>>>>> transformed before the subclass Apple is transformed?
>>>>>
>>>>> I have the impression that the Java agent technology is very limited
>>>>> and that the only way to solve these issues is
>>>>> to do a static transformation. The big problem is that it makes the
>>>>> system much harder to work with from an IDE; normally
>>>>> you only need to register some javaagent and you are done. But adding
>>>>> a postcompilation step is much harder.
>>>>>
>>>>>         
>>>> Rémi
>>>>
>>>>
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>>>>
>>>>       
>>   
>

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