Fwd: Re: subclass and superclass transformation question

Hi Tronje,

I'm still strugling with the same issues, but I have one answer to
your questions.

It is about adding new classes to the classloader, I use the following
code for that:

 private final static Method defineClassMethod;

   static {
       try {
           defineClassMethod = ClassLoader.class.getDeclaredMethod(
                   "defineClass",
                   String.class,
                   byte[].class,
                   int.class,
                   int.class);
           defineClassMethod.setAccessible(true);
       } catch (NoSuchMethodException e) {
           throw new RuntimeException(e);
       }
   }

   public static Class defineClass(ClassLoader classLoader, String
className, byte[] bytecode) {
       //System.out.println("definingClass: "+className);
       try {
           return (Class) defineClassMethod.invoke(
                   classLoader,
                   className.replace("/", "."),
                   bytecode,
                   0,
                   bytecode.length);
       } catch (IllegalAccessException e) {
           throw new RuntimeException(format("A problem occurred
while defining class '%s'", className), e);
       } catch (InvocationTargetException e) {
           throw new RuntimeException(format("A problem occurred
while defining class '%s'", className), e);
       }
   }

This works for me. This code can be called from your Javaagent when it
wants to create additional classes.

This is good enough for IDE support.. I'm also working on a
compiletime instrumentation
for production environments.

On Thu, Feb 25, 2010 at 12:23 AM, Tronje Krop <[email protected]> wrote:
> 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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