Enhancing using ClassFileTransformer.

[Richard Schilling <rs...@cognitiongroup.biz>]

Right.  After today's meeting, I now understand what 
ClassFileTransformer is.  It's pretty handy for what we are doing.  Per 
Craig's e-mail with the notes from today's JDO meeting, I'll be taking a 
closer look at instrumentation for this purpose. This is a great 
approach, because it allows us a concrete specification to implement 
enhancement into build tools.  Enhancing during run-time of the 
application post-build is another issue, and my understanding from Craig 
is that restricting enhancement during the build process is the goal.  
That's great too.

The instrumentation interface is so new, that it's kind of hard to find 
anything with background information.  So, I'll be filling out the JDO 
wiki as I go. 

Quick question: Can anyone post a link with a description (or just 
describe it here) of how the VM interacts with the instrumentation API 
to process the bytestream of a class using ClassFileTransformer? 

Does the VM invoke the class loader to find a class first, before 
handing the class definition off to the ClassFileTransformer?  I assume 
this is so but I want to be sure.

Richard


Andy Jefferson wrote:
> Hi Richard,
>
>   
>> So, how do we take advantage of the concept of instrumentation and
>> incorporate it into JDO?
>>     
>
> "JDOEnhancer" interface extends ClassFileTransformer, consequently it provides 
> runtime enhancement also
>
>   

Re: Enhancing using ClassFileTransformer.

[Andy Jefferson <an...@datanucleus.org>]

> Quick question: Can anyone post a link with a description (or just
> describe it here) of how the VM interacts with the instrumentation API
> to process the bytestream of a class using ClassFileTransformer?
> Does the VM invoke the class loader to find a class first, before
> handing the class definition off to the ClassFileTransformer?  I assume
> this is so but I want to be sure.

From what I remember the JVM will receive a request to load a class. It 
retrieves the bytes of the class. It then calls the method "transform" of 
the "Premain-Class" (specified in the MANIFEST.MF of the enhancer jar) with 
the input bytes for the class. This method can transform the bytecode of that 
class and return it, or return null (meaning there is no change to the class 
definition).

See
http://datanucleus.svn.sourceforge.net/viewvc/datanucleus/platform/enhancer/trunk/src/java/org/datanucleus/enhancer/DataNucleusClassFileTransformer.java?revision=2302&view=markup

That's about it.

-- 
Andy  (DataNucleus - http://www.datanucleus.org)