Wednesday, July 30, 2003

AspectWerkz online architecture - better than JVMPI - Sun needs to think about it for java 1.5

(imported from, read comments there)

I already talked about the coming AspectWerkz new architecture for online mode (on the fly weaving). The key advantages will be as I explained:
- java 1.3 support
- debug mode for JVM not required
- option for single jvm
You may think its useless since the online weaving is already working fine (almost) with JMangler 3.x, which came with the original terrific idea of hotswapping the java.lang.ClassLoader, but I can bet you will change your position once you will have played with it a little more, thru the official 0.8 release.

Today I gave a look at the JVMPI API. Its a C layer receiving events from the JVM. Two events could be usefull to handle for AOP implementations: JVMPI_EVENT_CLASS_LOAD (sent when a class is loaded in the VM) and
JVMPI_EVENT_CLASS_LOAD_HOOK (sent when the VM obtains a class file data, but before it constructs the in-memory representation for that class).

Coding a layer receiving JVMPI_EVENT_CLASS_LOAD_HOOK to transform the bytecode could be usefull, since it could be enabled with just a -XrunMyStuff option and a shared library. This is the approach many commercial or open source debugging and profiling tools have choosen.

But giving a deeper look implementing an AOP layer on it will fail shortly for the following reasons:
- the event does not give the caller classloader information
- it might be a hell to call the java implemented AOP layer (AspectWerkz off course) from the C/C++ MyStuff JVMPI addon. Unless we setup a kind of client / server mode with a protocol to transmit bytecode back and forth (see AdaptJ for this sort of things).
Could lead to the idea of a distributed weaving engine... but do you need it ? And how will we be able to handle the calller classloader distinction, for example to weave struts classes in a specifc war only ?

So I definetely think the JMangler idea was good, even if the implementation is not that much good. And thus I am sure the new AspectWerkz architecture which unleash the idea will be and is already killing...

adaptJ project
JMangler project

As a ending note, I am really surprised Sun implemented the JVMPI_EVENT_CLASS_LOAD_HOOK in JVMPI and not directly (and even both) in the java.lang.ClassLoader... since its almost the single issue an on the fly weaver has to solve... since we have to override a class of rt.jar for java 1.3 compatibility... since BEA has implemented it since more than 2 years (except for the system classloader, just for the custom classloader deploying application like ear, war, ejb-jar, rar, but this might change with WebLogic 9)... since several project tries to address this lack (JMangler, beSee, AspectWerkz, JBoss, AspectJ)

Sun engineers - wake up - put that stuff in java 1.5 - java AOP needs it !
Ready to make a lobby with me ?