[Bluej-discuss] CheckStyle extension

[Ian Utting]

Hi Steve -

> Universally in the checkstyle code base, everything uses
> Thread.currentThread().getContextClassLoader() to look up
> the appropriate class loader.  It appears that for extensions,
> the classloader associated with the thread invoking extension
> methods is *not* the one that contains the classpath for the
> extension's classes.  I ended up changing all references to
> use getClass().getClassLoader().  This means that, regardless
> of the class loader in effect for the current thread, the
> class loader used to load the checkstyle classes would be used
> to search for resources.

Ooh. Nice bug :-)

 > For the extension mechanism, the question is really
 > this: is there a useful reason (other than an accident of how the
 > code was developed) for these two class loaders to be different?

Probably not, but here's how it happened:

BlueJ launches extensions using a ClassLoader designed to separate 
extensions from each other and provide them with a "clean" view of their 
environment. This seems desirable -- we have had problems in the past 
with extensions accidentally sharing a parser library with BlueJ itself. 
We certainly don't want separately developed extensions to share a 
ClassLoader with each other.

But callbacks into extensions are (by and large) made on BlueJ's AWT 
event dispatch thread, which was loaded by BlueJ when it started, and so 
has a completely different ClassLoader (which really shouldn't be 
visible to extensions).

I suspect that callbacks to extensions should be disptached from a 
thread which shares the extension's ClassLoader, but we'll need to give 
some thoughts to the implications of that. I have (still in my inbox) a 
detailed note from Davin about other "dispatch thread" issues in 
extensions, and potential deadlocks arising from them.

I'm not sure that we can entirely fix the problem you've identified 
(bluej.getCurrentFrame().getClass().getClassLoader()?), but we should at 
least document the problem to save those who follow the pain you must 
have suffered.

Ian.