2009/2/27 Mike Kienenberger <[email protected]>
> A simple pluggable API sounds like a reasonable approach to me. For
> some situations, a pure reflection scanner might work better. For
> others, it won't scale. You might need to temporarily switch to a
> different scanner to determine if it's the cause of some particular
> bug. Or perhaps the target appserver itself provides some form of
> high-optimized scanner.
>
> On Fri, Feb 27, 2009 at 4:31 PM, Curtiss Howard
> <[email protected]> wrote:
> > Hi all,
> >
> > I'd like to take this opportunity to address some concerns I have with
> > the way annotation processing is done. I take interest in this
> > subject due to the extensive amount of work I have done in this area,
> > and I feel that I can shed some light on possible performance issues.
> > This message is mainly addressed to Jan-Kees van Andel as he is doing
> > the work on the MyFaces annotation processing engine, but of course
> > community input is always welcome. I will address some points that JK
> > raised in a previous thread on the user mailing list:
> >
> > "- Currently, I'm using the xbean-finder library from Geronimo for the
> > processing stuff. This library uses ASM under the hood which worries
> > me, since ASM is a widely used library and it has lead to many
> > classloading issues because of other frameworks packaging different
> > versions of ASM. "
> >
> > I would strongly recommend that MyFaces does NOT move away from doing
> > class scanning with ASM (or some other bytecode scanning library) as
> > the only other alternative is pure reflection and the performance in
> > that case does not scale well at all.
> >
> > "- I have also created an annotation scanner of my own, which is
> > actually quite fast and doesn't have a dependency on any library,
> > except Java. "
> >
> > Since this scanner has no external dependencies, I'm tempted to
> > believe this is a reflective scanner. If not, did you write a
> > bytecode scanning library?
> >
>
First, for archive purposes. This conversation contains an earlier
discussion about the subject. [1]
Yep. Writing a byte code scanner is not that difficult. Java provides a
class (DataInputStream) which handles most difficulties.
Having a byte code scanner inside the MyFaces codebase has the advantage of
less dependencies and (thus) less classloading issues with different ASM
versions. I've had such issues in the past with some Hibernate and Spring
combinations. Both were dependent on ASM and CGLIB and choosing the right
version of those dependencies was not trivial because both frameworks were
very picky. The same is true when running in Geronimo or JBoss, since they
use ASM internally.
>
> > "I don't think that making the annotation scanning code pluggable is
> > very useful since the xbean-finder library is already very easy to
> > use. "
> >
> > The one major concern with the xbean-finder library is that it
> > assembles a list of all the JARs on the classpath and scans all of
> > them. This is bad for a number of reasons:
> >
> > 1. The number of JARs to scan is _enormous_ in a typical customer
> > application running on a typical (i.e., bloated) appserver.
> > 2. By indiscriminately scanning JARs that are on the classpath but not
> > necessarily part of the application you open yourself up to
> > potentially pulling in random annotations that could have negative
> > effects on the application.
> >
> > Now, xbean-finder has a way for you to specify that you only want to
> > include JARs up to a certain point in the Classloader hierarchy, but
> > FaceConfigurator is not currently doing this :) and there is at least
> > one appserver I know of where even that approach wouldn't work since
> > applications are allowed to reference "shared libraries" that sit
> > apart from the normal classloader hierarchy. At a minimum, I think
> > FacesConfigurator would need to allow pluggability for integrators to
> > decide how to populate xbean-finder with its set of Classloaders.
> >
> > In terms of pluggability, what I'd like to see is a simple interface
> > that abstracts away annotation scanning. This would be good for two
> > reasons: first, appservers that integrate MyFaces would be able to
> > take advantage of performance benefits that may arise from tight
> > coupling with their runtime (not to mention that they may need to
> > customize the classpath that xbean-finder or the like searches). And
> > second, this is a good practice in general as MyFaces shouldn't
> > necessarily be tied to xbean-finder. There may very well be better
> > solutions, and being able to plug them in would help in the long run.
> >
> > My proposal for such an interface is this:
> >
> > public interface AnnotationScanner {
> > public Collection<Class> getAnnotatedClasses (Class
> annotationClass);
> > public Collection<Method> getAnnotatedMethods (Class
> annotationClass);
> > public Collection<Field> getAnnotatedFields (Class
> annotationClass);
> > public Collection<Package> getAnnotatedPackages (Class
> annotationClass);
> > }
> >
> > This interface is very similar to xbean-finder, and with good reason:
> > as you stated, it's a simple API to work with. FacesConfigurator
> > would need minimal changes to adapt to this interface. The advantage
> > here is, again, more freedom in how annotation scanning is performed.
> >
> > Also, I would propose a factory that creates these instances with the
> > FacesContext (that should be enough context for implementers to work
> > with).
> >
> > Please let me know what you think. Thanks,
> >
>
Sounds like a good plan. We can elaborate on this. But first, I'd like to
clean up the current FacesConfigurator class (split up into some
ConfiguratorStrategy classes). This will make it easier to work on the
codebase together without having annoying merge conflicts. I can do this
quite quickly I think...
Ps. I know the current patch in Jira [2] is far from ready (also stated in
the Jira comments). It was just meant to get things rolling.
[1]
http://www.nabble.com/Scanning-for-annotated-classes-in-MyFaces-2-to21318418.html
[2] https://issues.apache.org/jira/browse/MYFACES-2140
Regards,
Jan-Kees
