Re: Annotation & Injection in Geronimo + Tomcat + Jaspe

[Paul McMahan <pa...@gmail.com>]

On 3/23/07, David Jencks <da...@yahoo.com> wrote:
> I've been working on connecting geronimo annotation processing and
> injection support to tomcat and jasper and studying how the current
> tomcat and jasper support this and have been working on a proposal
> for changing how tomcat deals with this area that I hope will be seen
> as an improvement.
>
> I've generally removed exceptions from the method signatures below in
> an attempt to make the situation a little clearer.
>
> As far as I can tell (so far) there are four kinds of objects
> relevant here:
> servlets
> filters
> listeners
> tags
>
> The first three are created more or less directly by tomcat whereas
> tags are created by some code in jasper and by code generated by jasper.
>
> Currently tomcat and jasper use a very wide variety of techniques to
> create the objects and then use an instance of
>
> public interface AnnotationProcessor {
>      public void postConstruct(Object instance);
>      public void preDestroy(Object instance);
>      public void processAnnotations(Object instance);
> }
>
> in a sequence like this:
>
> Object o = <create a new instance through various kinds of magic>
> annotationProcessor.processAnnotations(o);
> annotationProcessor.postConstruct(o);
>
> When its time to toss the object they call
> annotationProcessor.preDestroy(o);
>
>
> What I would like to do is replace the AnnotationProcessor with a
> different simpler interface like this: (the name is not important...)
>
> public interface LifecycleProvider {
>      Object newInstance(String fqcn, ClassLoader classLoader);
>      void destroyInstance(Object o);
> }
>
> The idea is that the newInstance method does everything necessary to
> construct a fully injected and post-constructed instance, and the
> destroyInstance does everything necessary to stop it.  Its very easy
> to write an adapter between this proposed interface and the
> AnnotationProcessor interface, so tomcat and jasper would continue to
> support the AnnotationProcessor approach just as they do today.
>
> The reason use this interface from geronimo's perspective is that we
> have some very nice code that can do the object creation and
> injection in one step.  It's designed to support constructor
> injection as well as property injection, so the object instance
> construction and injection aren't really separable.
>
> Aside from altruism the reason I think the tomcat developers might be
> interested in this is that there is such a wide variety of code in
> the <create a new instance through various kinds of magic> step and
> it looks to me as if this is most likely a consequence of less and
> less attention getting paid as new kinds of objects need to be
> created.  This would put all the managed object creation code in one
> place so each object creation would get the same level of attention.
>
> For instance, while listeners and tags are created with a simple
> clazz.newInstance(), the servlet construction code checks a lot of
> conditions before deciding how to construct the object: in particular
> security settings might cause it to be in a PrivilegedAction and if
> it is available in the same classloader as tomcat then special
> actions are taken.  While I don't entirely understand the point of
> some of this it seems highly unlikely that it is appropriate only for
> servlets and not filters, listeners and tags.

I have been following developments in this area very closely over the
past few weeks and am  convinced that your proposal for handling
dependency injection is absolutely right on.  For Geronimo we should
definitely try to align our dependency injection strategy across the
JEE stack  and obviously servlet and jsp are two of the most important
components.

My impression was that MyFaces adopted this proposal based on its own
merit and not just as a better way to integrate with Geronimo.  I
think that bodes well for Tomcat accepting this proposal as well.

> I've been working on this approach for about a week now and think I
> have most everything related to tomcat changes working.  There are
> still some problems with things like tld schema upgrades which are
> not related to tomcat code.  I would be more comfortable proposing
> this to the tomcat community after we've ironed out more of the
> geronimo problems, but I'd like to start some discussion on this and
> also commit my code so everyone can see at least the geronimo side
> clearly.  Since my copy of geronimo doesn't build without my changes
> to tomcat/jasper, to proceed with this I need to get my version of
> tomcat out somehow.
>
> Here are some possibilities I've thought of, maybe someone can think
> of something even better:
>
> - attach my tomcat patch to a geronimo jira issue, maybe check the
> patch into svn somewhere, build tomcat + jasper locally, and put them
> into our "repository" module (mabye with source jars if I can figure
> out how to create them)

I prefer this approach because Tomcat doesn't build with maven and
uses a technique for caching build dependencies that I think will be a
little unusual to Geronimo developers.  I'm not criticizing Tomcat's
build process -- that project has been successful enough to stay
around a long time and has legacy build structure to maintain.  Also,
just from a vibe perspective this approach feels less like a fork to
me, which I think we would all agree is ultimately not our intention.

> - svn copy tomcat to our tree and apply the patch, check it in, and
> push "org.apache.geronimo.tomcat/jasper" jars to the snapshot repo
>
> In any case I'd prefer to check my geronimo changes into trunk in the
> optimistic expectation that the Tomcat community will accept
> something like this proposal and that if they don't it will still be
> easier to adapt to the AnnotationProcessor approach in trunk than to
> deal with a geronimo branch.
>
> Anyway I started GERONIMO-3010 and I'll attach my tomcat patches
> there even though they aren't quite ready to propose to the tomcat
> community.  (AFAIK they work but especially the jasper code
> generation needs to be cleaned up)
>
>
> Thoughts?

I'm really excited about getting my hands on this stuff and look
forward to seeing it in svn.

Best wishes,
Paul