Bored? Want to learn something new? Or waste some time? Need quality headache?
My advice: look at web services and try to make something just a bit more complicated than "hello world" under JBoss. You will never forget it.
JBoss 6 uses CXF under the hood to support WS-related sections of EJB specification. Both products have their strong points but JBoss is definitely the leader in the "most creative WTF" contest.
JBoss supports declarative configuration of both web service implementation and web service client. All you need is to create a JBoss specific deployment descriptor, a file called jbossws-cxf.xml, and package it with your application. The nice touch here is that this deployment descriptor is Spring-based. This alone makes JBoss leading contester.
It is not that I have anything against Spring. Well, I actually have a lot to say about it, but it deserves a separate post.
No, really, just think about it: first you come up with a way to configure the server using XML, supporting hundreds of different schemas, all based on the same XML configuration parser library. And then you say "screw it, I fancy Spring here".
They did not go as far as packaging Spring with JBoss. Why? Who knows. Having said A they might just well have said B. But no, they use classloading to detect at runtime if Spring is present. Only if Spring is there JBoss goes ahead and reads jbossws-cxf.xml. Wow, say good bye to all hard work you put into creating that jbossws-cxf.xml - it is not used anyway. But you do not know that yet.
After some googling and digging you realize that you need Spring. Sigh. But you do need that configuration from jbossws-cxf.xml! Being "standard obeying" you bite the bullet package Spring with your application. (Being lazy you just drop Spring into server/<server_name>/lib. You save yourself a lot of time but miss a lot of fun.) Redeploy and ... your precious jbossws-cxf.xml is not loaded.
Things are looking up: you are not bored any more. You spend some time double checking your deployment and redeploying it. Nope, still not loaded. If you study jboss logs carefully you might spot this small message Spring not available, skipping check for user provided jbossws-cxf.xml / cxf.xml configuration files. If you know your way about JBoss jmx console you go and check your application's classloader and try to load some Spring classes via it. They are loaded just fine. WTF?
Remember that runtime classloading check I mentioned above? Turns out it runs very early during application deployment, before the classloaders for the application are set up. As a result the check is using the classloader of the WS deployer coming from server/<server_name>/deployers/jbossws.deployer. Surprise!
It is getting more and more interesting. You have to have Spring and you can't package it with your application. Right, but you need to make this application work. Forget about standards and keeping things where they belong. Spring to server/<server_name>/lib, JBoss restart, wow! Congratulations, you made it.
I leave the question of the proper place for Spring jars as an exercise. Instead let's look at more fun that JBoss-CXF integration might bring.
This is how I first got involved in this matter: I had an EJB application that used JMS. For reasons not important here it was decided that the application should use Apache ActiveMQ. No problem, I tested it locally under JBoss 6.1, it looked OK, and so the application was deployed in an integration environment, together with ActiveMQ resource adapter. So far so good. Later more applications were added to that environment. One of them failed to start. There were some classloading errors involving Spring classes. The application was a war that internally used Spring so Spring was packaged into WEB-INF/lib and the classloading was properly configured (WEB-INF/lib jars first). I was asked to look into the problem because there was another Spring in classpath: from ActiveMQ which is using Spring internally.
Of course removing ActiveMQ RA solved the problem for that application. Changing classloading rules of the application (to parent first) "solved" the problem as well.
Digging further I realized what has happened. JBoss classloader architecture is notoriously broken so the Spring classes from ActiveMQ leak into every application. When this failing application (with the original configuration WEB-INF/lib jars first) was starting JBoss WS deployer noticed some WS annotations and went ahead with whatever it supposed to do, including looking for Spring and jbossws-cxf.xml. Spring was there leaked from ActiveMQ. (Remember, the classloader at this moment is the one from jbossws.deployer.) The application did not have jbossws-cxf.xml but the harm was done: some instances of Spring classes were created along the way and remained referenced from CXF objects.
The application startup continued and JBoss finally created the application specific classloader and the rest of the startup code was executed using this classloader. Along the way CXF was involved again and it noticed those Spring instances created before so it went ahead with more Spring dance. But this time all the Spring classes were coming from WEB-INF/lib with predictable result: ClassCastException.
Of course changing classloading configuration would fix the problem in this case. Spring classes would always come from ActiveMQ keeping CXF happy. Well, until some other application, RA, whatever, that has Spring packaged is deployed in the same server. And do not forget that it can be a different version of Spring jars. Care to resolve this mess?
Priceless! Never attribute to malice that which ... I am not sure. Sometimes I think somebody at RedHat came up with this brilliant business idea of how to make more money out of JBoss support ...