I’ve been professionally working with Java application servers for five years, but I have only experienced my self two vendors: Weblogic and JBoss, and mainly the last.
Focusing in JBoss I’m currently migrating (or at least trying to) some apps from 4.2.2 version to the first “stable” version of 6.x series. However I feel cluttered with issues far more complicated that I expected. Class loading, logging, startup process, everything seams to have changed, I’m sure for better but will always be like this?
So I question myself what are the other real alternatives out there? The existing alternatives have the same problems?
You may ask what features do I use for requiring a application server like JBoss: currently JMS, XA Transactions, Datasources.
Many of the features of full-blown applications servers are provided individually, and can be bundled with simpler alternatives like Tomcat or Jetty. JMS has multiple implementations, like ActiveMQ, HornetQ, etc. Same goes for JTA.
Spring is a nice alternative to JavaEE in that regard – it can run on a simple Tomcat and supports datasources, JTA and messaging. SprignSource also develops the tc server, which is a “enterprise-like” tomcat.
(I noticed you omitted Glasfish from the list – it is an application server worth taking a look at.)