Java EE application servers - learning from the past

(Edit 15 July 2011: JBoss AS 7 is here, and brings a huge improvement to class loading and memory management. It's not full isolation, but it limits the exposed contact surface between app server and app greatly, and massively improves class loading. Brilliant!)

If you've used a Java EE application server like Glassfish or JBoss AS for long, you will have experienced classloader leaks, though you may not have realized it.

If you've ever seen the error java.lang.OutOfMemoryError: PermGen space at deploy-time, read the linked article above. If you have ever worked on the JVM, on app servers, or on EE applications, please read on.

Even if you haven't hit classloader leaks, you should be aware of the various ways Java EE applications can cause memory leaks in the server and what to do about them.

For those coming here for help fixing an immediate issue with their app: Read the links above, and this article on using jhat's JavaScript interface to find likely leaks. More fancy JavaScript OQL tricks are here.

Database preferences and product selection methodolgy

I recently stumbled across an interesting weblog post by a DBA who expresses a strong preference for Oracle over PostgreSQL. I thought I'd respond to it with a few thoughts here, not so much because of the opinion expressed as the reasons given for it.