This blog post is a re-posting of my recent answer on StackOverflow regarding ThreadLocal objects and supposed PermGen memory leaks. Essentially, despite what Tomcat may say, ThreadLocal usage in a container is not always a memory leak. Repost of my answer below the line:
Every now and then I discover a new class in the Java standard library that I’ve not needed to use before. Today, that class was CopyOnWriteArrayList (or rather the CopyOnWriteArraySet wrapper). This is a good choice when implementing a set of subscribers in an observer pattern, as typically the set of subscribers doesn’t change very often but we iterate through the set regularly to broadcast events. The COW implementations are optimised for exactly this case, as the iterator only hits a volatile read rather than needing a lock.
Being curious, I decided to have a look at the implementation of this class to verify that it behaved as I expected from the docs. Sure enough, everything is as I would expect. However, something did stick out immediately as a bit odd. Continue reading “The curious case of transient final locks and CopyOnWriteArrayList”