To answer Manik's question for him, I ran some tests in JBCache and the performance for a plain synchronized HashSet compared to CopyOnWriteArraySet was worse, by about 10-20%. This is on a dual-core laptop, and probably for a four or eight-way, the difference might be more noticeable. COWAL actually does have some heavy synchronization for add and remove in JDK 1.5. For JDK 1.6, a read-write lock is used. (not sure this would help., as every operation done is a add or remove.) COWAL is more designed for more readers than writers. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4069904#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...