AFAIK plain HashMap is holding a _map wide_ lock for any retrieval operation or iterator traversal. ConcurrentHashMap doesn't do it (well, at least more fine grained). I am not looking at write operations. Performance tests showed that concurrent requests to the same service (shared meta data) suffered from synchronisation issues. That's why i replaced the plain HashMap. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4014634#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...