Exception as below happens in doing some configuration update in parallel:
In a constructor of Configuration - public Configuration(Configuration other), the copy of updatingResource data structure in copy constructor is not synchronized properly.
Configuration.get() eventually calls loadProperty() where updatingResource gets updated. So, whats happening here is one thread is trying to do copy of Configuration as demonstrated in stack trace and other thread is doing Configuration.get(key) and than ConcurrentModificationException occurs because copying of updatingResource is not synchronized in constructor.
We should make the update to updatingResource get synchronized, and also fix other tiny synchronized issues there.