The org.apache.log4j.NDC logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/NDC.html) class is holding references to a lot of dead threads in Hashtable. NDC uses the current thread as key in this hashtable and a Stack as value. Elements are only removed from this hashtable trough the public call remove()

NDC use can lead to memory leaks if you do not periodically call the NDC.remove() method. The current NDC implementation maintains a static hard link to the thread for which it is storing context. So, when the thread is released by its creator, the NDC maintains the link and the thread (and its related memory) is not released and garbage collected like one might expect. NDC.remove() fixes this by periodically checking the threads referenced by NDC and releasing the references of "dead" threads.