The method com.mongodb.Mongo.close() sets a signal that eventually tells the internal DBCleaner thread to close; which it correctly does. However, since the DBCleaner thread often sleeps, and cleaning up cursors could take a while, the thread will frequently not finish execution until after the close method returns. When this happens in apache tomcat, the following error is seen in tomcat's log files:

SEVERE: The web application [/someWebApp] appears to have started a thread named [MongoCleaner892931837] but has failed to stop it. This is very likely to create a memory leak.