com.mongodb.ReplicaSetStatus.Updater thread should be interrupted and closed gracefully upon Mongo.close() call. I know it's a daemon thread, but in Tomcat, when redeploying I get:

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

Also see: JAVA-191