The DBCursor object of the java driver api doesn't seem to be thread-safe:

I execute dbcursorobject.next() within the run() method of a Runnable in order to get back each document of a collection within a separate thread.

As a result I get some documents duplicate, others are missing and randomly a NullPointerException occurs.

I guess neither next() nor hasNext() are thread-safe here.
Since the java-driver claims to be thread-safe that might have to be fixed.