ArrayList<ServerAddress> servers = new ArrayList<ServerAddress>();
servers.add(new ServerAddress("s231", 20050));
servers.add(new ServerAddress("s231", 20051));
servers.add(new ServerAddress("s231", 20052));

Mongo m = new Mongo(servers);

m.setReadPreference(ReadPreference.SECONDARY);
final BasicDBObject query = new BasicDBObject();
final BasicDBObject fields = new BasicDBObject();

query.put("_id", new BasicDBObject("$gte", Integer.valueOf(1)));
fields.put("_id", Long.valueOf(1));

final DB db = m.getDB("offerStore");
boolean ok = db.authenticate("user", "pw".toCharArray());
System.out.println("ok: " + ok);
final DBCursor c = db.getCollection("offer").find(query, fields).limit(10);

while(c.hasNext()) {
DBObject offer = c.next();
System.out.println(offer);
}
c.close();

ok: false
Exception in thread "main" com.mongodb.MongoException: unauthorized db:offerStore ns:offerStore.offer lock type:0 client:192.x.y.z
        at com.mongodb.MongoException.parse(MongoException.java:82)
        at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:317)
        at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:298)
        at com.mongodb.DBCursor._check(DBCursor.java:369)
        at com.mongodb.DBCursor._hasNext(DBCursor.java:492)
        at com.mongodb.DBCursor.hasNext(DBCursor.java:517)
        at de.idealo.offerstore.mongo.TempTest.authenticationTest(TempTest.java:507)
        at de.idealo.offerstore.mongo.TempTest.main(TempTest.java:716)