I wasted literally two hours on this one, so putting down a note to hopefully help future Googlers.

Symptom

Here’s all the various errors that I got in the hive-server2.log during my attempts to get a CREATE EXTERNABLE TABLE to work against a MongoDB table in Hive:

Caused by: java.lang.ClassNotFoundException: com.mongodb.hadoop.io.BSONWritable
Caused by: java.lang.ClassNotFoundException: com.mongodb.util.JSON
Caused by: java.lang.ClassNotFoundException: org.bson.conversions.Bson
Caused by: java.lang.ClassNotFoundException: org.bson.io.OutputBuffer

Whilst Hive would throw errors along the lines of: