As of MongoDB 3.4 the username is optional for the X.509 authentication mechanism, and the Java driver supports that as of version 3.4.0 when using the MongoCredential API. But when using a connection string like:

mongodb://host/?authMechanism=MONGODB-X509&ssl=true

the driver silently ignores the authMechanism.

The driver should instead respect the auth mechanism, treating it as equivalent to:

MongoCredential.createMongoX509Credential()

For all other authMechanism, an IllegalArgumentException should be thrown if no username is specified.