In the official mongodb jstest dbref3.js (https://github.com/mongodb/mongo/blob/r2.4.5/jstests/dbref3.js) a DBRef must meet the following criteria:

• contain $ref and $id fields
• $ref must be the first field in the JavaScript object
• $id must be the second field in the JavaScript object
• the value associated with $ref must be a String

Currently the Java driver only checks if the two fields exist and performs a naive toString() on the $ref value.