Currently, you get the code and the err fields from the result document, and the code is not included in the exception message so it's generally lost.

To fix, adding a new MongoException subclass called WriteConcernException, which DuplicateKey now extends. WriteConcernException takes the command result and passes toString() of it to MongoException, so the JSON representation of the entire document will appear in logs, e.g.

com.mongodb.MongoException$DuplicateKey: { 
  "serverUsed" : "/127.0.0.1:27017" ,
  "err" : "E11000 duplicate key error index:    
           com_mongodb_unittest_DBCollectionTest.testDuplicateKey.$_id_ 
           dup key: { : ObjectId('511a75c1eaa895f019e01b78') }" , 
  "code" : 11000 , 
  "n" : 0 , 
  "connectionId" : 76 , 
  "ok" : 1.0}