Voici ma solution :
MongoClient mcAdmin = new MongoClient(
configuration.getServerAddresses(),
Arrays.asList(MongoCredential.createMongoCRCredential(
MONGODB_ADMIN_USERNAME, "admin",
MONGODB_ADMIN_PASSWORD.toCharArray())));
try {
mcAdmin.setWriteConcern(WriteConcern.JOURNALED);
DB db = mcAdmin.getDB(userDbName);
BasicDBObject commandArguments = new BasicDBObject();
commandArguments.put("user", userUsername);
commandArguments.put("pwd", userPassword);
String[] roles = { "readWrite" };
commandArguments.put("roles", roles);
BasicDBObject command = new BasicDBObject("createUser",
commandArguments);
db.command(command);
} finally {
mcAdmin.close();
}