AuthenticationFailed to mongoDB with Spring boot

so maybe that's what I need from Spring

Of course, you are saying spring.data.mongodb.authentication-database = mydatabase and it's incorrect!

You want to use spring.data.mongodb.authentication-database = admin instead, and it will work.

If you are using spring.data.mongodb.uri method,

spring.data.mongodb.uri=mongodb://stuart:stuartdto@localhost:27017/mydatabase?authSource=admin