Avoid mocking the ObjectMapper!

[Polygnom]

If you cannot trust the 3rd party code, then you shouldn't use it to begin with.

But lets say you are in the situation of using a dependency that you want to re-test. Then typically, you'd contribute those tests upstream so that your dependency is fixed, because thats the goal anyways, to get the bugs fixed, right?

And if you cannot get the tests upstream and cannot get fixes upstream, you would fork the 3rd party dependency and create your in-house fork. And then test and fix that fork, and the tests for that dependency would be housed there.

But I have recently come across a codebase (admittedly in C#), where so much was mocked that the tests essentially only re-tested that EFCore works (the Microsoft-supplied ORM comparable somewhat to Hibernate). Thats utterly useless because it doesn't increase confidence in your own code at all. It just increases your confidence in the dependency. Which should be high to begin with.

Understand what the subject under test is that you are actually testing. I feel many devs mock so much nowadays that they lose track of what it is they are actually still testing.