Mocking ScheduledExecutorService vs the “Don't mock type you don't own” philosophy

Question

Mocking ScheduledExecutorService would really make testing my classes easier, but according to the mockito recommendations this seems a bad idea, as the logic of the mocked clas

Answer 1

It's more of a guideline than a rule; do the thing that will most likely result in a clean, reliable, and non-brittle test. As in the document you quoted:

This is not a hard line, but crossing this line may have repercussions! (it most likely will)

One important thing here is that "don't mock types you don't own" usually refers to concrete or internal types, because those are much more likely to change their behavior between versions, or to gain or lose modifiers like final or static that Mockito's dynamic overrides might not pick up on. After all, if you were to subclass a third-party class manually, Java would throw a compiler error; Mockito's syntax would hide that from you until test runtime.

To list out the factors I think of:

• As assylias pointed out in the comments, you're referring to a Java interface, which insulates you from common changes to final methods or method visibility.
• The interface is well-documented and designed for third-party extension, providing yet another reason that Java would be unlikely to make breaking changes to the general contract of the interface.
• The interface in question is a very well-used interface in Java, which overall has a lot of users, and a lot of backwards-compatibility concerns. It is very unlikely that you'd be subject to breaking changes, compared to a smaller library, or one under active development. One might even say that the JRE is in such lock step to the Java language, you have as much to worry about from breaking syntax changes than from breaking interface changes.

Though I believe strongly in "don't mock types you don't own" as a general heuristic or code smell, I'd agree with you here that the type is worth mocking, and that—unless you were to write and test a full implementation to be used in other tests—it's the best path forward for you here.