Mocking the system clock

[igorakkerman]

Is there a way to mock the system clock in order to test functionality that relies on the current time?

Specifically, I would like Zoned::now() to return a specific date, time and timezone in a unit test.

[BurntSushi]

Not presently. I am open to adding it, but I am highly skeptical of it. In order to be convinced I'll likely want to see detailed use cases and why alternatives aren't approrpriate. And I'll also want to know why the standard library doesn't need to provide this and what the consequences of that are.

[igorakkerman]

The use case is to determine the time passed between a specific instant and now. Currently, I can write and test a function with two parameters then and now. The functions calling that function also need to have the now parameter in order to be testable. Ultimately, the whole call stack would need that parameter. While this is possible, IMO it goes against good design principles.

I agree that mocking the system clock could be a feature of the standard library.

[BurntSushi]

While this is possible, IMO it goes against good design principles.

Wait, really? Sincerely, I feel like it's exactly the opposite. It's like implicitly depending on a global rng, which I consider to be typically poor design. Same kind of deal with the "current" time.

My impression has been that if this mocking functionality were introduced, it would either be for some specific use case that I don't know about it, or an acknowledgment that not everything is perfect and sometimes we need to hack around bad design because it's otherwise too costly to fix.

Anyway, my answer here is basically, "possibly, but there is a very high bar." If you (or anyone else) wants to try to meet that high bar, I'm happy to have an issue tracking it.

[igorakkerman]

Many RNGs use the system clock as their seed, and how would you design the system clock other than as a global singleton?
Another approach in large systems to avoid such a parameter chain is to use dependency injection, which also comes down to a global registry.

Anyway, in the Java world, the JDK allows you to use a modified Clock: https://www.baeldung.com/java-override-system-time

[codeslubber]

I never go on StackOverflow anymore, but I kind of remembered this Q/A, note the first response is literally from the dude who ranks #1 on SO. I also did this in Swift, and Kotlin, would have to look at how, but I think it might have been a Decorator.

https://stackoverflow.com/questions/5622194/time-dependent-unit-tests