java - Time dependent unit tests

Question

I need to test a function that whose result will depend on current time (using Joda time's isBeforeNow(), it so happens).

public boolean isAvailable() {
    return (this.someDate.isBeforeNow());
}

Is it possible to stub/mock out the system time with (using Mockito, for example) so that I can reliably test the function?

Answer 1

The best way (IMO) of making your code testable is to extract the dependency of "what's the current time" into its own interface, with an implementation which uses the current system time (used normally) and an implementation which lets you set the time, advance it as you want etc.

I've used this approach in various situations, and it's worked well. It's easy to set up - just create an interface (e.g. Clock) which has a single method to give you the current instant in whatever format you want (e.g. using Joda Time, or possibly a Date).