I am testing code that runs in its own thread in a while loop. The code under test does some stuff based on the current time, then updates an AtomicReference<Integer>

The application logic is an algorithm that runs for days. I need to simulate the behavior by controlling the time readings, and checking the computed results.

I can’t figure out how to mock the time. I provided simplified code examples below.

Code under test

class ClassUnderTest {
    private ExecutorService executorService = ...
    private MyClock clock;
    private AtomicReference<Integer> result = ...

    public ClassUnderTest(MyClock clock) {
        this.clock = clock;

        // start the thread
        this.executorService.submit(this::run);
    }

    public void run() {
        while (true) {
            if (hasThreeHoursPassed(clock.getTime()) { // <-- PROBLEM, can't mock getTime() from Spock thread
                // ... core application logic that requires testing ...
                result.set(...);
            }
        }
   }
}

Spock test

def 'test thread' () {
    given:
    MyClock clockMock = Mock(MyClock)
    ClassUnderTest classUnderTest = ClassUnderTest(clockMock)
    long mockedTime = 1L;
    clockMock.getTime() >> mockedTime

    expect:
    for (int i = 0; i < 100; i++) {
        mockedTime += 1000 * 60 * 60 * 3 // 3 hours

        clockMock.getTime() >> mockedTime // <-- PROBLEM, this does not work. The thread does not see this value

        classUnderTest.getResult() == ... expected value ...
    }


}