Question

I have a method which implements different logic on data fetched from a DB depending on what the current date is.

I want to test it by having the unit test create objects, save them in the DB and invoke the tested method. However, in order to have predictable results I need to change the system date each time and I don’t know how to do that in Java.

Suggestions?

Answer 1

You can generate your expected results using the current date.

Or you write your system to use a date/time you give it when being tested (rather than the clock) That way the time is always what the test expects.

I use something like

interface TimeSource {
    long currentTimeMS(); // actually I have currentTimeNS
    void currentTimeMS(long currentTimeMS);
}

enum VanillaTimeSource implements TimeSource {
    INSTANCE;

    @Override
    public long currentTimeMS() {
        return System.currentTimeMillis();
    }

    @Override
    public void currentTimeMS(long currentTimeMS) {
        // ignored
    }
}

class FixedTimeSource implements TimeSource {
    private long currentTimeMS;
    @Override
    public long currentTimeMS() {
        return currentTimeMS;
    }

    @Override
    public void currentTimeMS(long currentTimeMS) {
        this.currentTimeMS =              currentTimeMS;
    }
}

In tests I use a FixedTimeSource which can be data driven e.g. set by inputs/events. In production I use a VanillaTimeSource.INSTANCE which ignores times in inputs/events and uses the current time.