How do you automatically test a time check?

Question

Say you have a property startTime. Then you have a method doSomething:

doSomething() {
   //...stuff
   startTime = System.currentTimeMillis();
   //... more stuff
}

How do you test that startTime was assigned correctly? You can't test against an absolute timestamp - it'll likely change between assignment and test. Maybe use a range?

Answer 1

One simple way is to pass in the time from outside the method:

doSomething(long currentTime) {
   //...stuff
   startTime = currentTime;
   //... more stuff
}

Your production code that calls the method can pass in the real value for System.currentTimeMillis(), whereas your unit test(s) can pass in a specific known value.

Of course, this isn't quite the same as your original. Another possibility that is a little more complex but should mimic the behaviour of your of your original more closely:

doSomething(ITimeGetter timeGetter) {
   //...stuff
   startTime = timeGetter.GetCurrentTime;
   //... more stuff
}

Define the interface:

interface ITimeGetter
{
   long GetCurrentTime();
}

Two implementations, one for the production code:

class RealTimeGetter implements ITimeGetter
{
  long GetCurrentTime()
  {
      return System.currentTimeMillis();
  }
}

One for the unit tests:

class TimeGetterForTests implements ITimeGetter
{
  long GetCurrentTime()
  {
     // return a known value.
  }
}

Instantiate an ITimeGetter appropriate to your scenario and pass it in to the method you want to test.

Answer 2

Martin Fowler just wrote an interesting blog post about tests that may fail. He also writes about how you should handle time i.e. by wrapping the System call in a separate object.

Generally speaking, when you communicate with outside resources, for testing and encapsulation, it always helps to wrap them into objects.

Answer 3

While I personally favor Kramii's answer, there are also frameworks like Moles that allow detouring of static system calls.

Answer 4

You need to abstract out the timestamp mechanism so you can mock it.

This will allow you to generate exactly those results from currentTimeMillis that you need for testing, so you can be certain that values are correct.