Unit tests and system clock

It took me way to long to learn this. Your code (and their unit tests) should inject the system clock as a dependency.

An example, let’s say you have a service that writes a record to the database with the system clock.

public void save(String userName) {
	long currentTimeMs = System.currentTimeMillis();
	User user = User.builder()
	    .name(userName)
		.updateTimeMs(currentTimeMs);
	database.save(user);
}

How would you test this? You can inject a mock database instance and use it to verify that it got a User object. Great! You can verify the username is as expected. How do you verify that tricky business rule that updateTimeMS is the “current time”?

For too long, I simply didn’t test that. Just verify that updateTimeMS > 0, it’s good enough, right? In this case, you should inject the system clock as an explicit dependency instead.

private Clock clock;

public void save(String userName) {
	long currentTimeMs = clock.getCurrentTimeMilis();
	User user = User.builder()
	    .name(userName)
		.updateTimeMs(currentTimeMs);
	database.save(user);
}

In fact, Java introduced the java.time.Clock class (introduced in Java 8!) as object oriented API to replace code like System.getCurrentTimeMillis().

The concept applies across different languages but Java holds a special place in my heart.

In Python, the situation is a bit different. Python allows you to monkey patch module functions. It seems that a full OOP idiom is not dogmatic in Python. So the alternative would be to rely on unittest.mock.patch

def save(user_name: str) -> None:
  database.save(name=user_name, time=time.time())

...

@patch('time.time', return_value=123)
def test_save_user(self) -> None:
   mock_db = MagicMock()
   user_repository = UserRepository(database=mock_db)
   user_repository.save("foo")
   
   mock_db.save.assert_called_with(name="foo", time=123)
   

Not my style. But it’s Pythonic.

• programming
• Java
• Python