Testing concurrent code question 167 of 180 mid

Can you unit test code that uses the current time or a scheduler?

No, you must sleep
Correct: Yes, by injecting the clock and the executor: a fixed Clock and a deterministic or manually driven executor make the behaviour reproducible
Only with a mocking framework that rewrites bytecode
Only in integration tests

Hard-coded System.currentTimeMillis() and a privately constructed ScheduledExecutorService are what make a class untestable. Inject both and the tests become ordinary:

A java.time.Clock you can fix or advance, so expiry and timeout logic is exact.
An Executor you can substitute with Runnable::run to make everything synchronous, or with a deterministic scheduler that runs queued tasks when you tell it to.

This is a design answer as much as a testing one: the same injection that makes the class testable also makes it configurable and lets the application own thread lifecycle in one place. If a class creates its own threads, nobody can control them.

testing design

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