Override Java System.currentTimeMillis for testing time sensitive code

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I strongly recommend that instead of messing with the system clock, you bite the bullet and refactor that legacy code to use a replaceable clock. Ideally that should be done with dependency injection, but even if you used a replaceable singleton you would gain testability.

This could almost be automated with search and replace for the singleton version:

  • Replace Calendar.getInstance() with Clock.getInstance().getCalendarInstance().
  • Replace new Date() with Clock.getInstance().newDate()
  • Replace System.currentTimeMillis() with Clock.getInstance().currentTimeMillis()

(etc as required)

Once you’ve taken that first step, you can replace the singleton with DI a bit at a time.

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