java - Will System.currentTimeMillis always return a value >= previous calls?

Question

https://docs.oracle.com/javase/6/docs/api/java/lang/System.html#currentTimeMillis() says:

Returns the current time in milliseconds. Note that while the unit of time of the return value is a millisecond, the granularity of the value depends on the underlying operating system and may be larger. For example, many operating systems measure time in units of tens of milliseconds.

It is not clear to me if I am guaranteed that this code will always print ever increasing (or the same) numbers.

while (1) { 
    System.out.println(System.currentTimeMillis() );
}

Answer 1

The short answer is no, System.currentTimeMillis() is not monotonic. It is based on system time, and hence can be subject to variation either way (forward or backward) in the case of clock adjustments (e.g. via NTP).

System.nanoTime() is monotonic, if and only if the underlying platform supports CLOCK_MONOTONIC -- see the comments on Java bug report 6458294 for a good writeup on some circumstances where this is/isn't true.

(And, as an additional anecdote, I have personally observed (several times) System.currentTimeMillis() run 'backwards', in the absence of clock adjustments, across threads -- that is, a call to that method in one thread returned a lower value than a call in another thread, even though it occurred chronologically after it in 'real-time')

If you need a monotonic source, System.nanoTime() on a platform supporting monotonicity is your best option.