System.currenTimeInMillis() vs System.nanoTime()

Question

I know that System.nanoTime() is now the preferred method for measuring time over System.currentTimeInMillis() . The first obvious reason is nanoTime() gives more precise timing and the other reason I read that the latter is affected by adjustments to the system’s real-time clock. What does "getting affected by systems real-time clock " mean ?

Answer 1

In this case I've found following blog post excerpt useful:

If you are interested in measuring absolute time then always use System.currentTimeMillis(). Be aware that its resolution may be quite coarse (though this is rarely an issue for absolute times.)

If you are interested in measuring/calculating elapsed time, then always use System.nanoTime(). On most systems it will give a resolution on the order of microseconds. Be aware though, this call can also take microseconds to execute on some platforms.

Clocks and Timers - General Overview by David Holmes

Since System.currentTimeMillis() is relying on the systems time of day clock, adjustments to the time are legitimate, in order to keep it on time.

What means adjustments here? Take for instance a look at the description of CLOCK_REALTIME from Linux:

System-wide clock that measures real (i.e., wall-clock) time. Setting this clock requires appropriate privileges. This clock is affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), and by the incremental adjustments performed by adjtime(3) and NTP.

Answer 2

Just check the JavaDoc of the methods:

System.nanoTime()

"... This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. ..."

System.currentTimeMillis()

"... Returns the current time in milliseconds. ..."

So as you can see if the system time changes during the measurement using the System.currentTimeMillis(), the interval you measure will change too. However, it will not change when measuring the interval using the System.nanoTime() method.