Setting milliseconds of System time with java

Question

Would it be possible to set system time with milliseconds component on Windows OS using Java? I am trying to synchronize clocks between couple of computers but the OS API seems to offer only set time in format: HH:MM:SS.

This is what i tried:

public static void main(String[] args) throws InterruptedException, IOException { String time="10:00:20"; // this works String timeWithMiliseconds = "10:00:20.100"; // this doesn't change time at all Runtime rt = Runtime.getRuntime(); rt.exec("cmd /C time " + time); }

I am wondering how do NTP clients work if it's not possible to set milliseconds component ?

One way to deal with this issue could be to calculate time in milliseconds when server should reach next second, and sleep for that time. Is there a better, more direct way to achieve this ?

Because they don't use the console command to set the time; they probably use the SetSystemTime function, which (like any other Windows function) you can't easily access from Java. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Nov 10, 2015 at 9:53
Thank you for comment. So it wouldn't be possible to call that function with ProcessBuilder/Runtime, have to use JNI ? — John
– John, Commented Nov 10, 2015 at 9:55
It's just an assignment i am working on and am not allowed to do so. — John
– John, Commented Nov 10, 2015 at 9:56
To call methods that are not part of Java (but C ish) you'll have to use JNI stackoverflow.com/a/17035731/995891 (or jna). Or maybe you could start a script / custom exe via Runtime that allows to set time in ms precision — zapl
– zapl, Commented Nov 10, 2015 at 10:02

SubOptimal · Accepted Answer · 2015-11-10 10:40:27Z

As mentioned by immibis you could use the Windows function SetSystemTime to set the time.

Find below a snippet which call the Windows function using JNA 4.2

Kernel32 kernel = Kernel32.INSTANCE; WinBase.SYSTEMTIME newTime = new WinBase.SYSTEMTIME(); newTime.wYear = 2015; newTime.wMonth = 11; newTime.wDay = 10; newTime.wHour = 12; newTime.wMinute = 0; newTime.wSecond = 0; newTime.wMilliseconds = 0; kernel.SetSystemTime(newTime);

For further information have a look into the sources of JNA and on those links SetSystemTime Windows function and SYSTEMTIME structure.

An introduction to JNA from 2009. Simplify Native Code Access with JNA

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