On 16/11/2011 6:09 PM, Weijun Wang wrote:
Yes, I know nanoTime() is precise, but how can I get a clock time from it? I
have tried to record a currentTimeMillis() value when the program starts and
then use the elapse of the nanaTime() to get a current time. This will break
if the user adjusts the system clock during the program execution.
One solution is to keep tracking the changing of both currentTimeMillis()
and nanoTime(). If the change of one has a difference (say, > 1 sec) from
the other one, it means a system clock change and I can quickly reset my
time to currentTimeMillis().
What you have tried is pretty much all you can do. nanoTime is not connected
to to the time-of-day. If you need to make a connection then you need to
make it and keep it synchronized.
David
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Thanks
Max
On 11/16/2011 03:23 PM, David Holmes wrote:
Hi Max,
On 16/11/2011 2:55 PM, Weijun Wang wrote:
I need a precise time, and is currently using java.util.Date, which knows
about milliseconds, but unfortunately the precision is only 10-15
milliseconds on a Windows.
In fact, I don't really need it to be so correct. My requirements are:
1. It's enough correct, say, at least as correct as Date.
2. It's precise in a relative sense, i.e. it changes fast
3. It should be monotonic, i.e. it grows, unless the user adjusts the
system
clock
There are only two time source available:
1. The time-of-day clock
This is what Date reports and is also what System.currentTimeMillis
reports. It only has millisecond precision. It's rate of update is
dependent on the OS - for Windows that is typically every 10ms or every
15ms depending on version.
2. The high resolution time source
This is what System.nanoTime reports. It has nanosecond precision, but
again depending on the OS it's resolution (update rate) will vary. The
update rate should easily be in the tens of microseconds. It should be
monotonic non-decreasing but it is not connected to the time-of-day
clock (and so should not be affected by any changes therein).
I have an old blog entry on this:
http://blogs.oracle.com/dholmes/entry/inside_the_hotspot_vm_clocks
David
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