Clocks and Time

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Intro

  • Absolute Time - Represents a specific point in time given by milliseconds plus nanoseconds past some point in time fixed by the clock. For the default real-time clock the fixed point is the Epoch (January 1, 1970, 00:00:00 GMT)
  • Relative Time - Represents a time interval milliseconds + nanoseconds seconds long. It generally is used to represent a time relative to now.
  • Standard Java only has a wall clock - calendar time. But real time systems require:
    • A monotonic clock: A monotonic clock that progresses at a constant rate and is not subject to the insertion of extra ticks to reflect leap seconds (as UTC clocks are). A constant rate is needed for control algorithms which want to executed on a regular basis.
    • A countdown clock: A countdown clock that can be paused, continued or reset.
    • A CPU execution time clock - measures amount of CPU time consumed by a particular thread or object.
  • Time is supported through the HighResolutionTime abstract class. Three implementing concrete classes - absolute, relative and rational.
  • Clock is supported through an abstract Clock class.
  • The realtime clock advances monotonically.
  • HighResolutionTime offers conversion between absolute and relative time values.
import java.util.Calendar;
import java.util.Date;
 
import javax.realtime.*;
 
public class TimeTest2 {
 
	public static void main(String[] args) {
 
		Clock clock = Clock.getRealtimeClock();
 
		Calendar cal = Calendar.getInstance();
		cal.set(2012, 0, 1, 0, 0, 0);
		Date newYear2012 = cal.getTime();
 
		AbsoluteTime at = new AbsoluteTime(newYear2012);
 
		//Convert absolute to relative
		RelativeTime rt = at.relative(clock);
 
		System.out.println("The number of ms between now and new year 2012 " + rt);
 
		//To confirm - add the current time and the relative time and we'll get new year 2012.
		Date d = new Date(System.currentTimeMillis() + rt.getMilliseconds());
		System.out.println(d);
 
		long oneweekms = 86400 * 7 * 1000;
		RelativeTime oneweek = new RelativeTime(oneweekms, 0);
 
		//Convert relative to absolute
		AbsoluteTime abs = oneweek.absolute(clock);
 
		//Will print an absolute date one week from now
		System.out.println(abs.getDate());
	}
}
  • Dont' change the clock associated with a time value because it is potentially unsafe. Particularly for absolute time values:
    • This is because absolute time values are internally represented as a number of milliseconds and nanoseconds since an epoch.
    • and different clocks may have different epochs.
    • The HighResolutionTime has a static waitForObject(Object, HighResolutionTime) method which is calls Object.wait() but with a HighResolutionTime parameter instead.
  • Rational Time: Is a Relative Time type with an associated frequency. Used to represent the rate at which certain events occur.
  • A rational time with an interval of 1 second and a frequency of 100ms, will have an inter arrival time of 10ms.
  • However note that the inter arrival time (the release) is not guaranteed only the frequency is.