Clocks and Time

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 countdown clock.
  • 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.