Killer Game Programming in Java

Sleeping Better

The animation loop in run() depends on a good timer and the accuracy of the sleep() call. The previous major section dealt with alternatives to currentTimeMillis(). In this section, I consider ways of improving the sleep() code in run(), so the required frame rate is consistently achieved.

The SleepAcc class measures sleep accuracy. Example 2-4 calls sleep() with increasingly small values and measures the actual sleep time using the Java 3D timer.

Example 2-4. Measuring sleep() accuracy

import java.text.DecimalFormat;
import com.sun.j3d.utils.timer.J3DTimer;

public class SleepAcc
{
  private static DecimalFormat df;

  public static void main(String args[])
  {
    df = new DecimalFormat("0.##");  // 2 dp

    // test various sleep values
    sleepTest(1000);
    sleepTest(500);
    sleepTest(200);
    sleepTest(100);
    sleepTest(50);
    sleepTest(20);
    sleepTest(10);
    sleepTest(5);
    sleepTest(1);
  } // end of main()


  private static void sleepTest(int delay)
  {
    long timeStart = J3DTimer.getValue();

    try {
      Thread.sleep(delay);
    }
    catch(InterruptedException e) {}

    double timeDiff =
       ((double)(J3DTimer.getValue() - timeStart))/(1000000L);
    double err = ((delay - timeDiff)/timeDiff) * 100;

    System.out.println("Slept: " + delay + " ms  J3D: " +
                          df.format(timeDiff) + " ms  err: " +
                          df.format(err) + " %" );
  }  // end of sleepTest()

} // end of SleepAcc class

The difference between the requested and actual sleep delay is negligible for times of 50 ms or more and gradually increases to a +/-10 to 20 percent error at 1 ms. A typical ...