Why does my Java game loop randomly jump to 62 FPS and crash?

Question

I've been working on a Java game, but I couldn't keep it from stuttering. I did some research, and found out that I was using a bad game loop. I've been working on a new game loop, which works a little better, but my framerate often jumps from 60 FPS to 62 FPS, and sometimes even crashes. This is the loop I'm using;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Frame;

@SuppressWarnings("serial")
public class GameLoop extends Applet implements Runnable {

    final int FRAMERATE = 60;

    @Override
    public void init() {
        setSize(1280, 720);
        setBackground(Color.WHITE);
        setFocusable(true);
        Frame frame = (Frame) this.getParent().getParent();
        frame.setTitle("Game Loop");
    }

    @Override
    public void start() {
        Thread thread = new Thread(this);
        thread.start();
    }

    @Override
    public void stop() {

    }

    @Override
    public void destroy() {

    }

    @Override
    public void run() {
        int framerate = 0;
        long counter = 0;

        long lastTime = System.nanoTime();
        long frameTime = 1000000000 / FRAMERATE;

        while (true) {
            long currentTime = System.nanoTime();
            long updateTime = currentTime - lastTime;
            lastTime = currentTime;

            counter += updateTime;
            framerate++;

            if (counter >= 1000000000) {
                System.out.println(framerate + " fps");
                counter = 0;
                framerate = 0;
            }

            try {
                Thread.sleep((frameTime * 2 - updateTime) / 1000000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

}

I've walked it through a couple of times, but I can't find out what's wrong. This is the console output I get;

60 fps
60 fps
60 fps
60 fps
62 fps
60 fps
62 fps
60 fps
60 fps
60 fps
... 
Exception in thread "Thread-3" java.lang.IllegalArgumentException: timeout value is negative
    at java.lang.Thread.sleep(Native Method)
    at GameLoop.run(GameLoop.java:58)
    at java.lang.Thread.run(Unknown Source)

As far as I can understand, the loop crashes because Thread.sleep((frameTime * 2 - updateTime) / 1000000); has a negative input, but I don't know why it is negative.

I've added;

if (updateTime < 0)
    updateTime = 0;

After;

long currentTime = System.nanoTime();
long updateTime = currentTime - lastTime;
lastTime = currentTime;

To prevent getting a negative input, but the problem still occurs. Also, I've tried to implement the loop into my game, but it's even laggier than before. The player seems to be shaking back and forth while moving.

Could anyone tell me what I'm doing wrong and how I can fix this?

Answer 1

You get that error because frameTime * 2 - updateTime is negative. This happens whenever the previous update took longer than 1/30 seconds. This can happen for all kinds of reasons. Maybe your game mechanics were a bit more computationally complex in that tick than you expected. Or maybe the operating system decided to idle your game thread because it wanted to execute some other task. This just happens and your render loop needs to be able to deal with it.

Try this instead:

long sleepDuration = (frameTime - updateTime) / 1000000;
if (sleepDuration > 0) {
    Thread.sleep(sleepDuration);
}

An even better solution might be to decouple your render-framerate from your game update-framerate. This means that your game won't run slower when the user's machine isn't powerful enough to render at 60 fps. You can do that by keeping track of game-time and render-time separately. Pseudocode:

realTime = 0;
updatedTime = 0;
while (game is running) {
     realTime += time the last loop took;
     while (updatedTime < realTime) {
         updateGameMechanics();
         updateTime += time one tick is supposed to take
     }
     renderGraphics();
     sleep if necessary
}

A different method is to use a variable lockstep. Instead of performing multiple updates, pass the time since the last update to updateGameMechanics and keep that time into account for all game calculations. I personally prefer a fixed lockstep because that way you don't need to take the deltaTime into account for absolutely everything in your game mechanics, but this is more of a religious question.