Game loop completely freezes my program

Question

Original Question

I'm currently working on a simple application that displays a map and will later implement pathfinding logic for units. I've implemented the map and view so far and everything runs just fine until I implemented the game loop.

With the game loop enabled, the program just freezes. I can't close the window anymore and the map isn't presented, even though the game loop is executed just fine. I've used this game loop twice in the past and never had any problems until now.

Edit: The game loop continues to execute just fine while everything else freezes.

Here are the two functions involved:

public GameController() {
    paused = true;

    frame = new GameFrame(this);

    map = new Map(500, 500);
    mvm = new MapViewModel(map.getMap(), map.getWidth(), map.getHeight());

    //TODO: gameLoop() currently breaks the game.

    gameLoop();
}

public void gameLoop() {
    double tickTime, lastTick;

    for (;;) {
        tickTime = System.nanoTime();
        lastTick = tickTime;

        //Repaints the frame
        update();

        while (tickTime - lastTick < NANOSECONDS_PER_UPDATE) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException ignored) {}

            tickTime = System.nanoTime();
        }
    }
}

edit2: I'm using Swing. The actual painting happens in the paintComponent method of my GamePanel (JPanel):

@Override
public void paintComponent(Graphics g) {
    Graphics2D g2 = (Graphics2D) g;

    //Paints the map
    painter.paintMap(g2, controller.getMvm());
}

Obviously, if you have any further questions feel free to ask. Thanks in advance.

Solution

Here's the code I'm using now, GameController and update haven't changed.

public void gameLoop() {
    timer = new Timer(MILLISECONDS_PER_UPDATE, updater);
    timer.start();
}

updater is an ActionListener that I have added as a private variable to the class.

private ActionListener updater = new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("test2");
        update();
    }
};

You could add updater locally but I prefer it this way.

Answer 1

You tell us nothing about what GUI library you might be using, but assuming that it might be Swing, it looks like you're running some long-running code on the Swing event thread, the main thread responsible for doing all Swing graphics and for interacting with the user. If so, then the code will prevent Swing from redrawing the GUI, freezing your application.

My suggestions are:

• Don't do this. Don't run any long-running code on the main event thread.
• Instead consider using a Swing Timer to do your "game loop".
• Or if you must use your while loop and Thread sleep, do it off of the event thread, but then be sure that all Swing calls that mutate the state of Swing objects be done on the event thread.
• For more on Swing threading, please read Concurrency in Swing.