repaint() in Java doesn't "re-paint" immediately?

Question

I have a code like that:

// In MyPanel.java
public void paintComponent(Graphics g)
{
    super.paintComponent(g);
    // Draw something
    mypanel_count++;
}

// In Test.java
public void testLargeData()
{
    while (notDone)
    {
        panel.repaint();
        // do huge work
        test_count++;
        System.out.println("Test_count: " + test_count + ", MyPanel_count: " + mypanel_count);
    }
}

// Output !!!
Test_count: 752, MyPanel_count: 23
Test_count: 753, MyPanel_count: 23
Test_count: 754, MyPanel_count: 23
Test_count: 755, MyPanel_count: 24

But when I change panel.repaint() to panel.paintComponent(panel.getGraphics()), the out is right:

Test_count: 752, MyPanel_count: 752
Test_count: 753, MyPanel_count: 753
Test_count: 754, MyPanel_count: 754
Test_count: 755, MyPanel_count: 755

Why? paintComponent method works, but sometimes it's blind, so I don't want to use it. Anybody can give me some suggestions? Thanks!

Answer 1

If you read the documentation of repaint carefully, you will notice that it states that (emphasis mine):

If this component is a lightweight component, this method causes a call to this component's paint method as soon as possible. Otherwise, this method causes a call to this component's update method as soon as possible.

This means that AWT/Swing is allowed to optimize repainting by merging repaints that are requested in a rapid succession. There is also a repaint(long time) method, which allows you to control how long AWT/Swing is allowed to wait with fullfilling your repaint request. It might still merge requests though, especially if you do them in a loop.

It might be helpful to read the article "Painting in AWT and Swing", which tries to explain the various concepts involved.

To get the panel repainted for every iteration, you would have to wait for a paint to happen and then proceed with your loop. This means you need some synchronization between your processing thread (the loop) and the AWT/Swing thread. As a rough idea, you could for example wait() on the panel object at the end of your loop if it has not been repainted since the last call to repaint() and call notifyAll() at the end of your panel's paintComponent() method. However, this can be tricky to implement right, so you should only do this if you really need "real-time" redrawing of your component. As an alternative, paintImmediately(...) could be used, but you would have to do all your processing in the event dispatching thread, like this:

SwingUtilities.invokeLater(new Runnable() {
    public void run() {
        while(notDone) {
            // Do your processing
            panel.paintImmediately(...);
        }
    }
});

Note that this will stop any event processing including mouse and keyboard input from being processed while your loop is running. You can read more about Swing and Threading in "Concurrency in Swing"