Java Swing会忽略repaint（）吗？

Question

Basically I need one of my panels to be repainted at least 60 times per second. However I noticed, that if I don't move my mouse, FPS drops to ~5. I wrote program to test it.

package test;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;

public class Test extends JFrame implements ActionListener {

    Timer t;
    JLabel l;
    JPanel p;
    long lastT;

    public static void main(String[] args){
        new Test();

    }

    public Test(){
        add(p = new JPanel());
        p.add(l = new JLabel("0000000000000000000000000000000"));
        pack();
        lastT = System.nanoTime();
        t = new Timer(10, this);
        t.setRepeats(true);
        t.start();
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        long time = System.nanoTime();
        l.setText(String.valueOf((time - lastT)));
        repaint();
        lastT = time;
    }

}

It shows the interval between frames. If my mouse is inside the window, it changes much faster, if it is outside of window, it changes much slower, similar to my original problem. But the thing is, the number itself is not much different no matter if mouse is inside or outside, which means that Timer is shooting event at same interval. So it means that the repaint() is ignored? How can it be fixed?

Answer 1

Please note that repaint is never guaranteed to work as the Swing repaint manager will ignore stacked repaint requests -- that is if repaint requests build up and are not able to be handled in a timely fashion due to code being run from the Swing event queue, only the last one is called. Please read Painting in AWT and Swing for more.

Note however that there is no need to call repaint() in your code above, since changing the state of the JLabel's model will trigger a repaint on its own, and this would be a repaint of just the label itself, something that should be run more efficiently than calling repaint on the entire GUI. Also note that 10 mSec is a very short time slice and a Swing Timer may not be accurately or reliably called at 10 msec.