Swing：为什么repaint()并没有调用paintComponent方法
这个程序想要画两个小球，一个平抛，一个自由落体。为什么在62行的repaint方法，并没有进入paintComponent()方法中。
请高手指教！
package org.net;import javax.swing.*;import java.awt.*;import java.awt.event.*;import java.awt.geom.Ellipse2D;public class BallFrame extends JFrame{ JPanel downPanel; JButton startButton; MyPanel upPanel; Ellipse2D.Double d1 ; Ellipse2D.Double d2 ; long t; long begin; double x; double y; public BallFrame(){ setSize(300,400); downPanel = new JPanel(); startButton = new JButton("start"); startButton.addActionListener(new ActionListener(){ public void actionPerformed(ActionEvent event){ MyThread t = new MyThread(); begin = System.currentTimeMillis(); t.start(); } }); downPanel.add(startButton); add(downPanel,BorderLayout.SOUTH); upPanel = new MyPanel(); d1 = new Ellipse2D.Double(0,0,15,15); d2 = new Ellipse2D.Double(0,0,15,15); upPanel.addBall(d1,d2); } public static void main(String[] args) { BallFrame f = new BallFrame(); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); f.setVisible(true); } class MyPanel extends JPanel{ Ellipse2D.Double d1 ; Ellipse2D.Double d2 ; public void paintComponent(Graphics g){ //???没有被调用 super.paintComponent(g); System.out.println("===进入paint方法==="); Graphics2D g2 = (Graphics2D)g; g2.fill(d1); g2.fill(d2); } public void addBall(Ellipse2D.Double d1,Ellipse2D.Double d2){ this.d1 = d1; this.d2 = d2; } } class MyThread extends Thread{ public void run(){ for(int i=0;i<1000;i++){ t = (System.currentTimeMillis()-begin)/1000; x = 26*t; y = 0.5 * 9.8 * Math.pow(t, 2); System.out.println(x+":"+y); repaint(); //????为什么调用repaint方法不能进入paintComponent方法呢？ try{ Thread.sleep(100); } catch(Exception e){ } } } }}