尝试使用Graphics2D在屏幕上绘制，但该程序无法正常工作

Question

如果我在不启动线程的情况下调用函数draw，它将起作用，但是如果我尝试建立游戏循环。因此，我认为错误在于我如何开始游戏循环，但我不知道如何纠正它。

import java.awt.Dimension;

import javax.swing.*;

public class GameWindow{

    public static final int WIDTH = 640;
    public static final int HEIGHT = 480;

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        GamePanel panel = new GamePanel();
        frame.setSize(new Dimension(WIDTH, HEIGHT));
        frame.setMaximumSize(new Dimension(WIDTH, HEIGHT));
        frame.setMinimumSize(new Dimension(WIDTH, HEIGHT));
        frame.setPreferredSize(new Dimension(WIDTH, HEIGHT));
        frame.setLocationRelativeTo(null);
        frame.setTitle("Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(panel);
        frame.setVisible(true);
    }
}

第二堂课

我也尝试只在一个类中编写所有内容，但仍然无法正常工作：

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;

import javax.swing.JComponent;

public class GamePanel extends JComponent implements Runnable{

    private Graphics2D g2;
    private boolean isRunning = false;

    public void paintComponent(Graphics g) {
        g2 = (Graphics2D) g;
        if(isRunning)
            return;
        isRunning = true;
        new Thread(this).start();
    }

    @Override
    public void run() {
        while(isRunning) {
            update();
            draw();
        }
    }

    private void draw() {
        Rectangle r = new Rectangle(10, 10, 10, 10);
        g2.setColor(Color.BLACK);
        g2.draw(r);
    }

    private void update() {
        // TODO Auto-generated method stub
    }
}