多层组件内的paintComponent方法内得到的height和width很小

在做一个类似略缩图功能的东西，自定义了一个继承自JComponent的Thumbnail类，当把这个Thumbnail类直接添加到JFrame时，图片显示没有问题，但是当我先添加到JPanel内，再将JPanel添加到JFrame时，图片只有一个点。如果Thumbnail类改为继承自JPanel，则图片大小只有10*10。求解，代码如下。import javax.swing.*;
import javax.imageio.*;;
import java.awt.*;
import java.awt.geom.*;
import java.io.*;public class TestTh extends JFrame{
  public static void main(String[] args){
    TestTh t = new TestTh();
    t.setSize(600, 400);
    t.setVisible(true);
    t.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    t.add(new Thumbnail("G:\\Image\\image.jpg"));
  }
}
class Thumbnail extends JComponent{
    private Image img;
    double ratio;
    String path;
    private AffineTransform af = AffineTransform.getScaleInstance(1,1);
    public Thumbnail(){
        setSize(180, 200);
    }    public void setPath(String path){
        this.path = path;
    }
    public Thumbnail(String path){
        this.path = path;
        setSize(180, 200);
        setImage();
    }
    private void setImage(){
        try{
            img = ImageIO.read(new File(path));
            ratio = culculateRatio(img.getWidth(null),img.getHeight(null));
            //paintImmediately(getBounds());
        }catch(IOException ex){
            System.out.println("Can not find this file!");
        }
        //paintImmediately(getBounds());
        concateZoom(ratio);
    }    private void concateZoom(double scale){
        if (img==null)return;
        af.preConcatenate(AffineTransform.getScaleInstance(scale,scale));
        paintImmediately(getBounds());
        System.out.println(getWidth() + "&" + getHeight());//此处输出180 & 200
    }    @Override
    protected void paintComponent(Graphics g) {
        System.out.println(getWidth() + "*" + getHeight());//此处输出1 * 1，最终画出的图只有一个点
        super.paintComponent(g);
        if (img==null) {
            g.setColor(Color.BLACK);
            g.fillRect(0,0,getWidth(),getHeight());
        }
        else {
            Graphics2D g2d = (Graphics2D)g;
            g2d.setTransform(af);
            g2d.drawImage(img, (int)(((getWidth()-img.getWidth(null)*ratio)/2)/ratio),
                    (int)(((getHeight()-img.getHeight(null)*ratio)/2)/ratio), this);
        }
    }//paintComponent    final double culculateRatio(int width, int height){//计算缩放比率
        //System.out.println(getWidth() + "*" + getHeight());
        if(getWidth()>=width && getHeight()>=height)
            return 1;
        if((double)getWidth()/width > (double)getHeight()/height)
            return (double)getHeight() / height;
        else return (double)getWidth() / width;
    }//culculateRatio
}//Thumbnail