动态加载图片显示不出来

显示图片扩展的panel
package com.xdg.graphic;import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;import javax.imageio.ImageIO;
import javax.swing.JPanel;public class ImageLoader extends JPanel {
private String imgPath;
private BufferedImage image;


public ImageLoader(String imgPath) {
this.imgPath = imgPath;
try {
this.image=ImageIO.read(new File(imgPath));
} catch (IOException e) {
e.printStackTrace();
}
}

public void paint(Graphics g) {
g.drawImage(image, 0, 0, this);
}

public Dimension getPreferredSize() {
        if (image == null) {
             return new Dimension(100,100);
        } else {
           return new Dimension(image.getWidth(null), image.getHeight(null));
       }
    } public String getImgPath() {
return imgPath;
} public void setImgPath(String imgPath) {
this.imgPath = imgPath;
}

}调用者窗体：
/*
 * FrmImgCropper.java
 *
 * Created on __DATE__, __TIME__
 */package com.xdg.image;import java.awt.BorderLayout;import com.xdg.graphic.ImageLoader;
import com.xdg.util.MiscUtil;/**
 *
 * @author  __USER__
 */
public class FrmImgCropper extends javax.swing.JFrame {
private ImageLoader imageLoader; /** Creates new form FrmImgCropper */
public FrmImgCropper() {
initComponents();
} /** This method is called from within the constructor to
 * initialize the form.
 * WARNING: Do NOT modify this code. The content of this method is
 * always regenerated by the Form Editor.
 */
//GEN-BEGIN:initComponents
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() { jPanel2 = new javax.swing.JPanel();
tfImagePath = new javax.swing.JTextField();
btnPreview = new javax.swing.JButton();
tfRatioW = new javax.swing.JTextField();
tfRatioH = new javax.swing.JTextField();
jLabel1 = new javax.swing.JLabel();
btnLoad = new javax.swing.JButton();
jLabel2 = new javax.swing.JLabel(); setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE); btnPreview.setText("Preview"); jLabel1.setText(":"); btnLoad.setText("Load");
btnLoad.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
btnLoadActionPerformed(evt);
}
}); jLabel2.setText("Image Path:"); javax.swing.GroupLayout jPanel2Layout = new javax.swing.GroupLayout(jPanel2);
jPanel2.setLayout(jPanel2Layout);
jPanel2Layout.setHorizontalGroup(jPanel2Layout
.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(
jPanel2Layout
.createSequentialGroup()
.addGap(34, 34, 34)
.addComponent(tfRatioW, javax.swing.GroupLayout.PREFERRED_SIZE, 55,
javax.swing.GroupLayout.PREFERRED_SIZE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addComponent(jLabel1)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
.addComponent(tfRatioH, javax.swing.GroupLayout.PREFERRED_SIZE, 53,
javax.swing.GroupLayout.PREFERRED_SIZE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 123,
Short.MAX_VALUE)
.addComponent(btnLoad, javax.swing.GroupLayout.PREFERRED_SIZE, 82,
javax.swing.GroupLayout.PREFERRED_SIZE).addGap(46, 46, 46)
.addComponent(btnPreview).addGap(276, 276, 276))
.addGroup(
jPanel2Layout.createSequentialGroup().addGap(66, 66, 66).addComponent(jLabel2)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
.addComponent(tfImagePath, javax.swing.GroupLayout.DEFAULT_SIZE, 593, Short.MAX_VALUE)
.addGap(29, 29, 29)));
jPanel2Layout.setVerticalGroup(jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(
javax.swing.GroupLayout.Alignment.TRAILING,
jPanel2Layout
.createSequentialGroup()
.addContainerGap(20, Short.MAX_VALUE)
.addGroup(
jPanel2Layout
.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jLabel2)
.addComponent(tfImagePath, javax.swing.GroupLayout.PREFERRED_SIZE,
javax.swing.GroupLayout.DEFAULT_SIZE,
javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(18, 18, 18)
.addGroup(
jPanel2Layout
.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jLabel1)
.addComponent(tfRatioW, javax.swing.GroupLayout.PREFERRED_SIZE,
javax.swing.GroupLayout.DEFAULT_SIZE,
javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(tfRatioH, javax.swing.GroupLayout.PREFERRED_SIZE,
javax.swing.GroupLayout.DEFAULT_SIZE,
javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(btnPreview).addComponent(btnLoad)).addGap(20, 20, 20))); getContentPane().add(jPanel2, java.awt.BorderLayout.SOUTH); pack();
}// </editor-fold>
//GEN-END:initComponents private void btnLoadActionPerformed(java.awt.event.ActionEvent evt) {
if (imageLoader == null) {
imageLoader = new ImageLoader(tfImagePath.getText());
getContentPane().add(imageLoader);
repaint();
} else {
imageLoader.setImgPath(tfImagePath.getText());
imageLoader.repaint();
} } /**
 * @param args the command line arguments
 */
public static void main(String args[]) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() { FrmImgCropper cropper = new FrmImgCropper();
cropper.setSize(800, 900);
cropper.setVisible(true);
}
});
} //GEN-BEGIN:variables
// Variables declaration - do not modify
private javax.swing.JButton btnLoad;
private javax.swing.JButton btnPreview;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JPanel jPanel2;
private javax.swing.JTextField tfImagePath;
private javax.swing.JTextField tfRatioH;
private javax.swing.JTextField tfRatioW;
// End of variables declaration//GEN-END:variables}
窗体运行时，输入图片路径，然后点击load按钮，显示不出来，但如果创建窗体实例的时候就将这个图片Panel加进来，则可以显示，这是为什么，还需要调用什么？