JAVA GUI custom JPanel drawing board background

import java.awt.Graphics; 
import javax.swing.ImageIcon; 
import javax.swing.JFrame; 
import javax.swing.JPanel; 
  
public class GUITest { 
   private static JFrame jframe; //声明一个窗体 
   private JPanel jpanel;     //声明一个画板 
   
   public GUITest(){       //构造方法 
     jframe = new JFrame(); 
     init(); 
   } 
   
   private void init(){ 
     jframe.setTitle("测试"); 
     jpanel = new JPanel(){//关键代码，就是重写了paint的一个方法 
        @Override
        protected void paintComponent(Graphics g) { 
          super.paintComponent(g); 
          ImageIcon img = new ImageIcon(GUITest.class.getResource("bg.png"));  
         /** 
          * bg.PNG这个地方换成自己的图片 
          * 此处使用的相对路径，bg.png跟该测试类在同一路径下 
          * 不过建议使用相对路径避免使用绝对路径 
          */
          img.paintIcon(this, g, 0, 0); 
        } 
     }; 
     jpanel.setOpaque(true); 
     jframe.setBounds(200, 200, 500, 400); //设置显示位置距离左边200像素距离上边200像素及屏幕大小500*400 
       
     jframe.add(jpanel); //添加画板到窗体 
       
     jframe.setVisible(true); //设置显示界面 
   } 
   
   public static void main(String[] args) { 
      new GUITest();      // 实例化对象  
   } 
}

import java.awt.Graphics; 
import java.awt.GridBagConstraints; 
import java.awt.GridBagLayout; 
import java.awt.Insets; 
import javax.swing.ImageIcon; 
import javax.swing.JButton; 
import javax.swing.JFrame; 
import javax.swing.JLabel; 
import javax.swing.JPanel; 
import javax.swing.JPasswordField; 
import javax.swing.JTextField; 
  
public class GUIT { 
  //声明窗体，面板及控件 
  private static JFrame jframe; 
  private JLabel jlabel,jlabel1; 
  private GridBagLayout gridbag; 
  private GridBagConstraints constraints; 
  private JTextField jtfield1; 
  private JPasswordField jpfield1; 
  private JButton jbutton1,jbutton2,jbutton3; 
  private JPanel jpanel; 
    
  public GUIT(){ 
    jframe = new JFrame(); 
    jlabel = new JLabel(); 
    jlabel1 = new JLabel(); 
    jtfield1 = new JTextField(); 
    jpfield1 = new JPasswordField(); 
    gridbag = new GridBagLayout(); 
    jbutton1 = new JButton(); 
    jbutton2 = new JButton(); 
    jbutton3 = new JButton(); 
    init(); 
  } 
   
   /** 
   * init()初始化并显示界面 
   */
  private void init(){ 
    jframe.setTitle("登录"); 
    /** 
     * 设置JPanel背景 
     */
    jpanel = new JPanel(){ 
      @Override
      protected void paintComponent(Graphics g) { 
        super.paintComponent(g); 
        ImageIcon img = new ImageIcon(GUITest.class.getResource("ddmbg.jpg")); 
        img.paintIcon(this, g, 0, 0); 
      } 
    }; 
    //为JLabel,JButton初始化文本 
    jlabel.setText("用户名："); 
    jlabel1.setText("密  码："); 
    jbutton1.setText("登录"); 
    jbutton2.setText("退出"); 
    jbutton3.setText("注册"); 
    
    //设置显示位置及屏幕大小500*400 
    jframe.setBounds(450, 240, 400, 240); 
    //jpanel采用GridBagLayout布局管理器 
    jpanel.setOpaque(false); 
    jpanel.setLayout(gridbag); 
      
    //初始化用户名label，并添加该控件到画板 
    constraints = getGridBagConstraints(0,0,1,1,0,0,GridBagConstraints.CENTER,GridBagConstraints.NONE,new Insets(10,0,10,0),0,0); 
    gridbag.setConstraints(jlabel, constraints); 
    jpanel.add(jlabel); 
      
    //初始化用户名文本框，并添加该组件到画板 
    constraints = getGridBagConstraints(1,0,1,1,0,0,GridBagConstraints.CENTER,GridBagConstraints.NONE,new Insets(10,0,10,0),100,0); 
    gridbag.setConstraints(jtfield1, constraints); 
    jpanel.add(jtfield1); 
       
    //初始化密码label 
    constraints = getGridBagConstraints(0,1,1,1,0,0,GridBagConstraints.CENTER,GridBagConstraints.NONE,new Insets(10,0,10,0),0,0); 
    gridbag.setConstraints(jlabel1, constraints); 
    jpanel.add(jlabel1); 
     
    //初始化密码文本框 
    constraints = getGridBagConstraints(1,1,1,1,0,0,GridBagConstraints.CENTER,GridBagConstraints.NONE,new Insets(10,0,10,0),100,0); 
    gridbag.setConstraints(jpfield1, constraints); 
    jpanel.add(jpfield1); 
     
    //初始化注册按钮，并添加该控件到画板 
    constraints = getGridBagConstraints(0,2,1,1,0,0,GridBagConstraints.CENTER,GridBagConstraints.NONE,new Insets(10,0,10,0),0,0); 
    gridbag.setConstraints(jbutton3, constraints); 
    jpanel.add(jbutton3); 
    
    //初始化登录按钮 
    constraints = getGridBagConstraints(1,2,1,1,0,0,GridBagConstraints.CENTER,GridBagConstraints.NONE,new Insets(10,0,10,0),0,0); 
    gridbag.setConstraints(jbutton1, constraints); 
    jpanel.add(jbutton1); 
    
    //初始化退出按钮 
    constraints = getGridBagConstraints(2,2,1,1,0,0,GridBagConstraints.CENTER,GridBagConstraints.NONE,new Insets(10,0,10,0),0,0); 
    gridbag.setConstraints(jbutton2, constraints); 
    jpanel.add(jbutton2); 
      
    //添加画板到窗体 
    jframe.add(jpanel); 
    //窗体初始化完成 
  } 
   
   private static GridBagConstraints getGridBagConstraints(int gridx,int gridy,int gridwidth,int gridheight,double weightx,double weighty,int anchor,int fill,Insets insets,int ipadx,int ipady){ 
     return new GridBagConstraints(gridx, gridy, gridwidth, gridheight, weightx, weighty, anchor, fill, insets, ipadx, ipady); 
   } 
   
   public static void main(String[] args) { 
     new GUIT(); 
     jframe.setVisible(true); 
   } 
}