当我运行jar文件时，我的图像消失了

Question

这里是我制作的课程..

package testdnevnik;

import javax.swing.JFrame;

public class Diary {

  StartWindow startWindow = new StartWindow();

  static JFrame frame = new JFrame("Diary");

  int n = 0;

  public Diary() {

     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     frame.setResizable(false);
     frame.setSize(800,600);
     frame.setLocationRelativeTo(null);
     //frame.add(new ImagePanel("Img//bg2.jpeg"));
     frame.setVisible(true);
     frame.add(startWindow);

     startWindow.checkStudents(n);
     startWindow.chooseOption();
  }

  public static void main(String[] args) {
     new Diary();  
  }

}

这是主类，这里是开始窗口的面板..

package testdnevnik;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JPasswordField;
import javax.swing.JTextField;
import static testdnevnik.Diary.frame;

public class StartWindow extends JPanel {

  JPanel jp = new JPanel();
  JLabel jl = new JLabel();
  JLabel headline = new JLabel();
  JButton viewStudents = new JButton();
  JButton editStudents = new JButton();
  JButton addStudents = new JButton();
  JButton removeStudents = new JButton();
  JLabel usernameLbl = new JLabel();
  JLabel passwordLbl = new JLabel();
  JTextField usernameFld = new JTextField();
  JPasswordField passwordFld = new JPasswordField();
  JButton logIn = new JButton("Log In");
  JButton signUp = new JButton("Sign Up");

  public StartWindow () {


  jp.setLayout( new BorderLayout() );

 // headline
 headline.setText("Diary");
 headline.setLocation(200, 30);
 headline.setHorizontalAlignment(JLabel.CENTER);
 headline.setFont(new Font("Serif", Font.BOLD, 60));
 headline.setForeground(Color.ORANGE);
 headline.setSize(300, 80);
 headline.setVisible(true);

  // Buttons
 //View Students - Button
 viewStudents.setText("View All Students");
 viewStudents.setSize(200,40);
 viewStudents.setLocation(70 , 200);
 viewStudents.setVisible(true);

  // Edit Students - Button
 editStudents.setText("Edit Students");
 editStudents.setSize(200,40);
 editStudents.setLocation(70 , 300);
 editStudents.setVisible(true);

  // Add Students - Button
 addStudents.setText("Add New Students");
 addStudents.setSize(200,40);
 addStudents.setLocation(70 ,400);
 addStudents.setVisible(true);

  // Remove Students - Button
 removeStudents.setText("Remove Students");
 removeStudents.setSize(200,40);
 removeStudents.setLocation(70 , 500);
 removeStudents.setVisible(true);

 // username and password labels and input field
 usernameLbl.setText("Username: ");
 usernameLbl.setBounds(320, 200, 108, 40);
 usernameLbl.setFont(new Font(null,Font.BOLD, 20));
 usernameLbl.setVisible(true);
 usernameFld.setBounds(450, 205, 200, 30); // username field input
 usernameFld.setVisible(true);

 passwordLbl.setText("Password: ");
 passwordLbl.setBounds(320, 300, 108, 40);
 passwordLbl.setFont(new Font(null,Font.BOLD, 20));
 passwordLbl.setVisible(true);
 passwordFld.setBounds(450, 305, 200, 30); // password field input
 passwordFld.setVisible(true);

 // button for log in and sign up
 logIn.setBounds(580, 400, 70, 40);
 logIn.setVisible(true);
 signUp.setBounds(480, 400, 90, 40);
 signUp.setVisible(true);

 // panel
 jp.setSize(new Dimension(800,600));

 jp.add(viewStudents); // add ViewStudents button to the panel
 jp.add(editStudents); // add EditStudents button to the panel
 jp.add(addStudents); // add AddStudents button to the panel
 jp.add(removeStudents); // add RemoveStudents button to the panel
 jp.add(usernameLbl); // username label
 jp.add(passwordLbl); // password label
 jp.add(usernameFld); // username input field
 jp.add(passwordFld); // password input field
 jp.add(logIn); //add log in button to the panel
 jp.add(signUp); //add sign up button to the panel
 jp.add(headline); // add headline label to the panel
 jp.add(new ImagePanel("Img//bg2.jpg")); // add background image to the panel

 // adding panel to the frame
 frame.add(jp);

 validate();
}

  public void checkStudents(int n) {
     int numberOfStudents = n;
        if (numberOfStudents == 0) {
           removeStudents.setEnabled(false);
     }
  }

  public void chooseOption() {

     viewStudents.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent evt) {
           JOptionPane.showMessageDialog(null, "view students", "alert", JOptionPane.INFORMATION_MESSAGE);
        }
  });

     editStudents.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent evt) {
           JOptionPane.showMessageDialog(null, "edit students", "alert", JOptionPane.INFORMATION_MESSAGE);
        }
  });

     addStudents.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent evt) {
           JOptionPane.showMessageDialog(null, "add students", "alert", JOptionPane.INFORMATION_MESSAGE);
        }
  });

     removeStudents.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent evt) {
           JOptionPane.showMessageDialog(null, "remove students", "alert", JOptionPane.INFORMATION_MESSAGE);
        }
  });

     logIn.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent evt) {
           JOptionPane.showMessageDialog(null, "succesful log in", "alert", JOptionPane.INFORMATION_MESSAGE);
        }
     });

     signUp.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent evt) {
           JOptionPane.showMessageDialog(null, "sign up form", "alert", JOptionPane.INFORMATION_MESSAGE);
        }
     });

  }

}

这里是图像背景的类

package testdnevnik;

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import javax.swing.ImageIcon;
import javax.swing.JPanel;

public class ImagePanel extends JPanel {

private Image img;

public ImagePanel(String img) {
    this(new ImageIcon(img).getImage());
}

public ImagePanel(Image img) {
    this.img = img;
    Dimension size = new Dimension(img.getWidth(null), img.getHeight(null));
    setPreferredSize(size);
}

public void paintComponent(Graphics g) {
    g.drawImage(img, 0, 0, null);
}
}

当我从NetBeans运行程序时，一切正常，运行完美，但是当我在NetBeans中单击“清理并生成项目”并从文件夹中的jar文件运行时，背景为白色，但组件已添加到面板很好，只是背景图像消失了，为什么??