为什么我不能从Main类显示此自定义JFrame？

我在Java Swing开发中是一个新手，并且遇到以下问题

我有以下3个课程：

1）一种主要类，简单地示出了LoginFrame类：

package com.test.login4;

import javax.swing.JFrame;


public class Main {

    private static final LoginFrame loginFrame = new LoginFrame();

    //private static final GUI gui = new GUI();

    public static void main(String[] args) {
          System.out.println("Main ---> main()");   

          loginFrame.setVisible(true);


    }

}

2）然后，我有了LoginFrame类，该类简单地扩展了经典的Swing JFrame，它显示了用户可以在其中插入用户名和密码的登录表单：

package com.test.login4;

import java.awt.Container;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.IOException;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;



public class LoginFrame extends JFrame implements ActionListener {

    private static final int FIXED_WIDTH = 550;
    private static final Dimension INITAL_SIZE = new Dimension(FIXED_WIDTH, 230);

    private boolean loginResult = true;

    public LoginFrame() {

        System.out.println("Inside LoginFrame ---> LoginFrame()");

        this.setTitle("XCloud Login");

        this.setPreferredSize(INITAL_SIZE);
        this.setResizable(false);

        Container mainContainer = this.getContentPane(); // main Container into
                                                            // the main JFrame

        // JPanel creation and settings of the MigLayout on it:
        // JPanel externalPanel = new JPanel();
        JPanelWithBackground externalPanel = null;

        try {
            // externalPanel = new JPanelWithBackground("resources/logo.png");
            externalPanel = new JPanelWithBackground("src/com/test/resources/logo.png");
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        externalPanel.setLayout(new net.miginfocom.swing.MigLayout("fill"));

        externalPanel.add(new JLabel("Username"), "w 50%, wrap");
        JTextField userNameTextField = new JTextField(20);

        externalPanel.add(userNameTextField, "w 90%, wrap");

        externalPanel.add(new JLabel("Password"), "w 50%, wrap");
        JTextField pswdTextField = new JTextField(20);
        externalPanel.add(pswdTextField, "w 90%, wrap");

        JButton loginButton = new JButton("Login");
        loginButton.setActionCommand("loginAction");
        loginButton.addActionListener(this);

        externalPanel.add(loginButton, "w 25%, wrap");

        mainContainer.add(externalPanel);
        // mainFrame.add(mainContainer);
        // loginFrame.setVisible(true);
        this.setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub

    }

}

3）此类使用JPanelWithBackground对象具有背景图像

package com.test.login4;

import java.awt.Graphics;
import java.awt.Image;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.JPanel;

public class JPanelWithBackground extends JPanel {

      private Image backgroundImage;

      // Some code to initialize the background image.
      // Here, we use the constructor to load the image. This
      // can vary depending on the use case of the panel.
      public JPanelWithBackground(String fileName) throws IOException {
        backgroundImage = ImageIO.read(new File(fileName));
      }

      public void paintComponent(Graphics g) {
        super.paintComponent(g);

        // Draw the background image.
        //g.drawImage(backgroundImage, 0, 0, this);
        g.drawImage(backgroundImage, 0, 0, 550, 230, this);
      }
    }

我的问题是，当我执行Main类时，它向我显示Java应用程序的图标（在Ubuntu栏上），但无法显示LoginFrame窗口。

为什么？我想念什么？

我尝试创建并显示一个经典的JFrame而不是我的LoginFrame（在Main类中），并且我没有问题。