Java如何添加背景。。。。。。。

Question

如何为面板添加背景图片，求大神指导，代码如下：（自己添加的时候总是把组件遮挡了或者添加不上去）
万分感谢！！
import java.awt.*;
import java.awt.event.*;
import java.io.File;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.*;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.NodeList;

public class Main extends JFrame implements ActionListener {
/**
* @param args
*/
JLabel l1 = new JLabel("武汉纺织大学学生信息查询");
JLabel l2 = new JLabel("用户登陆");
JLabel l3 = new JLabel("用户名 ：");
JLabel l4 = new JLabel("密 码 ：");
JTextField t1 = new JTextField();
JPasswordField t2 = new JPasswordField();
JButton button1 = new JButton("登陆");
JButton button2 = new JButton("重置");
static JPanel p1 = new JPanel(new FlowLayout(FlowLayout.CENTER, 2, 2));
Font f1 = new Font("宋体", Font.BOLD, 20);
Font f2 = new Font("宋体", Font.BOLD, 30);
Font f3 = new Font("宋体", Font.BOLD, 15);
public static String currentSID = "";

public Main() {
    GUI();
}
public void GUI() { // 面板设计
    p1.add(l1);
    p1.add(l2);
    p1.add(l3);
    p1.add(l4);// 文字
    p1.add(t1);
    p1.add(t2);// 文本框
    p1.add(button1);
    p1.add(button2);// 按钮
    add(p1);
    p1.setBackground(Color.black);
    p1.setLayout(null);
    l1.setBounds(100, 50, 400, 30);
    l1.setFont(f2);
    l1.setForeground(Color.cyan);
    l2.setBounds(250, 150, 130, 40);
    l2.setFont(f1);
    l2.setForeground(Color.cyan);
    l3.setBounds(170, 230, 100, 40);
    l3.setFont(f1);
    l3.setForeground(Color.cyan);
    l4.setBounds(170, 280, 100, 40);
    l4.setFont(f1);
    l4.setForeground(Color.cyan);
    t1.setBounds(280, 240, 130, 20);
    t1.setFont(f3);
    t2.setBounds(280, 290, 130, 20);
    t2.setFont(f3);
    button1.setBounds(200, 350, 80, 30);
    button1.setFont(f1);
    button2.setBounds(300, 350, 80, 30);
    button2.setFont(f1);
    button1.addActionListener(this);
    button2.addActionListener(this);
}

public void actionPerformed(ActionEvent e) { // 登陆消息响应
    // TODO Auto-generated method stub  
    Object source = e.getSource();
    if (source == button1) {
        String user = t1.getText();
        String password = t2.getText();
        if((user.equals("admin")==true)&&(password.equals("123")==true))
         {
        setVisible(false);
        new panel();
         }
         else if(login(user,password))
         {
             setVisible(false);
             new vo();
         }
         else if((user.equals("")==true&&(password.equals("")==true)))
         {
             JOptionPane.showMessageDialog(null, "请输入用户名和密码！", "提示", 2);
         }
         else{
             JOptionPane.showMessageDialog(null, "输入的用户名或密码错误！请重新输入！", "提示", 2);
             t1.setText(null);
             t2.setText(null);
         }
    }
    if (source == button2) {
        t1.setText(null);
        t2.setText(null);
    }
}

public boolean login(String name,String password){ // 调用xml文档中的参数登陆
    try {
        DocumentBuilderFactory factory = DocumentBuilderFactory
                .newInstance();
        DocumentBuilder domParser = factory.newDocumentBuilder();
        Document document = domParser.parse(new File("class.xml"));
        try {
            NodeList imags = document.getElementsByTagName("student");
            int j = imags.getLength();
            for (int k = 0; k < j; k++) {
                Element elink = (Element) imags.item(k);
                String number = elink.getElementsByTagName("number").item(0)
                        .getTextContent();
                String classes = elink.getElementsByTagName("classes")
                        .item(0).getTextContent();
                if(name.equals(number) && password.equals(classes)){
                    currentSID = number;
                    return true;
                }
            }
        } catch (Exception e) {
            System.out.println(e);
            System.out.println("错误");
        }

    } catch (Exception e) {
        System.out.println(e);
        System.out.println("错误");
    }
    return false;
}

public static void main(String[] args) {
    // TODO Auto-generated method stub 
    Main f = new Main();
    f.setTitle("武汉纺织大学");
    f.add(p1);  
    f.setSize(600, 600);
    f.setLocationRelativeTo(null);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setVisible(true);
    try {
        // 设置界面的外观，为系统外观
        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        SwingUtilities.updateComponentTreeUI(f);
    } catch (Exception e) {
        e.printStackTrace();
    }

}
}

Answer 1

http://blog.csdn.net/huai814586181/article/details/45788263