JPanel

class NewPanel extends JPanel{
    public NewPanel() {}

    public void paintComponent(Graphics g)
    {
       int x=0,y=0;
       java.net.URL imgURL=getClass().getResource("login.jpg");

       //test.jpg是测试图片，与Demo.java放在同一目录下
        ImageIcon icon=new ImageIcon(imgURL);//this is really useful
        g.drawImage(icon.getImage(),x,y,getSize().width,getSize().height,this);
        while(true)
        {
          g.drawImage(icon.getImage(),x,y,this);
          if(x>getSize().width && y>getSize().height)break;
         //这段代码是为了保证在窗口大于图片时，图片仍能覆盖整个窗口
         if(x>getSize().width)
          {
             x=0;
             y+=icon.getIconHeight();
          }
          else
           x+=icon.getIconWidth();
        }
     }
   }

实现背景的设置