验证码基本原理

	response.setContentType("image/jpeg");
		BufferedImage bi=new BufferedImage(100,80,1);
		Graphics gra=bi.getGraphics();
		gra.setColor(new Color(255,0,0));
		gra.drawRect(0, 0,100 , 80);
		gra.setColor(new Color(55,55,55));
		gra.fillRect(0, 0, 100, 80);
		gra.setColor(new Color(0,255,0));
		gra.drawLine(0, 0,100, 80);
		Font font=new Font("Fixedsys",Font.PLAIN,80-2);
		gra.setColor(new Color(255,0,0));
		gra.setFont(font);
		gra.drawString("ST", 0, 80);
		OutputStream os=response.getOutputStream();
		ImageIO.write(bi, "jpeg",os);
		os.flush();os.close();