验证码的生成，并且“看不清，换一张”

public class AuthImg extends HttpServlet {
	private static final String CONTENT_TYPE = "text/html; charset=UTF-8";

	private Font mFont;

	public AuthImg() {
		mFont = new Font("Times New Roman", 0, 18);
	}

	public void init() throws ServletException {
	}

	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		String authCode = genAuthCode(4);
		AuthCodeCookie authCodeCookie = new AuthCodeCookie(request, response);
		authCodeCookie.addAuthCode(authCode);//保存验证码到cookies
		response.setContentType("image/jpeg");
		javax.servlet.ServletOutputStream out = response.getOutputStream();
		int width = 60;
		int height = 20;
		BufferedImage image = new BufferedImage(width, height, 1);
		Graphics gra = image.getGraphics();
		Random random = new Random();
		gra.setColor(getRandColor(200, 250));
		gra.fillRect(1, 1, width, height);
		gra.setColor(getRandColor(160, 200));
		for (int i = 0; i < 155; i++) {
			int x = random.nextInt(width);
			int y = random.nextInt(height);
			int xl = random.nextInt(12);
			int yl = random.nextInt(12);
			gra.drawLine(x, y, x + xl, y + yl);
		}

		gra.setFont(mFont);
		for (int i = 0; i < 4; i++) {
			char c = authCode.charAt(i);
			gra.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
			gra.drawString(String.valueOf(c), 13 * i + 6, 16);
		}

		gra.dispose();
		ImageIO.write(image, "JPEG", out);
	}

	Color getRandColor(int fc, int bc) {
		Random random = new Random();
		if (fc > 255)
			fc = 255;
		if (bc > 255)
			bc = 255;
		int r = fc + random.nextInt(bc - fc);
		int g = fc + random.nextInt(bc - fc);
		int b = fc + random.nextInt(bc - fc);
		return new Color(r, g, b);
	}

	public String genAuthCode(int length) {
		if (length < 1)
			return null;
		String strChars[] = { "1", "2", "3", "4", "5", "6", "7", "8", "9" };
		StringBuffer strPassword = new StringBuffer();
		int nRand = (int) Math.round(Math.random() * 100D);
		for (int i = 0; i < length; i++) {
			nRand = (int) Math.round(Math.random() * 100D);
			strPassword.append(strChars[nRand % (strChars.length - 1)]);
		}

		return strPassword.toString();
	}

	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		doGet(request, response);
	}

	public void destroy() {
	}

}

html:
function refresh() {
var img = document.getElementById("checkCodeImg");
now = new Date();
img.src = "authimg?code="+now.getTime();
}

<IMG height=18 src='authimg' id="checkCodeImg" >
<a href="#" onclick="javascript:refresh();">看不清,换一张</a>

web.xml:
<servlet>
    <servlet-name>authimg</servlet-name>
    <servlet-class>com.worthtech.app.servlet.AuthImg</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>authimg</servlet-name>
    <url-pattern>/authimg</url-pattern>
  </servlet-mapping>