在SIS2下，验证码的实现
按照整个执行过程来描述的，自己备注一下，呵呵

一、页面链接
<img id="vCode" src="validateImage.jpg" align="absmiddle" /> 
<a href="#" onclick="changeCode();">看不清,换一个</a>

二、在Web.xml中配置validateImg的映射路径

<servlet><servlet-name>validateImg</servlet-name><servlet-class>com.cytest.ValidateImg</servlet-class></servlet><servlet-mapping><servlet-name>validateImg</servlet-name><url-pattern>/validateImage.jpg</url-pattern></servlet-mapping>

三、ValidateImg.java的具体实现

package com.cytest;import javax.servlet.*;import javax.servlet.http.*;import java.io.*;import java.awt.*;import java.awt.image.*;import java.util.*;import javax.imageio.*; public class ValidateImg extends HttpServlet {//定义图形验证码中绘制字符的字体private final Font mFont = new Font("Arial Black", Font.PLAIN, 16);//定义图形验证码的大小private final int IMG_WIDTH = 70;private final int IMG_HEIGTH = 20;/** * 定义一个获取随机颜色的方法 */private 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);//得到随机颜色return new Color(r , g , b);}/** * 重写service方法，生成对客户端的响应 */public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//设置禁止缓存response.setHeader("Pragma","No-cache");response.setHeader("Cache-Control","no-cache");response.setDateHeader("Expires", 0);response.setContentType("image/jpeg");BufferedImage image = new BufferedImage(IMG_WIDTH , IMG_HEIGTH , BufferedImage.TYPE_INT_RGB);Graphics g = image.getGraphics();Random random = new Random();g.setColor(getRandColor(200 , 250));//填充背景色g.fillRect(1, 1, IMG_WIDTH - 1, IMG_HEIGTH - 1); //为图形验证码绘制边框g.setColor(new Color(102 , 102 , 102));g.drawRect(0, 0, IMG_WIDTH - 1, IMG_HEIGTH - 1);g.setColor(getRandColor(160,200));//生成随机干扰线for (int i = 0 ; i < 80 ; i++) {int x = random.nextInt(IMG_WIDTH - 1);int y = random.nextInt(IMG_HEIGTH - 1);int xl = random.nextInt(6) + 1;int yl = random.nextInt(12) + 1;g.drawLine(x , y , x + xl , y + yl);}g.setColor(getRandColor(160,200));//生成随机干扰线for (int i = 0 ; i < 80 ; i++) {int x = random.nextInt(IMG_WIDTH - 1);int y = random.nextInt(IMG_HEIGTH - 1);int xl = random.nextInt(12) + 1;int yl = random.nextInt(6) + 1;g.drawLine(x , y , x - xl , y - yl);}//设置绘制字符的字体g.setFont(mFont);//用于保存系统生成的随机字符串String sRand = "";for (int i = 0 ; i < 4 ; i++) {String tmp = getRandomChar();sRand += tmp;//获取随机颜色g.setColor(new Color(20 + random.nextInt(110),20 + random.nextInt(110),20 + random.nextInt(110)));//在图片上绘制系统生成的随机字符g.drawString(tmp , 15 * i + 10,15);}//获取HttpSesssion对象HttpSession session = request.getSession(true);//将随机字符串放入HttpSesssion对象中 session.setAttribute("randNum" , sRand);g.dispose();//向输出流中输出图片ImageIO.write(image, "JPEG", response.getOutputStream());}/** * 获取随机字符串 * @return */private String getRandomChar() {//生成一个0、1、2的随机数字int rand = (int)Math.round(Math.random() * 2);long itmp = 0;char ctmp = '\u0000';switch (rand) {//生成大写字母case 1:itmp = Math.round(Math.random() * 25 + 65);ctmp = (char)itmp;return String.valueOf(ctmp);//生成小写字母case 2:itmp = Math.round(Math.random() * 25 + 97);ctmp = (char)itmp;return String.valueOf(ctmp);//生成数字default :itmp = Math.round(Math.random() * 9);return  itmp + "";}}}

四、不小心误删Tomcat下的temp，出现了验证码图片无法加载的错误
原因：ImageIO.write(image, "JPEG", response.getOutputStream());此句需要写缓冲文件，该文件保存在temp下，当其不存在时，导致can't create output stream!

1.在Tomcat下手动创建一个temp
2.将ImageIO.write(image, "JPEG", response.getOutputStream());换成如下语句：
引用JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(response.getOutputStream());
encoder.encode(image);

PS：以上是解决此问题的2种方式