网友写的验证码生成方案，可防止绝大多数机械识别。

web.xml 

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
    <!-- ******************************************** -->
    <!-- **********验证码生成器************************** -->
    <!-- ******************************************** -->
    <servlet>
        <servlet-name>CaptchaServlet</servlet-name>
        <servlet-class>com.lowca.activity.web.servlet.CaptchaServlet</servlet-class>
        <init-param>
            <param-name>width</param-name>
            <param-value>150</param-value>
        </init-param>
        <init-param>
            <param-name>height</param-name>
            <param-value>100</param-value>
        </init-param>
        <init-param>
            <param-name>bgColor</param-name>
            <param-value>255,255,255</param-value>
        </init-param>
        <init-param>
            <param-name>fontColor</param-name>
            <param-value>0,255,0</param-value>
        </init-param>
        <init-param>
            <param-name>fontSize</param-name>
            <param-value>44</param-value>
        </init-param>
        <init-param>
            <param-name>fontCount</param-name>
            <param-value>4</param-value>
        </init-param>
        <init-param>
            <param-name>fontName</param-name>
            <param-value>宋体</param-value>
        </init-param>
        <init-param>
            <param-name>chars</param-name>
            <!-- 去掉了容易造成误解的字符，例如：i o z 和 1 0 2 -->
            <param-value>ABCDEFGHJKLMNPQRSTUVWXYabcdefghjkmnpqrstuvwxy3456789</param-value>
        </init-param>
        <init-param>
            <param-name>sessionKey</param-name>
            <param-value>com.lowca.activity.captcha</param-value>
        </init-param>
        <init-param>
            <param-name>maxRotateAngle</param-name>
            <param-value>40</param-value>
        </init-param>
        <init-param>
            <param-name>fontPadding</param-name>
            <param-value>-15</param-value>
        </init-param>
    </servlet>

    <servlet-mapping>
        <servlet-name>CaptchaServlet</servlet-name>
        <url-pattern>/captcha.jpg</url-pattern>
    </servlet-mapping>
</web-app>

servlet类文件 

package com.lowca.activity.web.servlet;

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import org.apache.commons.lang.StringUtils;
import org.apache.commons.lang.math.NumberUtils;

public class CaptchaServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    private static final String CONTENT_TYPE = "image/jpeg";

    // ==================以下变量用于保存配置参数的默认值 ==================
    /**
     * 验证码的宽度
     */
    private int width = 200;
    /**
     * 验证码的高度
     */
    private int height = 150;
    /**
     * 验证码背景色
     */
    private Color bgColor = Color.WHITE;
    /**
     * 文字颜色
     */
    private Color fontColor = Color.GREEN;
    /**
     * 验证码字符字号
     */
    private int fontSize = 42;
    /**
     * 验证码字符个数
     */
    private int fontCount = 4;
    /**
     * 验证码字符间距，单位是像素
     */
    private int fontPadding = -15;
    /**
     * 验证码字符最大旋转角度
     */
    private int maxRotateAngle = 40;
    /**
     * 验证码字体
     */
    private String fontName = "宋体";
    /**
     * 验证码用到的字符
     */
    private String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    /**
     * 验证码在session中的键名
     */
    private String sessionKey = "com.lowca.activity.captcha";

    // ==================以下变量用于在初始化时候保存数据，减少重复计算 ==================

    private Font font;

    private char[] seedArray;

    private int offsetX = 0;

    private int offsetY = 0;

    /**
     * 生成由数字和字母随机组成的图片
     */
    protected void service(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, java.io.IOException {
        BufferedImage buffImg = new BufferedImage(width, height,
                BufferedImage.TYPE_INT_RGB);
        Graphics2D g2d = buffImg.createGraphics();
        g2d.setColor(bgColor);
        g2d.fillRect(0, 0, width, height);
        g2d.setPaint(fontColor);
        g2d.setFont(font);
        // 旋转文本

        StringBuffer randCode = new StringBuffer();
        for (int i = 0; i < fontCount; i++) {
            g2d.setStroke(new BasicStroke((float) (Math.random() * 5)));
            double angle = Math.random() * maxRotateAngle * Math.PI / 180;
            int p = (int) (Math.random() * chars.length());
            String word = String.valueOf(seedArray[p]);
            randCode.append(word);
            int x = offsetX + i * (fontSize + fontPadding);

            AffineTransform origXform = g2d.getTransform();
            AffineTransform newXform = (AffineTransform) (origXform.clone());
            newXform.rotate(angle, x, offsetY);
            g2d.setTransform(newXform);
            g2d.drawString(word, x, offsetY);
            g2d.setTransform(origXform);
        }
        g2d.setStroke(new BasicStroke(2.0f));
        int x = offsetX - (int) (20 * Math.random());
        int y = offsetY - (int) (20 * Math.random());
        int w = (int) (width - 10 - Math.random() * 6);
        int h = (int) (height - 10 - Math.random() * 6);
        int startAngle = (int) (Math.random() * 16 + 45);
        int arcAngle = (int) (Math.random() * 45 + 90);
        g2d.drawArc(x, y, w, h, startAngle, arcAngle);
        g2d.dispose();

        // 将四位数字的验证码保存到Session里面
        HttpSession session = req.getSession();
        session.setAttribute(sessionKey, randCode);

        // 禁止图像缓存
        resp.setHeader("Prama", "no-cache");
        resp.setHeader("Coche-Control", "no-cache");
        resp.setDateHeader("Expires", 0);

        // 将图像输出到Servelt输出流中
        ServletOutputStream out = resp.getOutputStream();
        ImageIO.write(buffImg, "jpeg", out);
        out.close();
    }

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

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

    public void init() throws ServletException {
        // 初始化配置参数
        String param = getInitParameter("width");
        if (NumberUtils.isDigits(param)) {
            width = Integer.parseInt(param);
        }
        param = getInitParameter("height");
        if (NumberUtils.isDigits(param)) {
            height = Integer.parseInt(param);
        }
        param = getInitParameter("bgColor");
        if (param != null && param.matches("^\d+,\d+,\d+$")) {
            bgColor = getColor(param);
        }
        param = getInitParameter("fontColor");
        if (param != null && param.matches("^\d+,\d+,\d+$")) {
            fontColor = getColor(param);
        }
        param = getInitParameter("fontSize");
        if (NumberUtils.isDigits(param)) {
            fontSize = Integer.parseInt(param);
        }
        param = getInitParameter("maxRotateAngle");
        if (NumberUtils.isDigits(param)) {
            maxRotateAngle = Integer.parseInt(param);
        }
        param = getInitParameter("fontPadding");
        if (param != null && param.matches("-?\d+")) {
            fontPadding = Integer.parseInt(param);
        }
        param = getInitParameter("fontName");
        if (StringUtils.isNotBlank(param)) {
            fontName = param;
        }
        param = getInitParameter("chars");
        if (StringUtils.isNotBlank(param)) {
            chars = param;
        }
        param = getInitParameter("sessionKey");
        if (StringUtils.isNotBlank(sessionKey)) {
            sessionKey = param;
        }
        // 初始化状态变量
        font = new Font(fontName, Font.ITALIC, fontSize);
        seedArray = chars.toCharArray();
        offsetX = width / 2 - fontCount * fontSize / 2 - (fontCount - 1)
                * fontPadding / 2 - 8;
        offsetY = height / 2 + fontSize / 2 - 8;
    }

    private Color getColor(String rgb) {
        String[] array = rgb.split(",");
        return new Color(Integer.parseInt(array[0]),
                Integer.parseInt(array[1]), Integer.parseInt(array[2]));
    }

}

在我的机器上，这个程序每秒钟可以产生大约83张验证码。验证码图样如下

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• 大小: 2.6 KB

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• 大小: 2.5 KB

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• 大小: 2.4 KB

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• 大小: 2.4 KB