Patchca是Piotr Piastucki写的一个java验证码开源库，打包成jar文件发布，patchca使用简单但功能强大。

本例实现了自定义背景，由于生成图片较小，波动太大时会导致部分文字显示不全，所以更改了滤镜属性。

效果图：

代码如下：

package com.ninemax.cul.servlet;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.awt.image.BufferedImageOp;
import java.io.IOException;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

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

import org.patchca.background.BackgroundFactory;
import org.patchca.color.ColorFactory;
import org.patchca.color.RandomColorFactory;
import org.patchca.filter.ConfigurableFilterFactory;
import org.patchca.filter.library.AbstractImageOp;
import org.patchca.filter.library.WobbleImageOp;
import org.patchca.font.RandomFontFactory;
import org.patchca.service.Captcha;
import org.patchca.service.ConfigurableCaptchaService;
import org.patchca.text.renderer.BestFitTextRenderer;
import org.patchca.text.renderer.TextRenderer;
import org.patchca.word.RandomWordFactory;

/** 
 * 验证码生成类 
 * 
 * 使用开源验证码项目patchca生成 
 * 依赖jar包：patchca-0.5.0.jar 
 * 项目网址：https://code.google.com/p/patchca/ 
 * 
 * @author zyh
 * @version 1.00  2012-7-12 New 
 */
public class ValidationCodeServlet extends HttpServlet {
	private static final long serialVersionUID = 5126616339795936447L;
	
	private ConfigurableCaptchaService configurableCaptchaService = null;
	private ColorFactory colorFactory = null;
	private RandomFontFactory fontFactory = null;
	private RandomWordFactory wordFactory = null;
	private TextRenderer textRenderer = null;
	
	public ValidationCodeServlet() {
		super();
	}

	/**
	 * Servlet销毁方法,负责销毁所使用资源. <br>
	 */
	public void destroy() {
		wordFactory = null;
		colorFactory = null;
		fontFactory = null;
		textRenderer = null;
		configurableCaptchaService = null;
		super.destroy(); // Just puts "destroy" string in log
	}

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

	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		response.setContentType("image/png");
		response.setHeader("cache", "no-cache");
		
		HttpSession session = request.getSession(true);
		OutputStream outputStream = response.getOutputStream();
		
		// 得到验证码对象,有验证码图片和验证码字符串
		Captcha captcha = configurableCaptchaService.getCaptcha();
		// 取得验证码字符串放入Session
		String validationCode = captcha.getChallenge();
		session.setAttribute("validationCode", validationCode);
		// 取得验证码图片并输出
		BufferedImage bufferedImage = captcha.getImage();
		ImageIO.write(bufferedImage, "png", outputStream);
		
		outputStream.flush();
		outputStream.close();
	}

	/**
	 * Servlet初始化方法
	 */
	public void init() throws ServletException {
		configurableCaptchaService = new ConfigurableCaptchaService();
		
		// 颜色创建工厂,使用一定范围内的随机色
		colorFactory = new RandomColorFactory();
		configurableCaptchaService.setColorFactory(colorFactory);
		
		// 随机字体生成器
		fontFactory = new RandomFontFactory();
		fontFactory.setMaxSize(32);
		fontFactory.setMinSize(28);
		configurableCaptchaService.setFontFactory(fontFactory);
		
		// 随机字符生成器,去除掉容易混淆的字母和数字,如o和0等
		wordFactory = new RandomWordFactory();
		wordFactory.setCharacters("abcdefghkmnpqstwxyz23456789");
		wordFactory.setMaxLength(5);
		wordFactory.setMinLength(4);
		configurableCaptchaService.setWordFactory(wordFactory);
		
		// 自定义验证码图片背景
		MyCustomBackgroundFactory backgroundFactory = new MyCustomBackgroundFactory();
		configurableCaptchaService.setBackgroundFactory(backgroundFactory);
		
		// 图片滤镜设置
		ConfigurableFilterFactory filterFactory = new ConfigurableFilterFactory();
		
		List<BufferedImageOp> filters = new ArrayList<BufferedImageOp>();
		WobbleImageOp wobbleImageOp = new WobbleImageOp();
		wobbleImageOp.setEdgeMode(AbstractImageOp.EDGE_MIRROR);
		wobbleImageOp.setxAmplitude(2.0);
		wobbleImageOp.setyAmplitude(1.0);
		filters.add(wobbleImageOp);
		filterFactory.setFilters(filters);
		
		configurableCaptchaService.setFilterFactory(filterFactory);
		
		// 文字渲染器设置
		textRenderer = new BestFitTextRenderer();
		textRenderer.setBottomMargin(3);
		textRenderer.setTopMargin(3);
		configurableCaptchaService.setTextRenderer(textRenderer);
		
		// 验证码图片的大小
		configurableCaptchaService.setWidth(82);
		configurableCaptchaService.setHeight(32);
	}
	
	/**
	 * 自定义验证码图片背景,主要画一些噪点和干扰线
	 */
	private class MyCustomBackgroundFactory implements BackgroundFactory {
		private Random random = new Random();

		public void fillBackground(BufferedImage image) {
			Graphics graphics = image.getGraphics();
			
			// 验证码图片的宽高
			int imgWidth = image.getWidth();
			int imgHeight = image.getHeight();
			
			// 填充为白色背景
			graphics.setColor(Color.WHITE);
			graphics.fillRect(0, 0, imgWidth, imgHeight);
			
			// 画100个噪点(颜色及位置随机)
			for(int i = 0; i < 100; i++) {
				// 随机颜色
				int rInt = random.nextInt(255);
				int gInt = random.nextInt(255);
				int bInt = random.nextInt(255);
				
				graphics.setColor(new Color(rInt, gInt, bInt));
				
				// 随机位置
				int xInt = random.nextInt(imgWidth - 3);
				int yInt = random.nextInt(imgHeight - 2);
				
				// 随机旋转角度
				int sAngleInt = random.nextInt(360);
				int eAngleInt = random.nextInt(360);
				
				// 随机大小
				int wInt = random.nextInt(6);
				int hInt = random.nextInt(6);
				
				graphics.fillArc(xInt, yInt, wInt, hInt, sAngleInt, eAngleInt);
				
				// 画5条干扰线
				if (i % 20 == 0) {
					int xInt2 = random.nextInt(imgWidth);
					int yInt2 = random.nextInt(imgHeight);
					graphics.drawLine(xInt, yInt, xInt2, yInt2);
				}
			}
		}
	}
}

由于是个Servlet所以web.xml配置如下：

<servlet>
    <servlet-name>validationCode</servlet-name>
    <servlet-class>com.ninemax.cul.servlet.ValidationCodeServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>validationCode</servlet-name>
    <url-pattern>/validationCodeServlet.png</url-pattern>
  </servlet-mapping>

JSP引用(部分)：

<img id="validationCode" alt="验证码图片" title="验证码图片" src="<%=path %>/validationCodeServlet.png" οnclick="refreshCode(this)" />
<a id="aRecode" href="javascript:void(0);" οnclick="refreshCode()">换一张</a>

JS重新载入图片方法(参考)：

/**
 * 刷新验证码
 * @param imgObj 验证码Img元素
 */
function refreshCode(imgObj) {
	if (!imgObj) {
		imgObj = document.getElementById("validationCode");
	}
	var index = imgObj.src.indexOf("?");
	if(index != -1) {
		var url = imgObj.src.substring(0,index + 1);
		imgObj.src = url + Math.random();
	} else {
		imgObj.src = imgObj.src + "?" + Math.random();
	}
}