第一种方法 全是字母和数字组成 

package com.myFirstSpring.util;
 
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;
 
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;
 
/**
 * 
 * @Description 生成验证码
 * @author waa
 * @version V1.0, 日期2011-7-4
 */
public class ValidateCodeUtils extends HttpServlet {
 
 private static final long serialVersionUID = -1409007752285164213L;
 private static int width = 75;
 private static int height = 35;
 // private static char[] ch =
 // "AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz0123456789".toCharArray();
 private static char[] ch = "abcdefghijklmnopqrstuvwxyz0123456789".toCharArray();
 
 public void doGet(HttpServletRequest request, HttpServletResponse response)
 
 throws ServletException, IOException {
 
 this.doPost(request, response);
 
 }
 
 // 生成数字和字母的验证码
 public void doPost(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
 processRequest(request, response);
 }
 
 protected void processRequest(HttpServletRequest request,
  HttpServletResponse response) throws ServletException, IOException {
 response.setContentType("image/jpeg");
 response.setHeader("Pragma", "No-cache");
 response.setHeader("Cache-Control", "no-cache");
 response.setDateHeader("Expires", 0);
 // 在内存中创建图象
 BufferedImage image = new BufferedImage(width, height,
  BufferedImage.TYPE_INT_RGB);
 
 // 获取图形上下文
 Graphics g = image.getGraphics();
 
 // 生成随机类
 Random random = new Random();
 
 // 设定背景色
 g.setColor(getRandColor(200, 250));
 g.fillRect(0, 0, width, height);
 
 // 设定字体
 g.setFont(new Font("Times New Roman", Font.PLAIN, 18));
 
 // 画边框
 // g.setColor(new Color());
 // g.drawRect(0,0,width-1,height-1);
 
 // 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
 g.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);
  g.drawLine(x, y, x + xl, y + yl);
 }
 
 // 取随机产生的认证码(4位数字)
 StringBuffer sb = new StringBuffer();
 int index, len = ch.length;
 for (int i = 0; i < 4; i++) {
  index = random.nextInt(len);
  g.setColor(new Color(random.nextInt(88), random.nextInt(188),
   random.nextInt(255)));
  g.setFont(new Font("Arial", Font.ITALIC, 28));// 输出的字体和大小
  g.drawString("" + ch[index], (i * 15) + 6, 26);// 写什么数字，在图片的什么位置画
  sb.append(ch[index]);
 }
 // 将认证码存入SESSION
 request.getSession().setAttribute("piccode", sb.toString());
 request.getSession().setAttribute(request.getSession().getId(), sb.toString());
 
 // 图象生效
 g.dispose();
 ServletOutputStream responseOutputStream = response.getOutputStream();
 // 输出图象到页面
 ImageIO.write(image, "JPEG", responseOutputStream);
 
 // 以下关闭输入流！
 responseOutputStream.flush();
 responseOutputStream.close();
  
 }
 
 protected void ValidateCode(HttpServletRequest request,
  HttpServletResponse response) throws ServletException, IOException {
 // 在内存中创建图象
 BufferedImage img = new BufferedImage(width, height,
  BufferedImage.TYPE_INT_RGB);
 // 得到该图片的绘图对象
 Graphics g = img.getGraphics();
 
 Random r = new Random();
 Color c = new Color(200, 150, 255);
 g.setColor(c);
 
 // 填充整个图片的颜色
 g.fillRect(0, 0, width, height);
 
 // 向图片中输出数字和字母
 StringBuffer sb = new StringBuffer();
 int index, len = ch.length;
 for (int i = 0; i < 4; i++) {
  index = r.nextInt(len);
  g
   .setColor(new Color(r.nextInt(88), r.nextInt(188), r
    .nextInt(255)));
  g.setFont(new Font("Arial", Font.BOLD | Font.ITALIC, 22));// 输出的字体和大小
  g.drawString("" + ch[index], (i * 15) + 6, 18);// 写什么数字，在图片的什么位置画
  sb.append(ch[index]);
 }
 
 request.getSession().setAttribute("piccode", sb.toString());
 ServletOutputStream responseOutputStream = response.getOutputStream();
 ImageIO.write(img, "JPG", responseOutputStream);
 responseOutputStream.flush();
 responseOutputStream.close();
 }
 
 Color getRandColor(int fc, int bc) {// 给定范围获得随机颜色
 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);
 }
 
 /**
 * 得到某一个类的路径
 * 
 * @param name
 * @return
 */
 public static String getPath(Class name) {
 String strResult = null;
 if (System.getProperty("os.name").toLowerCase().indexOf("window") > -1) {
  strResult = name.getResource("/").toString().replace("file:/", "")
   .replace("%20", " ");
 } else {
  strResult = name.getResource("/").toString().replace("file:", "")
   .replace("%20", " ");
 }
 return strResult;
 }
}