<dependency>
      <groupId>com.octo.captcha</groupId>
      <artifactId>jcaptcha</artifactId>
      <version>1.0</version>
    </dependency>

import com.octo.captcha.component.image.backgroundgenerator.BackgroundGenerator;
import com.octo.captcha.component.image.backgroundgenerator.FileReaderRandomBackgroundGenerator;
import com.octo.captcha.component.image.color.RandomListColorGenerator;
import com.octo.captcha.component.image.fontgenerator.FontGenerator;
import com.octo.captcha.component.image.fontgenerator.RandomFontGenerator;
import com.octo.captcha.component.image.textpaster.DecoratedRandomTextPaster;
import com.octo.captcha.component.image.textpaster.TextPaster;
import com.octo.captcha.component.image.textpaster.textdecorator.TextDecorator;
import com.octo.captcha.component.image.wordtoimage.ComposedWordToImage;
import com.octo.captcha.component.image.wordtoimage.WordToImage;
import com.octo.captcha.component.word.wordgenerator.RandomWordGenerator;
import com.octo.captcha.component.word.wordgenerator.WordGenerator;
import com.octo.captcha.engine.CaptchaEngine;
import com.octo.captcha.engine.image.ListImageCaptchaEngine;
import com.octo.captcha.image.gimpy.GimpyFactory;

import java.awt.*;

/**
 * 产生验证码图片的类
 */
public class CapchaHelper {
  private static final Integer MIN_WORD_LENGTH = 4;// 验证码最小长度
  private static final Integer MAX_WORD_LENGTH = 4;// 验证码最大长度
  private static final Integer IMAGE_HEIGHT = 30;// 验证码图片高度
  private static final Integer IMAGE_WIDTH = 130;// 验证码图片宽度
  private static final Integer MIN_FONT_SIZE = 15;// 验证码最小字体
  private static final Integer MAX_FONT_SIZE = 15;// 验证码最大字体
  private static final String RANDOM_WORD = "0123456789";// 随机字符

  // 验证码随机字体
  private static final Font[] RANDOM_FONT = new Font[]{
      new Font("nyala", Font.BOLD, MIN_FONT_SIZE),
      new Font("Arial", Font.BOLD, MIN_FONT_SIZE),
      new Font("Bell MT", Font.BOLD, MIN_FONT_SIZE),
      new Font("Credit valley", Font.BOLD, MIN_FONT_SIZE),
      new Font("Impact", Font.BOLD, MIN_FONT_SIZE)
  };

  // 验证码随机颜色
  private static final Color[] RANDOM_COLOR = new Color[]{
      new Color(255, 255, 255),
      new Color(255, 220, 220),
      new Color(220, 255, 255),
      new Color(220, 220, 255),
      new Color(255, 255, 220),
      new Color(220, 255, 220)
  };

  private static ListImageCaptchaEngine captchaEngine;

  public static CaptchaEngine getCaptchaEngine(final String imgPath) {
    if (captchaEngine == null) {
      synchronized (CapchaHelper.class) {
        if (captchaEngine == null && imgPath != null) {
          captchaEngine = new ListImageCaptchaEngine() {
            @Override
            protected void buildInitialFactories() {
              RandomListColorGenerator randomListColorGenerator = new RandomListColorGenerator(RANDOM_COLOR);
              BackgroundGenerator backgroundGenerator = new FileReaderRandomBackgroundGenerator(IMAGE_WIDTH, IMAGE_HEIGHT, imgPath);
              WordGenerator wordGenerator = new RandomWordGenerator(RANDOM_WORD);
              FontGenerator fontGenerator = new RandomFontGenerator(MIN_FONT_SIZE, MAX_FONT_SIZE, RANDOM_FONT);
              TextDecorator[] textDecorator = new TextDecorator[]{};
              TextPaster textPaster = new DecoratedRandomTextPaster(MIN_WORD_LENGTH, MAX_WORD_LENGTH, randomListColorGenerator, textDecorator);
              WordToImage wordToImage = new ComposedWordToImage(fontGenerator, backgroundGenerator, textPaster);
              addFactory(new GimpyFactory(wordGenerator, wordToImage));
            }
          };
        }
      }
    }
    return captchaEngine;
  }
}

//获取验证码背景图片的路径，这路径放了很多作为背景的图像
    String captcha_backgrounds = session.getServletContext().getRealPath("/WEB-INF/img/captcha");
    CaptchaEngine ce = CapchaHelper.getCaptchaEngine(captcha_backgrounds);
    //需要admin网页中用js定时从服务端获取当前的验证码
    Captcha captcha = ce.getNextCaptcha();
    //为了验证，把captcha对象放到session中，以在客户端提交验证码时进行验证
    req.getSession().setAttribute("captcha", captcha);
    //获取验证码图片，这是未压缩的位图
    BufferedImage image = (BufferedImage) captcha.getChallenge();
    resp.setContentType("image/jpeg");
    ImageIO.write(image, "jpg", resp.getOutputStream());

//获取验证码背景图片的路径，这路径放了很多作为背景的图像
    String captcha_backgrounds = session.getServletContext().getRealPath("/WEB-INF/img/captcha");
    CaptchaEngine ce = CapchaHelper.getCaptchaEngine(captcha_backgrounds);
    //需要admin网页中用js定时从服务端获取当前的验证码
    Captcha captcha = ce.getNextCaptcha();
    //为了验证，把captcha对象放到session中，以在客户端提交验证码时进行验证
    session.setAttribute("captcha", captcha);
    //获取验证码图片，这是未压缩的位图
    BufferedImage image = (BufferedImage) captcha.getChallenge();
    ByteArrayOutputStream bao=new ByteArrayOutputStream();
    //应缩成jpg并写到输出流中
    ImageIO.write(image, "jpg", bao);
    return bao.toByteArray();

<img id="captcha" src="/captcha_img" onclick="refreshCaptchaImg()" />

function refreshCaptchaImg() {
      //从服务端重新下载验证码图片
      //给这个地加参数纯为了强制刷新，否则由于src指向的url地址没变,浏览器不会真正生刷新图片
      var now = new Date()
      $("#captcha").attr("src","/captcha_img?"+now.getTime());