项目参考：oschina开源项目若依

1.添加maven

<!--验证码 -->
<kaptcha.version>2.3.2</kaptcha.version>
<dependency>
	<groupId>com.github.penggle</groupId>
	<artifactId>kaptcha</artifactId>
	<version>${kaptcha.version}</version>
</dependency>

2.编写配置类

package com.ruoyi.framework.config;

import java.util.Properties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import com.google.code.kaptcha.impl.DefaultKaptcha;
import com.google.code.kaptcha.util.Config;
import static com.google.code.kaptcha.Constants.*;

/**
 * 验证码配置
 * 
 * @author ruoyi
 */
@Configuration
public class CaptchaConfig
{
    @Bean(name = "captchaProducer")
    public DefaultKaptcha getKaptchaBean()
    {
        DefaultKaptcha defaultKaptcha = new DefaultKaptcha();
        Properties properties = new Properties();
        // 是否有边框 默认为true 我们可以自己设置yes，no
        properties.setProperty(KAPTCHA_BORDER, "yes");
        // 验证码文本字符颜色 默认为Color.BLACK
        properties.setProperty(KAPTCHA_TEXTPRODUCER_FONT_COLOR, "black");
        // 验证码图片宽度 默认为200
        properties.setProperty(KAPTCHA_IMAGE_WIDTH, "160");
        // 验证码图片高度 默认为50
        properties.setProperty(KAPTCHA_IMAGE_HEIGHT, "60");
        // 验证码文本字符大小 默认为40
        properties.setProperty(KAPTCHA_TEXTPRODUCER_FONT_SIZE, "38");
        // KAPTCHA_SESSION_KEY
        properties.setProperty(KAPTCHA_SESSION_CONFIG_KEY, "kaptchaCode");
        // 验证码文本字符长度 默认为5
        properties.setProperty(KAPTCHA_TEXTPRODUCER_CHAR_LENGTH, "4");
        // 验证码文本字体样式 默认为new Font("Arial", 1, fontSize), new Font("Courier", 1, fontSize)
        properties.setProperty(KAPTCHA_TEXTPRODUCER_FONT_NAMES, "Arial,Courier");
        // 图片样式 水纹com.google.code.kaptcha.impl.WaterRipple 鱼眼com.google.code.kaptcha.impl.FishEyeGimpy 阴影com.google.code.kaptcha.impl.ShadowGimpy
        properties.setProperty(KAPTCHA_OBSCURIFICATOR_IMPL, "com.google.code.kaptcha.impl.ShadowGimpy");
        Config config = new Config(properties);
        defaultKaptcha.setConfig(config);
        return defaultKaptcha;
    }

    @Bean(name = "captchaProducerMath")
    public DefaultKaptcha getKaptchaBeanMath()
    {
        DefaultKaptcha defaultKaptcha = new DefaultKaptcha();
        Properties properties = new Properties();
        // 是否有边框 默认为true 我们可以自己设置yes，no
        properties.setProperty(KAPTCHA_BORDER, "yes");
        // 边框颜色 默认为Color.BLACK
        properties.setProperty(KAPTCHA_BORDER_COLOR, "105,179,90");
        // 验证码文本字符颜色 默认为Color.BLACK
        properties.setProperty(KAPTCHA_TEXTPRODUCER_FONT_COLOR, "blue");
        // 验证码图片宽度 默认为200
        properties.setProperty(KAPTCHA_IMAGE_WIDTH, "160");
        // 验证码图片高度 默认为50
        properties.setProperty(KAPTCHA_IMAGE_HEIGHT, "60");
        // 验证码文本字符大小 默认为40
        properties.setProperty(KAPTCHA_TEXTPRODUCER_FONT_SIZE, "35");
        // KAPTCHA_SESSION_KEY
        properties.setProperty(KAPTCHA_SESSION_CONFIG_KEY, "kaptchaCodeMath");
        // 验证码文本生成器
        properties.setProperty(KAPTCHA_TEXTPRODUCER_IMPL, "com.ruoyi.framework.config.KaptchaTextCreator");
        // 验证码文本字符间距 默认为2
        properties.setProperty(KAPTCHA_TEXTPRODUCER_CHAR_SPACE, "3");
        // 验证码文本字符长度 默认为5
        properties.setProperty(KAPTCHA_TEXTPRODUCER_CHAR_LENGTH, "6");
        // 验证码文本字体样式 默认为new Font("Arial", 1, fontSize), new Font("Courier", 1, fontSize)
        properties.setProperty(KAPTCHA_TEXTPRODUCER_FONT_NAMES, "Arial,Courier");
        // 验证码噪点颜色 默认为Color.BLACK
        properties.setProperty(KAPTCHA_NOISE_COLOR, "white");
        // 干扰实现类
        properties.setProperty(KAPTCHA_NOISE_IMPL, "com.google.code.kaptcha.impl.NoNoise");
        // 图片样式 水纹com.google.code.kaptcha.impl.WaterRipple 鱼眼com.google.code.kaptcha.impl.FishEyeGimpy 阴影com.google.code.kaptcha.impl.ShadowGimpy
        properties.setProperty(KAPTCHA_OBSCURIFICATOR_IMPL, "com.google.code.kaptcha.impl.ShadowGimpy");
        Config config = new Config(properties);
        defaultKaptcha.setConfig(config);
        return defaultKaptcha;
    }
}

3.编写验证码文本生成器类

package com.ruoyi.framework.config;

import java.security.SecureRandom;
import java.util.Random;
import com.google.code.kaptcha.text.impl.DefaultTextCreator;

/**
 * 验证码文本生成器
 * 
 * @author ruoyi
 */
public class KaptchaTextCreator extends DefaultTextCreator
{
    private static final String[] CNUMBERS = "0,1,2,3,4,5,6,7,8,9,10".split(",");

    @Override
    public String getText()
    {
        Integer result = 0;
        Random random = new SecureRandom();
        int x = random.nextInt(10);
        int y = random.nextInt(10);
        StringBuilder suChinese = new StringBuilder();
        int randomoperands = (int) Math.round(Math.random() * 2);
        if (randomoperands == 0)
        {
            result = x * y;
            suChinese.append(CNUMBERS[x]);
            suChinese.append("*");
            suChinese.append(CNUMBERS[y]);
        }
        else if (randomoperands == 1)
        {
            if (!(x == 0) && y % x == 0)
            {
                result = y / x;
                suChinese.append(CNUMBERS[y]);
                suChinese.append("/");
                suChinese.append(CNUMBERS[x]);
            }
            else
            {
                result = x + y;
                suChinese.append(CNUMBERS[x]);
                suChinese.append("+");
                suChinese.append(CNUMBERS[y]);
            }
        }
        else if (randomoperands == 2)
        {
            if (x >= y)
            {
                result = x - y;
                suChinese.append(CNUMBERS[x]);
                suChinese.append("-");
                suChinese.append(CNUMBERS[y]);
            }
            else
            {
                result = y - x;
                suChinese.append(CNUMBERS[y]);
                suChinese.append("-");
                suChinese.append(CNUMBERS[x]);
            }
        }
        else
        {
            result = x + y;
            suChinese.append(CNUMBERS[x]);
            suChinese.append("+");
            suChinese.append(CNUMBERS[y]);
        }
        suChinese.append("=?@" + result);
        return suChinese.toString();
    }
}

4.控制层使用

package com.ruoyi.web.controller.system;

import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.annotation.Resource;
import javax.imageio.ImageIO;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.servlet.ModelAndView;
import com.google.code.kaptcha.Constants;
import com.google.code.kaptcha.Producer;
import com.ruoyi.common.core.controller.BaseController;

/**
 * 图片验证码（支持算术形式）
 * 
 * @author ruoyi
 */
@Controller
@RequestMapping("/captcha")
public class SysCaptchaController extends BaseController
{
    @Resource(name = "captchaProducer")
    private Producer captchaProducer;

    @Resource(name = "captchaProducerMath")
    private Producer captchaProducerMath;

    /**
     * 验证码生成
     */
    @GetMapping(value = "/captchaImage")
    public ModelAndView getKaptchaImage(HttpServletRequest request, HttpServletResponse response)
    {
        ServletOutputStream out = null;
        try
        {
            HttpSession session = request.getSession();
            response.setDateHeader("Expires", 0);
            response.setHeader("Cache-Control", "no-store, no-cache, must-revalidate");
            response.addHeader("Cache-Control", "post-check=0, pre-check=0");
            response.setHeader("Pragma", "no-cache");
            response.setContentType("image/jpeg");

            String type = request.getParameter("type");
            String capStr = null;
            String code = null;
            BufferedImage bi = null;
            if ("math".equals(type))
            {
                String capText = captchaProducerMath.createText();
                capStr = capText.substring(0, capText.lastIndexOf("@"));
                code = capText.substring(capText.lastIndexOf("@") + 1);
                bi = captchaProducerMath.createImage(capStr);
            }
            else if ("char".equals(type))
            {
                capStr = code = captchaProducer.createText();
                bi = captchaProducer.createImage(capStr);
            }
            session.setAttribute(Constants.KAPTCHA_SESSION_KEY, code);
            out = response.getOutputStream();
            ImageIO.write(bi, "jpg", out);
            out.flush();

        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
        finally
        {
            try
            {
                if (out != null)
                {
                    out.close();
                }
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
        }
        return null;
    }
}

5. thymeleaf模板调用

<div class="col-xs-6">
	<a href="javascript:void(0);" title="点击更换验证码">
		<!--<img th:src="@{captcha/captchaImage?type=math}" class="imgcode" width="85%"/>-->
   		<img th:src="@{captcha/captchaImage(type=${captchaType})}" class="imgcode" width="85%"/>
	</a>
</div>