依赖

<!-- easy-captcha用来生成验证码，由于jdk9以后，内置JavaScript去掉了，所以需要导入这个org.openjdk.nashorn -->
<dependency>
    <groupId>com.github.whvcse</groupId>
    <artifactId>easy-captcha</artifactId>
    <version>${easy-captcha.version}</version>
</dependency>
<dependency>
    <groupId>org.openjdk.nashorn</groupId>
    <artifactId>nashorn-core</artifactId>
    <version>${nashorn-core.version}</version>
</dependency>

工具类

/**
 * 验证码工具类
 *
 * @author hc
 */
public class CaptchaUtil {

    /**
     * 默认字体大小
     */
    private static Integer fontSize = 45;

    private static List<Font> list = new ArrayList<>();

    static {
        //静态代码快初始化，下面字体之所以注释掉是因为不好区分
        try {
            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/actionj.ttf"))));
            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/epilog.ttf"))));
            // list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/fresnel.ttf"))));
//            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/headache.ttf"))));
//            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/lexo.ttf"))));
            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/prefix.ttf"))));
            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/progbot.ttf"))));
//            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/ransom.ttf"))));
            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/robot.ttf"))));
//            list.add(Font.createFont(Font.TRUETYPE_FONT, Objects.requireNonNull(Captcha.class.getResourceAsStream("/scandal.ttf"))));
        } catch (FontFormatException | IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * @param type   验证码图片的类型
     * @param width
     * @param height
     * @param length
     * @return
     */
    public static Captcha getCaptcha(int type, int width, int height, int length) {
        Captcha captcha;
        switch (type) {
            case 1:
                // 算术类型,固定设置为两位，图片为算数运算表达式
                captcha = new ArithmeticCaptcha(width, height, length);
                break;
            case 2:
                //中文
                captcha = new ChineseCaptcha(width, height, length);
                break;
            case 3:
                //中文gif
                captcha = new ChineseGifCaptcha(width, height, length);
                break;
            case 4:
                //gif英文
                captcha = new GifCaptcha(width, height, length);
                break;
            case 5: //默认
                captcha = new SpecCaptcha(width, height, length);
                break;
            default:
                throw new RuntimeException("验证码配置信息错误");
        }

        //也可以不设置字体，直接使用系统默认的字体
        // 随机字体
        final int index = (int) (Math.random() * list.size());
//        System.out.println("index  " + index);
        captcha.setFont(list.get(index).deriveFont(Font.BOLD, fontSize));

        return captcha;
    }

}

控制器

@RestController
@RequestMapping("/captcha")
public class CaptchaController {

    @GetMapping("/v1/get")
    public ResultBean<String> getCaptcha() {
        //生成验证码
        final Captcha captcha = CaptchaUtil.getCaptcha(1, 160, 55, 2);
        //验证码对应的文本
        System.out.println("验证码对应的文本: "+captcha.text());

        return captcha.toBase64();
    }

 }

效果