工具类源码

import org.apache.commons.codec.binary.Base64;

import javax.imageio.ImageIO;
import java.awt.*;
import java.awt.geom.QuadCurve2D;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;

/**
 * 生成验证码工具类
 *
 * @author whw
 * @date 2020/1/7 16:49
 */
public class CaptchaUtil {
    /**
     * 默认宽度
     */
    private static final int WIDTH = 108;
    /**
     * 默认高度
     */
    private static final int HEIGHT = 40;
    /**
     * 默认长度
     */
    private static final int CODE_SIZE = 4;

    /**
     * 验证码随机字符数组
     */
    //private static final char[] charArray = "23456789ABCDEFGHJKMNPQRSTUVWXY".toCharArray();
    private static final char[] CHAR_ARRAY = "0123456789".toCharArray();
    /**
     * 生成随机类
     */
    private static final Random RANDOM = new Random();
    /**
     * 验证码字体
     */
    private static final Font[] RANDOM_FONT = new Font[]{
            new Font(Font.DIALOG, Font.BOLD, 33),
            new Font(Font.DIALOG_INPUT, Font.BOLD, 34),
            new Font(Font.SERIF, Font.BOLD, 33),
            new Font(Font.SANS_SERIF, Font.BOLD, 34),
            new Font(Font.MONOSPACED, Font.BOLD, 34)
    };

    /**
     * 生成验证码字符串
     */
    public static String generateCode() {
        int count = CODE_SIZE;
        char[] buffer = new char[count];
        for (int i = 0; i < count; i++) {
            buffer[i] = CHAR_ARRAY[RANDOM.nextInt(CHAR_ARRAY.length)];
        }
        return new String(buffer);
    }

    public static Map<String, String> generate(String code) {
        Map<String, String> map = new HashMap<>(2);
        BufferedImage image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        try {
            drawGraphic(image, code);
            ImageIO.write(image, "jpeg", baos);
            //转换成字节
            byte[] bytes = baos.toByteArray();
            //转换成base64串
            String base64 = Base64.encodeBase64String(bytes);
            //删除 \r\n
            base64 = base64.replaceAll("\n", "").replaceAll("\r", "");
            map.put("base64", "data:image/jpeg;base64," + base64);
            map.put("code", code);
            baos.close();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
        return map;
    }

    private static void drawGraphic(BufferedImage image, String code) {
        // 获取图形上下文
        Graphics2D g = image.createGraphics();
        g.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_NEAREST_NEIGHBOR);
        // 图形抗锯齿
        g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        // 字体抗锯齿
        g.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);

        // 设定背景色，淡色
        g.setColor(getRandColor(210, 250));
        g.fillRect(0, 0, WIDTH, HEIGHT);

        // 画小字符背景
        Color color = null;
        for (int i = 0; i < 20; i++) {
            color = getRandColor(120, 200);
            g.setColor(color);
            String rand = String.valueOf(CHAR_ARRAY[RANDOM.nextInt(CHAR_ARRAY.length)]);
            g.drawString(rand, RANDOM.nextInt(WIDTH), RANDOM.nextInt(HEIGHT));
            color = null;
        }
        // 取随机产生的认证码(4位数字)
        char[] buffer = code.toCharArray();
        for (int i = 0; i < buffer.length; i++) {
            //旋转度数 最好小于45度
            int degree = RANDOM.nextInt(28);
            if (i % 2 == 0) {
                degree = degree * (-1);
            }
            //定义坐标
            int x = 22 * i, y = 21;
            //旋转区域
            g.rotate(Math.toRadians(degree), x, y);
            //设定字体颜色
            color = getRandColor(20, 130);
            g.setColor(color);
            //设定字体，每次随机
            g.setFont(RANDOM_FONT[RANDOM.nextInt(RANDOM_FONT.length)]);
            //将认证码显示到图象中
            g.drawString("" + buffer[i], x + 8, y + 10);
            //旋转之后，必须旋转回来
            g.rotate(-Math.toRadians(degree), x, y);
        }
        //图片中间曲线，使用上面缓存的color
        g.setColor(color);
        //width是线宽,float型
        BasicStroke bs = new BasicStroke(3);
        g.setStroke(bs);
        //画出曲线
        QuadCurve2D.Double curve = new QuadCurve2D.Double(0d, RANDOM.nextInt(HEIGHT - 8) + 4, WIDTH / 2, HEIGHT / 2, WIDTH, RANDOM.nextInt(HEIGHT - 8) + 4);
        g.draw(curve);
        // 销毁图像
        g.dispose();
    }

    /**
     * 给定范围获得随机颜色
     *
     * @param fc
     * @param bc
     * @return
     */
    private static Color getRandColor(int fc, int bc) {
        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);
    }
}

调用方法

// 获取验证码code
String code = CaptchaUtil.generateCode();
// 传入code生成验证码对象
Map<String, String> captchaMap = CaptchaUtil.generate(code);

生成之后返回的是一个Map，Map中包含了code和base64码两个对象：

前端渲染

直接将base64码放到img标签的src属性里即可