源码

import javax.imageio.ImageIO;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Random;

/**
 * @author Yawei Xi
 * @date 2018-10-24
 */
public class ImageCaptcha {
/**
 * 随机数
 */
private static final Random RANDOM = new Random();
/**
 * 字体
 */
private static final String[] FONT_NAMES = {"宋体", "华文楷体", "黑体", "微软雅黑", "楷体_GB2312"};
/**
 * 字母库
 */
private static final String CODE = "23456789abcdefghjkmnopqrstuvwxyzABCDEFGHJKMNPQRSTUVWXYZ";
/**
 * 字符串的长度
 */
private int length;
/**
 * 验证码图片的高度
 */
private int height;
/**
 * 验证码图片的宽度
 */
private int width;
/**
 * 默认背景颜色
 */
private Color bgColor;

public ImageCaptcha() {
    this.bgColor = new Color(255, 255, 255);
    this.length = 4;
    this.height = 35;
    this.width = 70;
}

public ImageCaptcha(int length) {
    this.bgColor = new Color(255, 255, 255);
    this.length = length;
    this.height = 35;
    this.width = 70;
}

public ImageCaptcha(int length, int width, int height) {
    this.bgColor = new Color(255, 255, 255);
    this.length = length;
    this.height = height;
    this.width = width;
}

/**
 * 获取指定长度的随机字符串
 *
 * @return 随机字符串
 */
private String getRandomString() {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < length; i++) {
        sb.append(CODE.charAt(RANDOM.nextInt(CODE.length())));
    }
    return sb.toString();
}

/**
 * 将验证码图片保存为图片
 *
 * @param filePath 需要保存的文件路径
 */
public void output(String filePath) {
    try {
        OutputStream os = new FileOutputStream(filePath);
        ImageIO.write(getCaptcha(), "JPEG", os);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

/**
 * 随机生成文字的颜色
 *
 * @return 颜色对象
 */
private Color randomColor() {
    int red = RANDOM.nextInt(150);
    int green = RANDOM.nextInt(150);
    int blue = RANDOM.nextInt(150);
    return new Color(red, green, blue);
}

/**
 * 随机选择字体
 *
 * @return 字体
 */
private Font randomFont() {
    int index = RANDOM.nextInt(FONT_NAMES.length);
    String fontName = FONT_NAMES[index];
    int style = RANDOM.nextInt(3);
    int size = RANDOM.nextInt(5) + 24;
    return new Font(fontName, style, size);
}

/**
 * 给验证码画干扰的线条
 *
 * @param image 验证码图片
 * @param w     验证码图片的宽度
 * @param h     验证码图片的高度
 */
private void drawLine(BufferedImage image, int w, int h) {
    int num = 3;// 画三条
    Graphics2D g2 = (Graphics2D) image.getGraphics();
    for (int i = 0; i < num; i++) {
        int x1 = RANDOM.nextInt(w);
        int y1 = RANDOM.nextInt(h);
        int x2 = RANDOM.nextInt(w);
        int y2 = RANDOM.nextInt(h);
        g2.setStroke(new BasicStroke(1.5F));
        g2.setColor(Color.BLUE);
        g2.drawLine(x1, y1, x2, y2);
    }
}

/**
 * 生成验证码的背景图片
 *
 * @param c 颜色
 * @param w 宽度
 * @param h 高度
 * @return 生成的图片
 */
private BufferedImage createBGImage(Color c, int w, int h) {
    BufferedImage image = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
    Graphics2D g2 = (Graphics2D) image.getGraphics();
    g2.setColor(c);
    g2.fillRect(0, 0, w, h);
    return image;
}

/**
 * 将文字写入图片
 *
 * @param s 需要写进图片的字符串
 * @param c 图片背景颜色
 * @param w 宽度
 * @param h 高度
 * @return 验证码
 */
private BufferedImage createCaptcha(String s, Color c, int w, int h) {
    BufferedImage image = createBGImage(c, w, h);
    Graphics2D g2 = (Graphics2D) image.getGraphics();
    // 向图片中画字符
    for (int i = 0; i < s.length(); i++) {
        float x = i * 1.0F * w / s.length();
        g2.setFont(randomFont());
        g2.setColor(randomColor());
        // 首字符的基线位于用户空间的 (x, h-5) 位置处 原点在左上角,X轴递增的方向是从左向右;Y轴是从上到下
        // 在提供的坐标位于基线上最左边字符的情况下，可以从右到左呈现字形 h-5表示y轴方向,向上偏移了5
        g2.drawString(String.valueOf(s.charAt(i)), x, h - 5);
    }
    drawLine(image, w, h);
    return image;
}

/**
 * 获取验证码图片
 *
 * @return 验证码图片对象
 */
public BufferedImage getCaptcha() {
    return createCaptcha(getRandomString(), bgColor, width, height);
}
}

测试用例

/**
 * @author Yawei Xi
 * @date 2018-10-24
 */
public class Test {
  public static void main(String[] args) throws Exception {
    String path = "D:\\test.jpg";
    ImageCaptcha ic = new ImageCaptcha();
    ic.output(path);
  }
}