效果图

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * 生成图形验证码
 */
public class AuthImg extends HttpServlet {

	private static final long serialVersionUID = 1L;
	// 定义图形验证码中绘制的字符的字体
	private final Font mFont = new Font("Arial Black", Font.PLAIN, 23);
	// 图形验证码的大小
	private final int IMG_WIDTH = 72;
	private final int IMG_HEIGTH = 27;

	// 获取随机颜色的方法
	private Color getRandColor(int fc, int bc) {
		Random random = new Random();
		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);
	}

	// 获取随机字符串
	private String getRandomChar() {
		int rand = (int) Math.round(Math.random() * 2);
		long itmp = 0;
		char ctmp = \'\u0000\';
		switch (rand) {
		case 1:
			itmp = Math.round(Math.random() * 25 + 65);
			ctmp = (char) itmp;
			return String.valueOf(ctmp);
		case 2:
			itmp = Math.round(Math.random() * 25 + 97);
			ctmp = (char) itmp;
			return String.valueOf(ctmp);
		default:
			itmp = Math.round(Math.random() * 9);
			return itmp + "";
		}
	}

	@Override
	public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// 设置禁止缓存
		response.setHeader("Pragma", "No-cache");
		response.setHeader("Cache-Control", "no-cache");
		response.setDateHeader("Expires", 0);
		response.setContentType("image/jpeg");
		BufferedImage image = new BufferedImage(IMG_WIDTH, IMG_HEIGTH, BufferedImage.TYPE_INT_RGB);
		Graphics g = image.getGraphics();
		Random random = new Random();
		g.setColor(getRandColor(200, 250));
		// 填充背景色
		g.fillRect(1, 1, IMG_WIDTH - 1, IMG_HEIGTH - 1);
		// 为图形验证码绘制边框
		g.setColor(new Color(102, 102, 102));
		g.drawRect(0, 0, IMG_WIDTH, IMG_HEIGTH);
		g.setColor(getRandColor(160, 200));
		// 生成随机干扰线
		for (int i = 0; i < 80; i++) {
			int x = random.nextInt(IMG_WIDTH - 1);
			int y = random.nextInt(IMG_HEIGTH - 1);
			int x1 = random.nextInt(6) + 1;
			int y1 = random.nextInt(12) + 1;
			g.drawLine(x, y, x + x1, y + y1);
		}
		g.setColor(getRandColor(160, 200));
		// 生成随机干扰线
		for (int i = 0; i < 80; i++) {
			int x = random.nextInt(IMG_WIDTH - 1);
			int y = random.nextInt(IMG_HEIGTH - 1);
			int x1 = random.nextInt(12) + 1;
			int y1 = random.nextInt(6) + 1;
			g.drawLine(x, y, x - x1, y - y1);
		}
		// 设置绘制字符的字体
		g.setFont(mFont);
		// 用于保存系统生成的随机字符串
		String sRand = "";
		for (int i = 0; i < 4; i++) {
			String tmp = getRandomChar();
			sRand += tmp;
			g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
			g.drawString(tmp, 15 * i + 10, 20);
		}
		// 获取HttpSession对象
		HttpSession session = request.getSession(true);
		session.removeAttribute("rand");
		session.setAttribute("rand", sRand);
		g.dispose();
		// 向输出流中输出图片
		ImageIO.write(image, "JPEG", response.getOutputStream());
	}

}