抽象的理解，要想要验证码图片就要用笔和纸来画出来

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
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;

public class CodeServlet extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

		// 使用java图形界面技术绘制一张图片

		int charNum = 4;
		int width = 30 * 4;
		int height = 30;

		// 1. 创建一张内存图片
		BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

		// 2.获得绘图对象
		Graphics graphics = bufferedImage.getGraphics();

		// 3、绘制背景颜色
		graphics.setColor(Color.YELLOW);
		graphics.fillRect(0, 0, width, height);

		// 4、绘制图片边框
		graphics.setColor(Color.BLUE);
		graphics.drawRect(0, 0, width - 1, height - 1);

		// 5、输出验证码内容
		graphics.setColor(Color.RED);
		graphics.setFont(new Font("宋体", Font.BOLD, 20));

		// 随机输出4个字符
		Graphics2D graphics2d = (Graphics2D) graphics;
		String s = "ABCDEFGHGKLMNPQRSTUVWXYZ23456789";
		Random random = new Random();
		// session中要用到
		String msg = "";
		int x = 5;
		for (int i = 0; i < 4; i++) {
			int index = random.nextInt(32);
			String content = String.valueOf(s.charAt(index));
			msg += content;
			double theta = random.nextInt(45) * Math.PI / 180;
			// 让字体扭曲
			graphics2d.rotate(theta, x, 18);
			graphics2d.drawString(content, x, 18);
			graphics2d.rotate(-theta, x, 18);
			x += 30;
		}

		// 6、绘制干扰线
		graphics.setColor(Color.GRAY);
		for (int i = 0; i < 5; i++) {
			int x1 = random.nextInt(width);
			int x2 = random.nextInt(width);
			
			int y1 = random.nextInt(height);
			int y2 = random.nextInt(height);
			graphics.drawLine(x1, y1, x2, y2);
		}

		// 释放资源
		graphics.dispose();

		// 图片输出 ImageIO
		ImageIO.write(bufferedImage, "jpg", response.getOutputStream());//以jpg的形式发送到

	}

}

html界面的调用

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
	<img alt="验证码" src="/code" title="看不清除,换一张" onclick="changeImg(this)">
</body>
<script type="text/javascript"> function changeImg(obj){ //操作src属性 obj.src="/code?i="+Math.random(); //图片是静态资源，要使用参数骗过服务器 } </script>
</html>

在web.xml配置servlet的访问路径或者用注解的形式也可以/code，