验证码插件代码：image.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@page import="java.awt.image.BufferedImage"%>
<%@page import="java.awt.Graphics2D"%>
<%@page import="java.awt.Color"%>
<%@page import="java.awt.Font"%>
<%@page import="javax.imageio.ImageIO"%>

<%
int width = 60;
int height = 20;
// 创建具有可访问图像数据缓冲区的Image
BufferedImage buffImg = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics2D g = buffImg.createGraphics();

// 创建一个随机数生成器
Random random = new Random();

g.setColor(Color.WHITE);
g.fillRect(0, 0, width, height);

// 创建字体，字体的大小应该根据图片的高度来定
Font font = new Font("Times New Roman", Font.PLAIN, 24);
// 设置字体
g.setFont(font);

// 画边框
g.setColor(Color.BLACK);
g.drawRect(0, 0, width - 1, height - 1);

// 随机产生160条干扰线
g.setColor(Color.LIGHT_GRAY);
for (int i = 0; i < 160; i++) {
int x = random.nextInt(width);
int y = random.nextInt(height);
int x1 = random.nextInt(12);
int y1 = random.nextInt(12);
g.drawLine(x, y, x + x1, y + y1);
}

// randomCode 用于保存随机产生的验证码
StringBuffer randomCode = new StringBuffer();
int red = 0, green = 0, blue = 0;

// 随机产生4位数字的验证码
for (int i = 0; i < 4; i++) {
// 得到随机产生的验证码数字
String strRand = String.valueOf(random.nextInt(10));

// 产生随机的颜色分量来构造颜色值
red = random.nextInt(110);
green = random.nextInt(50);
blue = random.nextInt(50);

// 用随机产生的颜色将验证码绘制到图像中
g.setColor(new Color(red, green, blue));
g.drawString(strRand, 13 * i + 6, 16);

randomCode.append(strRand);
}

// 将四位数字的验证码保存到session中
//HttpSession session1 = request.getSession();
session.setAttribute("randomCode", randomCode.toString());

// 禁止图像缓存
response.setHeader("Pragma", "no-cache");
response.setHeader("Cache-Control", "no-cache");
response.setDateHeader("Expires", 0);

response.setContentType("image/jpeg");
// 将图像输出到servlet输出流中
ServletOutputStream sos = response.getOutputStream();
ImageIO.write(buffImg, "jpeg", sos);
sos.close();
//sos = null;
out.clear();
out = pageContext.pushBody();
%>

前台页面调用：

<img src="<%=path %>/image.jsp" width="43" height="17" />