在<head>里添加：
<script type="text/javascript">
function changeImg(){
document.getElementById("Image1").src="image.jsp?"+Math.random();
}
</script>
在body>里添加：
<div class="p-n-tent clearfix validate">
验证码：<input type="password" id="validate" name="validate" placeholder="请输入验证码" />
<font color="red">*</font><img src="image.jsp" id="Image1" /> <a href="javascript:changeImg();" style="color:red; font-size:12px; text-decoration:none;">看不清，换一张</a><br />
<span style="color:Red;font-size:12px"></span>
</div>
其中image.jsp:
<%@ page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*,java.io.*" pageEncoding="GB2312"%> <%! Color getRandColor(int fc,int bc){//给定范围获得随机颜色 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); } %> <% //设置页面不缓存 response.setHeader("Pragma","No-cache"); response.setHeader("Cache-Control","no-cache"); response.setDateHeader("Expires", 0); // 在内存中创建图象 int width=60, height=20; BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB); // 得到 输出流 OutputStream os=response.getOutputStream(); // 获取图形上下文 Graphics g = image.getGraphics(); //生成随机类 Random random = new Random(); // 设定背景色 g.setColor(getRandColor(200,250)); g.fillRect(0, 0, width, height); //设定字体 g.setFont(new Font("Times New Roman",Font.PLAIN,18)); //画边框 //g.setColor(new Color()); //g.drawRect(0,0,width-1,height-1); // 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到 g.setColor(getRandColor(160,200)); for (int i=0;i<155;i++) { int x = random.nextInt(width); int y = random.nextInt(height); int xl = random.nextInt(12); int yl = random.nextInt(12); g.drawLine(x,y,x+xl,y+yl); } // 取随机产生的认证码(4位数字) String sRand=""; for (int i=0;i<4;i++){ String rand=String.valueOf(random.nextInt(10)); sRand+=rand; // 将认证码显示到图象中 g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));// 调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成 g.drawString(rand,13*i+6,16); } // 将认证码存入SESSION session.setAttribute("rand",sRand); // 图象生效 g.dispose(); // 输出图象到页面 ImageIO.write(image, "JPEG", os); os.flush(); os.close(); os=null; response.flushBuffer(); out.clear(); out = pageContext.pushBody(); %>