一.准备知识
随便一本JavaWeb书上都有，就那几步，看看就ok了，不过可能存在一些问题，路过的指教下。。。
二.代码
<%@ page language="java" contentType="text/html; charset=UTF-8"pageEncoding="UTF-8"%><html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>登陆页面</title><script type="text/javascript">function refresh() {loginForm.image.src = "creatImage.jsp";}</script></head><body><h1>欢迎登陆本系统</h1><br><form action="" method = "post" name="loginForm"><label>账号:<input name="username" type="text" /></label><br><label>密码:<input name="password" type="password" /></label><br><label>验证码:<input name="code" type="text" /></label><!-- 将验证码当做图片处理 --><img name="image" border="0" src="creatImage.jsp" onclick="refresh()" /><input type="submit" value="登陆" /></form></body></html>
<%@page import="java.util.Random"%><%@page import="java.awt.Graphics"%><%@page import="javax.imageio.*"%><%@page import="java.awt.*"%><%@page import="java.awt.image.BufferedImage"%><%@ page language="java" contentType="text/html; charset=UTF-8"pageEncoding="UTF-8"%><%final char[] str = {'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};int width=100,height=60;BufferedImage bi = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);Graphics g = bi.getGraphics();g.setColor(new Color(200,200,200));g.fillRect(0, 0, width, height);Random rnd = new Random();StringBuffer sb = new StringBuffer("");//产生四位数的字母数字验证码，各个数字的颜色也随即for(int i=0; i<4; i++) {int num = rnd.nextInt(str.length);Color c = new Color(rnd.nextInt(256),rnd.nextInt(256),rnd.nextInt(256));g.setColor(c);g.setFont(new Font("", Font.BOLD+Font.ITALIC, 20));g.drawString(str[num]+"", 10, 17);sb.append(str[num]);}//划干扰线for(int i=0; i<10; i++) {Color c = new Color(rnd.nextInt(256),rnd.nextInt(256),rnd.nextInt(256));g.setColor(c);g.drawLine(rnd.nextInt(width), rnd.nextInt(height),rnd.nextInt(width), rnd.nextInt(height));}String s = new String(sb);/*若是产生四位数字，则nextInt(8999) + 1000;然后String.valueOf转换为String*///验证码存入session里，方便在登陆校检页比对session.setAttribute("image",s);//输出到页面ImageIO.write(bi,"JPEG",response.getOutputStream());/*加入下面这两句什么作用呢？否则报异常: java.lang.IllegalStateException: getOutputStream()has already been called for this response不管原因了*/out.clear();out = pageContext.pushBody();%><html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>图片生成</title></head><body></body></html>