验证码虽然是降低了用户对软件（或者系统）的体验，产生的负面的影响，然而，验证码也有其必要的作用。验证码主要作用是区分了“机器”和“人”，在某些领域里，如果用“机器”去不断的注册账户或者登录账户，产生的弊端会很大。
如：购票网站，银行系统，网络购物，各大论坛。这些领域都在避免别人用“机器”去操作，所有使用验证码，增加了机器识别验证码 的难度，让真正的用户“人”去操作。
以下是具体的案例
环境和工具：
Myeclipse10_6
Tomcat7.x
jdk:myeclipse自带的jdk环境
index.jsp 如下
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<% String path = request.getContextPath(); String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>index.jsp</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<script type="text/javascript"> function reloadCode(){ var time = new Date().getTime(); document.getElementById("imagecode").src="<%=request.getContextPath() %>/servlet/ImageServlet?d="+time; } </script>
</head>
<body>
<form action="<%=request.getContextPath() %>/servlet/LoginServlet" method="get">
验证码：<input type="text" name="checkcode"/>
<img alt="验证码" id="imagecode" src="<%=request.getContextPath() %>/servlet/ImageServlet"/>
<a href="javascript: reloadCode();">看不清楚,点击刷新</a><br>
<input type="submit" value="提交">
</form>
</body>
</html>ImageServlet.java 如下
package com.dulu;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;
import javax.imageio.ImageIO;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ImageServlet extends HttpServlet{
public void doGet(HttpServletRequest request,HttpServletResponse response)throws IOException{
BufferedImage bi = new BufferedImage(68,22,BufferedImage.TYPE_INT_RGB);
Graphics g = bi.getGraphics();
Random randrgb = new Random();
//设置一个比较浅的背景色
Color c = new Color(randrgb.nextInt(10)+235,randrgb.nextInt(10)+235,randrgb.nextInt(10)+235);
g.setColor(c); // 为g设置背景色
g.fillRect(0,0,68,22); // 为g设置边框
//画验证码中的内容
//ch 得到含有字母和数字的数组
char[] ch = "AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz0123456789".toCharArray();
//随机获取数组中的4位
Random r =new Random();
int len = ch.length, index = 0;
StringBuffer sb = new StringBuffer();
for(int i=0;i<4;i++){
index = r.nextInt(len);// index是数组中随机位置的索引 从0开始
g.setColor(new Color(r.nextInt(88),r.nextInt(188),r.nextInt(255)));
g.drawString(ch[index]+"",(i*15)+ 3,18);//画出这个随机字符
sb.append(ch[index]); // 将随机字符添加在sb尾部
}
request.getSession().setAttribute("piccode",sb.toString());
ImageIO.write(bi,"JPG",response.getOutputStream());
}
}LoginServlet.java 如下
package com.dulu;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class LoginServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
// piccode 是产生的随机验证码
String piccode = (String) request.getSession().getAttribute("piccode");
String checkcode = request.getParameter("checkcode");
PrintWriter out = response.getWriter();
if(checkcode!= ""){ //如果用户有输入的信息
piccode = piccode.toUpperCase(); // 将系统产生的验证码字符大写
checkcode = checkcode.toUpperCase(); // 将用户输入的字符大写
// System.out.println("你输入的验证码是："+ checkcode);
if(checkcode.equals(piccode)){
System.out.println("验证码输入正确！");
}else{
System.out.println("验证码输入错误！");
}
}else{ // 用户什么也没有输入
System.out.println("你未输入验证码！");
}
out.flush();// 刷新输出流
out.close();// 关闭输出流
}
}web.xml 如下
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<servlet>
<servlet-name>ImageServlet</servlet-name>
<servlet-class>com.dulu.ImageServlet</servlet-class>
</servlet>
<servlet>
<description>This is the description of my J2EE component</description>
<display-name>This is the display name of my J2EE component</display-name>
<servlet-name>LoginServlet</servlet-name>
<servlet-class>com.dulu.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ImageServlet</servlet-name>
<url-pattern>/servlet/ImageServlet</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/servlet/LoginServlet</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>测试如下：
输入正确的测试：
输入错误的测试
控制台输出：
你输入的验证码是：OPLK
验证码输入错误！
用户无输入，直接提交时：
控制台输出：你未输入验证码！