jsp+servlet实现验证码功能

验证码的功能大多数人可能不都理解，但几乎每个安全网站都会有。验证码是用来防止非人为因素操作的行为，例如一个黑客要黑一个网站，怎么弄呢？最简单的思路当然是造成其网路拥堵直至系统瘫痪掉。如果没有验证码，那么我就可以在注册页面，写一个程序，只有注册表单，不断更换主键或不可重复的内容，不停的提交。那这样每秒可以注册几万次都有可能，这样服务器就大量负载，很容易就瘫痪并死掉。而达到这样的目的并不困难。
 
增加验证码之后，就不是机器人能识别的了，必须是人在操作系统，由于是图片形式的，而且歪歪斜斜乱七八糟，而现阶段图形模式识别技术还没做到能够识别验证码的程度，所以加上验证码就相对安全了很多。在软件开发中难免会遇到这样的情况，客户压根不知道验证码用来干什么的，但一定要加不可。为什么要加？因为大家都有。没法子总要加上才显得自己专业，其实这个东西很容易加，就是用数字或字母做模板用swing的图形API，画出一个图片出来。完整的逻辑和代码如下：
 
[html] 
<img id="validateCodeId" src="validateCodeServlet" 
                                    onclick="flushValidateCode(this);" title='看不清,点击刷新' 
                                    style="cursor: pointer;" /> 
 
[javascript] 
//刷新验证码 
flushValidateCode = function(obj) { 
    obj.src  ='validateCodeServlet?d='+new Date(); 
} 
 
请求的URL是validateCodeServlet，后面参数d=new Date()是保证每次点击刷新的时候URL都不一样，否则会不刷新。别的地方也使用Math.rand();这样的办法取得一个随机数，这样也是可以的。
web.xml这样来配置servlet的映射信息。
[html]
  <servlet> 
    <servlet-name>validateCode</servlet-name> 
    <servlet-class>com.xzfy.mainpage.web.action.ValidateCodeServlet</servlet-class> 
  </servlet> 
    <servlet-mapping> 
        <servlet-name>validateCode</servlet-name> 
        <url-pattern>/validateCodeServlet</url-pattern> 
    </servlet-mapping> 
ValidateCodeServlet
[java] 
package com.xzfy.mainpage.web.action; 
 
import java.awt.Color; 
import java.awt.Font; 
import java.awt.Graphics; 
import java.awt.image.BufferedImage; 
import java.io.IOException; 
import java.io.PrintWriter; 
import java.util.Random; 
import javax.imageio.ImageIO; 
import javax.servlet.ServletException; 
import javax.servlet.ServletOutputStream; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
import javax.servlet.http.HttpSession; 
 
import org.apache.commons.io.output.ByteArrayOutputStream; 
 
public class ValidateCodeServlet extends HttpServlet { 
 
    /**
     * Constructor of the object.
     */ 
    public ValidateCodeServlet() { 
        super(); 
    } 
 
    /**
     * Destruction of the servlet. <br>
     */ 
    public void destroy() { 
        super.destroy(); // Just puts "destroy" string in log 
        // Put your code here 
    } 
 
    /**
     * The doGet method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to get.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */ 
    public void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException { 
        response.setContentType("image/jpeg"); 
        response.setHeader("Pragma", "No-cache"); 
        response.setHeader("Cache-Control", "no-cache"); 
        response.setDateHeader("Expires", 0); 
        HttpSession session = request.getSession(); 
 
        int width = 60, height = 20; 
        BufferedImage image = new BufferedImage(width, height, 
                BufferedImage.TYPE_INT_RGB); 
 
        Graphics g = image.getGraphics(); 
 
        Random random = new Random(); 
 
        g.setColor(getRandColor(200, 250)); 
        g.fillRect(0, 0, width, height); 
 
        g.setFont(new Font("Times New Roman", Font.PLAIN, 18)); 
 
        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); 
        } 
 
        String sRand = ""; 
        for (int i = 0; i < 4; i++) { 
            String rand = String.valueOf(random.nextInt(10)); 
            sRand += rand; 
            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); 
        } 
        session.setAttribute("rand", sRand); 
        g.dispose(); 
        ServletOutputStream responseOutputStream = response.getOutputStream(); 
        ImageIO.write(image, "JPEG", responseOutputStream); 
 
        responseOutputStream.flush(); 
        responseOutputStream.close(); 
    } 
 
    /**
     * The doPost method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to post.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */ 
    public void doPost(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException { 
 
        doGet(request, response); 
    } 
 
    /**
     * Initialization of the servlet. <br>
     *
     * @throws ServletException if an error occurs
     */ 
    public void init() throws ServletException { 
        // Put your code here 
    }   
    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); 

    } 
}