关于图片验证码的若干问题

图片验证码是在登录、注册中防止机器注册、灌水等的常用一种小技术。现在贴出我的一种实现代码，也是借鉴别人的。现在主要的java类如下：

    public class RandImgCreater {     private static final String CODE_LIST = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";     private HttpServletResponse response = null;     private static final int HEIGHT = 20;     private static final int FONT_NUM = 4;     private int width = 0;     private int iNum = 0;     private String codeList = "";     private boolean drawBgFlag = false;     private int rBg = 0;     private int gBg = 0;     private int bBg = 0;     public RandImgCreater(HttpServletResponse response) {         this.response = response;         this.width = 13 * FONT_NUM + 12;         this.iNum = FONT_NUM;         this.codeList = CODE_LIST;     }     public RandImgCreater(HttpServletResponse response,int iNum,String codeList) {         this.response = response;         this.width = 13 * iNum + 12;         this.iNum = iNum;         this.codeList = codeList;            }     public String createRandImage(){         BufferedImage image = new BufferedImage(width, HEIGHT,                 BufferedImage.TYPE_INT_RGB);         Graphics g = image.getGraphics();         Random random = new Random();         if ( drawBgFlag ){             g.setColor(new Color(rBg,gBg,bBg));             g.fillRect(0, 0, width, HEIGHT);         }else{          g.setColor(getRandColor(200, 250));          g.fillRect(0, 0, width, HEIGHT);          for (int i = 0; i < 155; i++) {              g.setColor(getRandColor(140, 200));              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);          }         }         g.setFont(new Font("Times New Roman", Font.PLAIN, 18));         String sRand="";         for (int i=0;i             int rand=random.nextInt(codeList.length());             String strRand=codeList.substring(rand,rand+1);             sRand+=strRand;             g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));             g.drawString(strRand,13*i+6,16);         }

        g.dispose();

        try{             ImageIO.write(image, "JPEG", response.getOutputStream());         }catch(IOException e){         }         return sRand;     }     public void setBgColor(int r,int g,int b){         drawBgFlag = true;         this.rBg = r;         this.gBg = g;         this.bBg = b;     }

    private Color getRandColor(int fc, int bc) {         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);     } }

       凭java类就可以生成相应的代码，建立一个img.jsp就可以调用：具体代码如下：

       <%@ page import="com.register.img.RandImgCreater" %>        http://www.w3.org/1999/xhtml">        <%@ page contentType="text/html;charset=GBK"%>        <%                 out.clear();                 response.setHeader("Pragma","No-cache");                 response.setHeader("Cache-Control","no-cache");                 response.setDateHeader("Expires", 0);

                RandImgCreater rc = new RandImgCreater(response);                 String rand = rc.createRandImage();

                //如果没有下面两句代码，将会出现getOutputStream() has already been called for this response异常                 out.clear();                 out = pageContext.pushBody();                 session.setAttribute("rand",rand);        %>

          在这里提出几个问题，第一个，文中红线处，很重要，就这一句out.clear能有效防止在resin服务器下的乱码。没有这句，在tomcat服务器下是不存在问题的。还有下面红线句调用时能防止getOutputStream的异常错误。

          第二个问题，就是在调用验证码的时候，不能在本页面调用，如果这样，得到的验证码总会比输出的验证码慢半拍，即得到的是上次的验证码。并且第一次得到的就是空的，会抛异常。这样的话，如果只是刷页面，并且不会得到验证码。所以最好的办法是另外建一个action，在action中进行验证。代码也很简单，如下：

           <%

                //得到验证码的值                 String img=request.getParameter("code");                 String rand=session.getAttribute("rand").toString();           

                if(img.equals(rand)==false)                 {                       %>                             alert("验证码错误")                             history.go(-1)                       <%                 }else{      %>alert("验证码正确")<%               

}