jsp页面验证码完整实例

package com.servlet;

import java.awt.color;
import java.awt.font;
import java.awt.graphics2d;
import java.awt.image.bufferedimage;
import java.io.ioexception;
import java.io.printwriter;
import java.util.random;

import javax.servlet.servletexception;
import javax.servlet.servletoutputstream;
import javax.servlet.http.httpservlet;
import javax.servlet.http.httpservletrequest;
import javax.servlet.http.httpservletresponse;

import com.sun.image.codec.jpeg.jpegcodec;
import com.sun.image.codec.jpeg.jpegimageencoder;

public class identityservlet extends httpservlet {

  public static final char[] chars={'2','3','4','5','6','7','8','9','a'};//自定义验证码池
  public static random random=new random();　　//随机数
  
  public static string getrandomstring(){　　//获取6位随机数，放在图片里
    stringbuffer buffer=new stringbuffer();
    for(int i=0;i<6;i++){
      buffer.append(chars[random.nextint(chars.length)]);
    }
    return buffer.tostring();
  }
  
  public static color getrandomcolor(){　　//获取随机的颜色
    return new color(random.nextint(255), random.nextint(255), random.nextint(255));
  }
  
  public static color getreversecolor(color c){　　//返回某颜色的反色
    return new color(255 - c.getred(), 255 - c.getgreen(), 255 - c.getblue());
  }
  
  /**
   * constructor of the object.
   */
  public identityservlet() {
    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");　　//设置输出类型
    
    string randomstring = getrandomstring();　　//随机字符串
    request.getsession(true).setattribute("randomstring", randomstring);//放到session里
    
    int width=100;　　//图片宽度
    int height=30;　　//图片高度
    
    color color=getrandomcolor();　　//随机颜色，用于背景色
    color reverse=getreversecolor(color);//反色，用于前景色
    //创建一个彩色图片
    bufferedimage bi=new bufferedimage(width, height, bufferedimage.type_int_rgb);
    graphics2d g=bi.creategraphics();　　//绘图对象
    g.setfont(new font(font.sans_serif,font.bold,16));//设置字体
    g.setcolor(color);//设置颜色
    g.fillrect(0, 0, width, height);//绘制背景
    g.setcolor(reverse);
    g.drawstring(randomstring, 18, 20);//绘制随机字符
    for(int i=0,n=random.nextint(100);i<n;i++){　　//画最多100个噪音点
      g.drawrect(random.nextint(width), random.nextint(height), 1, 1);
    }
    servletoutputstream out= response.getoutputstream();//转成jpeg格式
    jpegimageencoder encoder=jpegcodec.createjpegencoder(out);//编码器
    encoder.encode(bi);　　//对图片进行编码
    out.flush();　　//输出到客户端
  }

  /**
   * 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
  }

}

<?xml version="1.0" encoding="utf-8"?>
<web-app version="3.0" 
  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_3_0.xsd">
 <display-name></display-name>
 <servlet>
  <servlet-name>identityservlet</servlet-name>
  <servlet-class>com.servlet.identityservlet</servlet-class>
 </servlet>

 <servlet-mapping>
  <servlet-name>identityservlet</servlet-name>
  <url-pattern>/servlet/identityservlet</url-pattern>
 </servlet-mapping>  
 <welcome-file-list>
  <welcome-file>index.jsp</welcome-file>
 </welcome-file-list>
</web-app>

<!doctype html public "-//w3c//dtd html 4.01 transitional//en" "http://www.w3.org/tr/html4/loose.dtd">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8">
<title>insert title here</title>
<script type="text/javascript">
function reloadimage(){
  document.getelementbyid('btn').disabled=true;
  document.getelementbyid('identity').src='servlet/identityservlet?ts='+new date().gettime();
}
</script>
</head>

<body>

<img src="servlet/identityservlet" id="identity" onload="btn.disabled=false;" />
<input type=button value="换个图片" onclick="reloadimage()" id="btn">
</body>
</html>