JSP中验证码的使用

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//////////////////////////////////////CheckCode.java/////////////////////////////////////////////////////
package com.shopping.controller.util;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;

/**
*
* @author Administrator
*         ==========================产生验证码============================
*/

public class CheckCode extends javax.servlet.http.HttpServlet implements
javax.servlet.Servlet {
private static final long serialVersionUID = -4545744597784634421L;

protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
// 设置响应类型为图片 , (以前是text/html)
response.setContentType("image/jpeg");

// 产生四位随机验证码
StringBuffer sb = new StringBuffer(4);
for (int i = 0; i < 4; i++) {
int n = (int) (Math.random() * 10);
sb.append(n);
}
String validateCode = sb.toString();
// 将验证码记录在session中，便于用户输入之后的验证
request.getSession().setAttribute("checkcode", validateCode);

// 创建缓存图片
BufferedImage image = new BufferedImage(80, 25,
BufferedImage.TYPE_INT_RGB);
Graphics g = image.getGraphics();

g.setColor(Color.LIGHT_GRAY);
g.fillRect(0, 0, 80, 25);
g.setColor(Color.BLACK);

g.drawString(validateCode, 10, 20);
g.dispose();
ServletOutputStream outStream = response.getOutputStream();
JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(outStream);
encoder.encode(image);
outStream.close();
}
}

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<!-------------------------------------------------------------------------------------------------------->
<!-- 配置验证码 -->
<servlet>
<servlet-name>CheckCode</servlet-name>
<servlet-class>com.shopping.controller.util.CheckCode</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>CheckCode</servlet-name>
<url-pattern>/checkcode</url-pattern>
</servlet-mapping>

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<!-------------------------------------------------------------------------------------------------------->
<tr>
<td>验证码:</td>
<td><input type="text" name="checkcode" id="checkcode" /></td>
<td><img src="<%=request.getContextPath()%>/checkcode" border="0" /></td>
</tr>