服务器端：Servlet 生成验证码
下面将使用Servlet输出图片验证码。图片验证码的原理是，服务器生成一个包含随机的字符串的图片发给客户端，客户端提交数据时需要填写字符串作为验证。由于字符串保存在图片中，因此机器很难识别，从而达到防止有人使用计算机程序恶意发送信息的目的。
注：Servlet输出图片时，需要调用getOutputStream输出图片，而不是getWriter
package com.fly.servlet;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
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 {
private static final long serialVersionUID = 1L;
//随即字符字典
public static final char[] CHARS = {'1','2','3','4','5','6','7','8','9','A','B','C','D',
'E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
public static Random random = new Random();//随机数
//获取六位随机数
public static String getRandomString(){
StringBuffer buffer = new StringBuffer();
for(int i=0;i<6;++i){
buffer.append(CHARS[random.nextInt(CHARS.length)]);
}
return buffer.toString();
}
//获取随机颜色
public static Color getRandomColor(){
return new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255));
}
//获取某颜色的反色
public static Color getReverseColor(Color c){
return new Color(255-c.getRed(),255-c.getGreen(),255-c.getBlue());
}
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);
int width = 100;//图片宽度
int height = 30;//图片高度
Color color = getRandomColor();
Color reverseColor = getReverseColor(color);
//创建一个彩色图片
BufferedImage bi = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
Graphics2D g = bi.createGraphics();//获取绘图对象
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);
g.setColor(reverseColor);//设置颜色
g.drawString(randomString, 18, 20);
//画最多100个噪音点
for(int i=0,n=random.nextInt(100);i<n;++i){
//随机噪音点
g.drawRect(random.nextInt(width), random.nextInt(width), 1, 1);
}
//转成JPEG格式
ServletOutputStream out = response.getOutputStream();
//编码器
JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
encoder.encode(bi);//对图片进行编码
out.flush();//输出到客户端
}
}
该Servlet在web.xml中的配置如下：<?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>identityServlet</servlet-name> <servlet-class>com.fly.servlet.IdentityServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>identityServlet</servlet-name> <url-pattern>/identity</url-pattern> </servlet-mapping> </web-app>
然后就可以直接访问该Servlet，浏览图片了。为了方便演示，下面给出一个HTML文件引用这个图片验证码，代码如下：<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>identity.html</title> <script type="text/javascript"> function reloadImage(){ //document.getElementById('btn').disabled = true; document.getElementById('identityimg').src = 'identity?ts='+new Date().getTime(); } </script> </head> <body> <img src="identity" id="identityimg" onclick="btn.disabled=false"/> <input type=button value="换个图片" onclick="reloadImage()" id="btn"/> </body> </html>
这样就可以访问该html页面，浏览验证码图片了。效果如图所示：