Servlet：response生成图片验证码

src 目录下com.xieyuan包MyServlet.java文件（Servlet文件）

package com.xieyuan;

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.net.URLEncoder;
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.corba.se.impl.javax.rmi.CORBA.Util;
import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;

public class MyServlet extends HttpServlet {

	/**
	 * Constructor of the object.
	 */
	public MyServlet() {
		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 {
		execute(request, response);
	}

	/**
	 * 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 {
		execute(request, response);
	}

	
	private static final char CHARS[]={'2','3','4','5','6','7','8','9','A','B','C','D','E',
		                               'F','G','H','J','K','L','M','N','P','Q','R','S','T','U','V',
		                               'W','X','Y','Z'
	                                  };
	public static Random random=new Random();
	//生成随机数字，len为需要随机数字的个数
	public static String getRandomString(int len)
	{
		StringBuilder builder=new StringBuilder();
		for(int i=0;i<len;i++)
		{
		    builder.append(CHARS[random.nextInt(CHARS.length)])	;
		}
	    return builder.toString();	
	}
	//随机生成颜色，座位背景色
	public static Color getColor()
	{
		return new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255));
	}
	//取颜色的反色
	public static Color getReverseColor(Color color)
	{
	    return new Color(255-color.getRed(),255-color.getGreen(),255-color.getBlue());	
	}
	
	private void execute(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
	{
		response.setCharacterEncoding("utf-8");
		//设置返回的文件编码
		response.setContentType("image/jpeg");
		
		//获取随机码
		String getRandomCode=getRandomString(5);
		//将随机码放到Session中
		request.getSession().setAttribute("randomcode", getRandomCode);
		int width=100;
		int height=30;
		Color color=getColor();
		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();
		g.setFont(new Font(null,Font.BOLD,16));
		g.setColor(color);
		g.fillRect(0,0,width,height);
		g.setColor(reverseColor);
		g.drawString(getRandomCode, 18,20);
		//绘制噪点,最多100个
		for(int i=0,n=random.nextInt(100);i<n;i++)
		{
			g.drawRect(random.nextInt(width), random.nextInt(height), 1,1);
		}
		ServletOutputStream out=response.getOutputStream();
		JPEGImageEncoder encoder=JPEGCodec.createJPEGEncoder(out);
		encoder.encode(bi);
		out.flush();
	}
	
	/**
	 * Initialization of the servlet. <br>
	 *
	 * @throws ServletException if an error occurs
	 */
	public void init() throws ServletException {
	}

}

配置好web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" 
	xmlns="http://java.sun.com/xml/ns/j2ee" 
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
	xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
	http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>MyServlet</servlet-name>
    <servlet-class>com.xieyuan.MyServlet</servlet-class>
  </servlet>

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

接着部署您的WEB 应用，此时访问：

http://127.0.0.1:8080/Test/servlet/MyServlet

可以看到一张图片验证码。但是，这种效果好吗？这,并不是我们想要的，图片验证码应该混搭在HTML页面里，做登录等功能才对。

那么，接下来我们来做个JSP页面，直接使用项目WebRoot目录下的，index.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
    
    <title>My JSP 'index.jsp' starting page</title>
	<meta http-equiv="pragma" content="no-cache">
	<meta http-equiv="cache-control" content="no-cache">
	<meta http-equiv="expires" content="0">    
	<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
	<meta http-equiv="description" content="This is my page">
	<!--
	<link rel="stylesheet" type="text/css" href="styles.css">
	-->
	<script language="JavaScript" >
	    function reloadImage()
	    {
	       <!--将按钮状态设置为不可用，当图片加载完成触发onload后，按钮状态就为可用了。这样可用避免重复获取-->
	       document.getElementById("btn").disabled=true;
	       <!--第一次连接的时候不会有问题，第二次连接时，假如你后面没有new Date().getTime(),加参数就会连接的时候拿缓存，没有连到服务器。加上时间函数就能保证你每次得到的不是浏览器的缓存。-->
	       document.getElementById("img").src="servlet/MyServlet?timestamp="+new Date().getTime();
	     }
	</script>
  </head>
  
  <body>
     <img src="servlet/MyServlet" id="img" onload="btn.disabled=false;" /><br/><br/>
     <input type="button" value="换一张图片" onClick="reloadImage()" id="btn" /><br/>
     <script>document.write("页面最后更新："+document.lastModified)</script>
  </body>
</html>

此时，我直接访问：http://127.0.0.1:8080/Test/  

结果就是我们想要的了

可以看到，无论我们点击多少次 “换一张图片”按钮，网页始终没有刷新，页面最后更新时间也没变，变的就是图片验证码！