驗證碼原理和實作

1、驗證碼的作用

驗證碼是目前大多網站所支持并使用于注冊登錄的，就在于其作用能有效防止暴力攻擊、惡意登錄注冊，驗證碼每次都不同,

這就可以排除,用其他病毒或者軟體自動申請用戶及自動登陸.有效防止這種問題，

2、原理及知識點分析

1.原理

驗證碼于服務器端生成，發送給客戶端，并以影像格式顯示，客戶端提交所顯示的驗證碼，客戶端接收并進行比較，若比對失敗則不能實作登錄或注冊，反之成功后跳轉相應界面，

2.主要知識

服務器端的相關實作，前端運用 HTML 5，CSS 3 ,(可空白留給美工實作，須有 HTML 基礎知識)，

主要是三部分:

(提交界面， 成功及失敗的) jsp

(后臺對應提交 jsp 驗證提交內容的) servlet (生成驗證碼的) servlet

XML 檔案

3、驗證碼實作

前臺代碼：

<a style="cursor: hand" onclick="reload()">
<img alt="加載中..." src="${pageContext.request.contextPath }/CheckCodeServlet" id="img" border="1">
</a>

<script type="text/javascript">
		function reload(){
			document.getElementById("img").src
             = document.getElementById("img").src+"?aa="+new Date().getTime();//nocache
		}
</script>

java代碼：

package com.school.controller;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Random;
import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javafx.scene.text.Font;

/**
 *驗證碼
 */
@WebServlet("/CheckCodeServlet")
public class CheckCodeServlet extends HttpServlet {
	
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//設定快取
		response.setHeader("pragma", "No-cache");
		response.setHeader("Cache-Control", "No-cache");
		response.setDateHeader("Expires", 0);
		//設定回傳資料的型別
		response.setContentType("image/gif");
		//設定畫板大小
		int width = 100;
		int height = 40;
		BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_BGR);
		//畫筆
		Graphics g = image.getGraphics();
		java.awt.Font font = new java.awt.Font("宋體",java.awt.Font.BOLD,25);
		g.setFont(font);
		String imagePath = request.getServletContext().getRealPath("/img/codeBackg.jpg");
		Image image1 = ImageIO.read(new File(imagePath));
		g.drawImage(image1,0,0,100,40,null);
		String source = "0123456789abcdefghijklmABCDEFGH好人壞人";
		String infor  = "";
		for (int i = 0; i < 4; i++) {
	        //隨機取一個字符
			int index =  new Random().nextInt(source.length()-1);
			char myCode = source.charAt(index);
			//讓字的顏色隨機
			Random random = new Random();
			g.setColor(new Color(20+random.nextInt(120),20+random.nextInt(120),20+random.nextInt(120)));
			//畫入文字
			g.drawString(myCode+"", 15+i*20, 20+new Random().nextInt(10));
			//畫入干擾線
			g.drawLine(random.nextInt(100), random.nextInt(40), random.nextInt(100), random.nextInt(40));
			infor += myCode;
		}
		//記錄到session，以備將來匹配
		request.getSession().setAttribute("infor", infor);
		//釋放資源
		g.dispose();
		OutputStream out  = response.getOutputStream();
		ImageIO.write(image,"gif", out);
		out.flush();
		out.close();	
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

//接收輸入的驗證碼
String code = request.getParameter("code");
//接收java生成的驗證碼
String infor = (String) request.getSession().getAttribute("infor");

//用equalsIgoreCase()方法判斷是否相等，不區分大小寫
if(!code.equalsIgnoreCase(infor)){
	//不正確
	loginError.put("code", "驗證碼不正確");
}