使用Servlet实现验证码

没有验证码带来的问题

1. 对特定用户不断登录破解密码。
2. 对某个网站创建账户。
3. 对某个网站提交垃圾数据。
4. 对某个网站刷票。

 通过验证码由用户肉眼识别其中的验证码信息，从而区分用户是人还是计算机。

定义：

• 验证码（CAPTCHA）:是一种区分用户是计算机还是人的公共全自动程序。

• 作用：防止恶意破解密码、刷票、论坛灌水，防止黑客暴力破解。

使用Servlet实现验证码

    实现图片验证码类GenerateImageCodeServlet.java

GenerateImageCodeServlet.java

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Date;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class GenerateImageCodeServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    private static final char[] CH = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".toCharArray();
    private static final int IMAGE_WIDTH = 73;
    private static final int IMAGE_HEIGHT = 28;
    private static final int LINE_NUM = 30;
    private static final int RANDOM_NUM = 4;
    Random random = new Random();

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("image/jpg");//设置相应类型,告诉浏览器输出的内容为图片
        response.setHeader("Pragma", "No-cache");//设置响应头信息，告诉浏览器不要缓存此内容
        response.setHeader("Cache-Control", "no-cache");
        response.setDateHeader("Expire", new Date().getTime());
        
        BufferedImage bi = new BufferedImage(IMAGE_WIDTH, IMAGE_HEIGHT, BufferedImage.TYPE_INT_BGR);
        Graphics g = bi.getGraphics();
        g.fillRect(0, 0, IMAGE_WIDTH, IMAGE_HEIGHT);
        g.setColor(getRandomColor(110, 133));
        g.setFont(new Font("Times New Roman", Font.ROMAN_BASELINE, 18));
        // 绘制干扰线
        for (int i = 1; i <= LINE_NUM; i++) {
            int x = random.nextInt(IMAGE_WIDTH);
            int y = random.nextInt(IMAGE_HEIGHT);
            int xl = random.nextInt(13);
            int yl = random.nextInt(15);
            g.drawLine(x, y, x + xl, y + yl);
        }

        // 绘制随机字符
        StringBuilder sb = new StringBuilder();
        String str = null;
        for (int i = 0; i < RANDOM_NUM; i++) {
            g.setFont(new Font("Fixedsys", Font.CENTER_BASELINE, 18));
            g.setColor(new Color(random.nextInt(101), random.nextInt(111), random.nextInt(121)));
            str = CH[random.nextInt(CH.length)] + "";
            g.drawString(str, 13 * i, 16);
            g.translate(random.nextInt(3), random.nextInt(3));
            sb.append(str);
        }
        g.dispose();
        request.getSession().setAttribute("safeCode", sb.toString());
        ImageIO.write(bi, "JPG", response.getOutputStream());
    }

    /**
     * 获得颜色
     */
    private Color getRandomColor(int fc, int bc) {
        if (fc > 255)
            fc = 255;
        if (bc > 255)
            bc = 255;
        int r = fc + random.nextInt(bc - fc - 16);
        int g = fc + random.nextInt(bc - fc - 14);
        int b = fc + random.nextInt(bc - fc - 18);
        return new Color(r, g, b);

    }

}

登录验证LoginServlet.java

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class LoginServlet extends HttpServlet{

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        
        resp.setContentType("text/html;charset=gbk");
        
        String safeCode = (String) req.getSession().getAttribute("safeCode");
        String checkcode = req.getParameter("checkcode");
        
        PrintWriter out = resp.getWriter();
        
        if (safeCode.equalsIgnoreCase(checkcode)) {
            out.println("验证码正确");
        } 
        else {
            out.println("验证码错误");
        }
        out.flush();
        out.close();
    }
    
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" 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">

    <servlet>
        <servlet-name>ImageCodeServlet</servlet-name>
        <servlet-class>com.lijy.servlet.GenerateImageCodeServlet</servlet-class>
    </servlet>
    
    <servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.lijy.servlet.LoginServlet</servlet-class>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>ImageCodeServlet</servlet-name>
        <url-pattern>/safe_code</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>LoginServlet</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>

    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
    
</web-app>

index.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!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=ISO-8859-1">
<title>checkcodes</title>
<script type="text/javascript">
    function reloadCode() {
        var time = new Date().getTime();
        document.getElementById("imagecode").src="<%=request.getContextPath()%>/safe_code?d="+time;
    }
</script>
</head>
<body>

<form action="<%=request.getContextPath()%>/login" method="get">
验证码：<input type="text" name="checkcode" />
    <img alt="验证码" id="imagecode" src="<%=request.getContextPath()%>/safe_code">
    <a href="javascript:reloadCode();">看不清楚</a><br/>
    <input type="submit" value="提交" />
<hr>

</form>

</body>
</html>

页面截图