3、验证码功能

验证码怎么来的？

• 前端实现
• 后端实现，需要用到java的图片类，生成一个图片

代码show

ImageServlet.java

package com.gongyi.servlet;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //如何让浏览器3秒自动刷新一次
        resp.setHeader("refresh", "3");
        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics();//笔
        //设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0, 0, 80, 20);
        //给图片写数据
        g.setColor(Color.blue);
        g.setFont(new Font(null, Font.BOLD, 20));
        g.drawString(makeNum(), 0, 20);
        //告诉浏览器，这个请求以图片的方式打开
        resp.setContentType("image/jpeg");
        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires", -1);
        resp.setHeader("Cache-Control", "no-cache");
        resp.setHeader("Pragma", "no-cache");
        //把图片写给浏览器
        ImageIO.write(image, "jpg", resp.getOutputStream());


        //
    }

    //生成随机数
    private String makeNum() {
        Random random = new Random();
        String num = random.nextInt(9999999) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7 - num.length(); i++) {
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;

    }

    @Override

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"
         metadata-complete="true">
    <servlet>
        <servlet-name>downFile</servlet-name>
        <servlet-class>com.gongyi.servlet.FileServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>downFile</servlet-name>
        <url-pattern>/downFile</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>imageServlet</servlet-name>
        <servlet-class>com.gongyi.servlet.ImageServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>imageServlet</servlet-name>
        <url-pattern>/image</url-pattern>
    </servlet-mapping>
</web-app>

效果图：