13：Response验证码实现

#### 3、验证码功能

验证怎么来的？

- 前端实现

- 后端实现，需要用到 Java 的图片类，生产一个图片

```java

package com.kuang.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，这个请求用图片的方式打开

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);

}

```

13：Response验证码实现