HttpServletResponse

web服务器接收到客户端http请求，针对这个请求，分别创建一个代表请求的HttpServletRequest对象，代表响应的一个HttpServletResponse对象

常见应用

1.向浏览器输出消息

2.下载文件

1.获取下载文件路径
2.下载文件名是啥？
3.设置想办法让浏览器能够支持下载我们需要的东西
4.获取下载文件的输入流
5.创建缓冲区
6.获取outputstream对象
7.将fileoutputstream流写到buffer缓冲区
8.使用outputstream将缓冲区中的数据输出到客户端！

下载文件代码：

package com.kuang.servlet;

import com.sun.deploy.net.URLEncoder;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;

public class FileServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        1.获取下载文件路径
        String realPath = "C:\\Users\\LLh\\Desktop\\1.jpg";
//        2.下载文件名是啥？
        System.out.println("下载文件的路径："+realPath);
        final String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
//        3.设置想办法让浏览器能够支持(Content-Disposition)下载我们需要的东西,中文文件名URLEncoder.encode编码，否则有可能乱码
        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
//        4.获取下载文件的输入流
        FileInputStream in = new FileInputStream(realPath);
//        5.创建缓冲区
        int len = 0;
        byte[] buffer = new byte[1024];
//        6.获取outputstream对象
        ServletOutputStream out = resp.getOutputStream();
//        7.将fileoutputstream流写到buffer缓冲区
        while ((len=in.read(buffer))>0){
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
//        8.使用outputstream将缓冲区中的数据输出到客户端！
    }

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

web.xml

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
  <display-name>Archetype Created Web Application</display-name>
  <servlet>
    <servlet-name>filedown</servlet-name>
    <servlet-class>com.kuang.servlet.FileServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>filedown</servlet-name>
    <url-pattern>/down</url-pattern>
  </servlet-mapping>
</web-app>

3.验证码功能

验证怎么来？
前端实现
后端实现，需要用到Java的图片类，生产一个图片。
Java代码如下：

     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 {
        //如果让浏览器3秒自动刷新一次；
        resp.setHeader("refesh","3");
        //在内存中创建一个图片
        BufferedImage bufferedImage = new BufferedImage(80,20,BufferedImage.TYPE_INT_BGR);
        //得到图片
        Graphics graphics = bufferedImage.getGraphics();//笔
        //设置图片的背景颜色
        graphics.setColor(Color.white);
        graphics.fillRect(0,0,80,20);
        //给图片写数据
        graphics.setColor(Color.BLUE);
        graphics.setFont(new Font(null,Font.BOLD,20));
        graphics.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(bufferedImage,"jpg",resp.getOutputStream());
    }
    //生产随机数
    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配置：

   <servlet>
    <servlet-name>ImageServlet</servlet-name>
    <servlet-class>com.kuang.servlet.ImageServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ImageServlet</servlet-name>
    <url-pattern>/img</url-pattern>
  </servlet-mapping>

4.实现重定向

B一个web资源收到客户端A请求后，B他会通知A客户端去访问另外一个web资源C,这个过程叫重定向

常见场景：
用户登录
代码如下：

package com.kuang.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class RequestTest extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进来了");
        //从request获取请求参数
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println("username = "+username+"  password = "+password);
        resp.sendRedirect("/success.jsp");//返回这个页面
    }
}

html

<html>
<body>
<h2>Hello World!</h2>
</body>
<%--这里提交的路径，需要寻找到项目的路径--%>
<%-- ${pageContext.reuqest.contextPath} 代表根路径--%>
<form action="/login" method="get">
    <input type="text" name="username" >br
    <input type="password" name="password">
    <input type="submit" value="提交">
</form>

</html>

web.xml

  <servlet>
    <servlet-name>login</servlet-name>
    <servlet-class>com.kuang.servlet.RequestTest</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>login</servlet-name>
    <url-pattern>/login</url-pattern>
  </servlet-mapping>