HttpServletResponse & HttpServletRequest

web服务器接收到客户端的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；

如果要获取客户端请求过来的参数：找HttpServletRequest

如果要给客户端响应一些信息：找HttpServletResponse

具体的常量和方法可以看源码

HttpServletResponse

常见应用

1.向浏览器发送消息

2.下载文件

下载路径   + 下载名  +setHeader设置头文件(下载指令+格式)+ 输入流获得数据,  缓存后以 输出流 传送给HttpServletResponse

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 1. 要获取下载文件的路径
//        String realPath = "E:\\amixi.jpg";   //在网页上显示图片
        String realPath = "E:\\note.txt";  //在网页上显示文件的内容
        System.out.println("下载文件的路径："+realPath);
// 2. 下载的文件名是啥？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
// 3. 设置想办法让浏览器能够支持(Content-Disposition)自动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流写入到buffer缓冲区,使用OutputStream将缓冲区中的数据输出到客户端！
        while ((len=in.read(buffer))>0){
            out.write(buffer,0,len);
        }
        in.close();
        out.close();

    }

生成验证码(动态生成图片)

package com.ljm.oo;

import javax.imageio.ImageIO;
import javax.servlet.ServletContext;
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 Captcha_ 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());

    }

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

    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;
    }

}

3.简单实现重定向(登录跳转)resp.sendRedirect("url")

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        resp.setHeader("Location","/servlet02_war/gethello");
//        resp.setStatus(302);
        resp.sendRedirect("/servlet02_war/baidu.jsp");
      //   /项目名字/web.xml中注册的url 丨 .jsp文件
    }

  String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username+":"+password);
        resp.sendRedirect("/servlet02_war/baidu.jsp");

<%@page contentType="text/html;" pageEncoding="UTF-8" %>
<%--{找到项目路径}/web.xml注册的url--%>
<form action="${pageContext.request.contextPath}/login" method="get">
    用户名：<input type="text" name="username"> <br>
    密码：<input type="password" name="password"> <br>
    <input type="submit">
</form>

后面在web.xml中注册/login

HttpServletRequest

可以从请求中获取数据: 单个的(getParameter) 和多个的(getParameterValues)

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
req.setCharacterEncoding("utf-8");
resp.setCharacterEncoding("utf-8");
String username = req.getParameter("username");
String password = req.getParameter("password");
String[] hobbys = req.getParameterValues("hobbys");
System.out.println("=============================");
//后台接收中文乱码问题
System.out.println(username);
System.out.println(password);
System.out.println(Arrays.toString(hobbys));
System.out.println("=============================");
System.out.println(req.getContextPath());
//通过请求转发
//这里的 / 代表当前的web应用
req.getRequestDispatcher("/success.jsp").forward(req,resp);
}