Servlet基础（续集2）

HttpServletResponse

web服务器接收到客户端的http的请求，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

简单分类：

负责向浏览器发送数据的方法:

ServletOutputStream getOutputStream() throws IOException;
PrintWriter getWriter() throws IOException;

负责向浏览器发送响应头的方法:

响应的状态码：

下载文件：

向浏览器输出消息
下载文件
1. 要获取下载文件的路径
2. 下载的文件名是什么
3. 设置想办法浏览器能够支持下载我们需要的文件
4. 获取下载文件的输入流
5. 创建缓冲区
6. 获取OujtputStream对象
7. 将FileOujtputStream流写入到buffer缓冲区
8. 使用OujtputStream将缓冲区中的数据输出到客户端!

public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        1.要获取下载文件的路径
        String realPath = "E:\\java project\\untitled\\note.md";
//        2.下载的文件名是什么
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
//        3.设置想办法浏览器能够支持(Content-Disposition)下载我们需要的文件,让中文文件名URLEncoder.encode编码，否则可能乱码
        resp.setHeader("Content-Disposition","attachment;filename"+ URLEncoder.encode(fileName, "UTF-8"));
//        下载文件
        FileInputStream in  = new FileInputStream(realPath);
//        4.获取下载文件的输入流
        int len = 0;
//        5.创建缓冲区
        byte[] buffer = new byte[1024];
//        6.获取OujtputStream对象
        ServletOutputStream out = resp.getOutputStream();
//        7.将FileOujtputStream流写入到buffer缓冲区,使用OujtputStream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();

    }

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

验证码功能：

验证怎么来的？

前端实现
后端实现，需要用到java图片类，生成一个图片

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5秒自动刷新一次；
        resp.setHeader("refresh","5");
        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80, 20,BufferedImage.TYPE_3BYTE_BGR);
        //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/jepg");
        //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);
    }
}

实现重定向：

B一个web资源收到客户端A请求后，B会通知A客户端去访问另一个web资源C，这个过程叫重定向

常见场景：

用户登录

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 RedirectServlet extends HttpServlet {


    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        resp.setHeader("Location", "/r/img");
//        resp.setStatus(302);
        resp.sendRedirect("/r/img");//重定向
    }

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

重定向与转发的区别：

相同点：页面都会实现跳转

不同点：

请求转发的时候，url不会发生变化 307
重定向的时候，url地址会发生变化 302

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 doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //处理请求
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username+"："+password);
        //重定向的时候一定要注意路径问题，否则会404
        resp.sendRedirect("./success.jsp");

    }

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

<html>
<body>
<h2>Hello World!</h2>


<%--这里提交的路径，需要寻找项目的路径--%>
<%--${pageContext.request.contextPath}代表当前项目--%>
<form action="${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>

</body>
</html>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>success</h1>
</body>
</html>

HttpServletReques

HttpServletRequest代表客户端的请求，用户通过Http协议访问服务器，HTTP请求中的所有信息会被封装到HttpServletRequest，通过这个HttpServletRequest的方法，获得客户端的所有信息。

获取前端传递的参数

请求转发

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;
import java.util.Arrays;

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobbys = req.getParameterValues("hobbys");
        System.out.println(username+":"+password+ Arrays.toString(hobbys));

        //resp.sendRedirect("./success");
        //通过请求转发
        req.getRequestDispatcher(req.getContextPath()+"/success.jsp").forward(req, resp);
    }

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

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录</title>
</head>
<body>
    <h1>登录</h1>
    <div style="text-align: center">
        <%--以post方式提交表单，提交我们的login请求--%>
        <form action="${pageContext.request.contextPath}/login" method="post">
            用户名：<input type="text" name="username"><br>
            密码：<input type="password" name="password"><br>
            爱好：
            <input type="checkbox" name="hobbys" value="唱歌">唱歌
            <input type="checkbox" name="hobbys" value="女孩">女孩
            <input type="checkbox" name="hobbys" value="代码">代码
            <input type="checkbox" name="hobbys" value="电影">电影
            <br>
            <input type="submit">
        </form>
    </div>
</body>
</html>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<H1>SUCCESS</H1>
</body>
</html>