【JavaWeb 第二章】Servlet 原理剖析

拼搏现实的明天。 2022-12-23 03:20 21阅读 0赞

### 文章目录 ###

*   *  Servlet 介绍
     *  Servlet 原理
     *  Mapping 问题
     *  ServletContext 对象
     *  ServletContext 应用
     *  HttpServlet Response
     *  HttpServlet Request
     *  实现重定向

## Servlet 介绍 ##

Servlet（Server Applet）是Java Servlet的简称，用 Java 编写的服务器端程序，具有独立于平台和协议的特性，主要功能在于交互式地浏览和生成数据，生成动态Web内容

我们把实现了 Servlet 接口的 Java 程序叫做 Servlet , 它运行于支持 Java 的应用服务器中。从原理上讲，Servlet 可以响应任何类型的请求，但绝大多数情况下 Servlet 只用来扩展基于 HTTP 协议的 Web 服务器

## Servlet 原理 ##

Servlet 是由 Web 服务器调用，web 服务器是收到浏览器请求后,会

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNzYzMzQ0_size_16_color_FFFFFF_t_70_pic_center]

## Mapping 问题 ##

一个 Servlet 请求可以指定一个映射路径

<servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.maze.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    
    <servlet>
        <servlet-name>error</servlet-name>
        <servlet-class>com.maze.servlet.ErrorServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>error</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

## ServletContext 对象 ##

web 容器在启动的时候，它会为每个 web 程序都创建一个对应的 getServletContext 对象，它代表了当前的 web 应用；

*  共享数据：不同 Servlet 对象可以互相共享数据

public class HelloServlet extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            ServletContext context = this.getServletContext();
            String username = "马泽";
            // 将一个数据保存在 context 中，名字为 username,值为 马泽
            context.setAttribute("username",username);
        }
    }

public class GetServlet extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            ServletContext context = this.getServletContext();
            String username = (String)context.getAttribute("username");
    
            resp.setContentType("text/html");
            resp.setCharacterEncoding("utf-8");
            resp.getWriter().print("名字:"+username);
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req,resp);
        }
    }

<servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.maze.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    
    <servlet>
        <servlet-name>getc</servlet-name>
        <servlet-class>com.maze.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getc</servlet-name>
        <url-pattern>/getc</url-pattern>
    </servlet-mapping>

## ServletContext 应用 ##

*  获取初始化参数

web.xml 文件

<context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>
    
    <servlet>
        <servlet-name>gp</servlet-name>
        <servlet-class>com.maze.servlet.Demo3Servlet</servlet-class>
    </servlet> 
    <servlet-mapping>
        <servlet-name>gp</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>

public class Demo3Servlet extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            ServletContext context = this.getServletContext();
    
            String url = context.getInitParameter("url");
            resp.getWriter().print(url);
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req,resp);
        }
    }

*  请求转发

使用方法： getRequestDispatcher("/gp").forward(req,resp);

![在这里插入图片描述][2020112723433220.png_pic_center]

*  重定向
 *  读取资源文件

当然以上这些其实几乎都不用 context 了，之后都会被替代

## HttpServlet Response ##

web 服务器接收到客户端请求，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 HttpServletRrquest
 *  如果要给客户端响应一些信息，找 HttpServletResponse

1、简单分类

负责向浏览器发送数据的方法

负责向浏览器发送响应头的方法

**下载文件**  
1、要获取下载文件的路径  
2、下载的文件名是啥？  
3、设置想办法让浏览器能够支持下载我们所需要的东西  
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 流写入 buffer 缓冲区  
8、使用 OutputStream 将缓冲区的数据输出到客户端  
9、关闭流

<?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>gd5</servlet-name>
            <servlet-class>com.maze.servlet.FileServlet</servlet-class>
        </servlet>
        <servlet-mapping>
            <servlet-name>gd5</servlet-name>
            <url-pattern>/gd5</url-pattern>
        </servlet-mapping>
    </web-app>

package com.maze.servlet;
    
    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;
    import java.net.URLEncoder;
    
    /** * @Title: * @Package * @Description: 下载文件 * @author: maze * @date 2020/11/29下午 14:29 */
    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 ="F:\\localcode\\javaweb-02-serlvet\\Response\\target\\classes\\下载文件1.txt";
            System.out.println("下载的路径是"+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下载我们所需要的东西
            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 buffer 缓冲区
            // 使用 OutputStream 将缓冲区的数据输出到客户端
            while((len=in.read(buffer))>0){ 
                out.write(buffer,0,len);
            }
            //8.关闭流
            in.close();
            out.close();
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

示意图

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNzYzMzQ0_size_16_color_FFFFFF_t_70_pic_center 1]

服务器文件保存在  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNzYzMzQ0_size_16_color_FFFFFF_t_70]

## HttpServlet Request ##

验证码实现

<servlet>
        <servlet-name>ImageServlet</servlet-name>
        <servlet-class>com.maze.servlet.ImageServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ImageServlet</servlet-name>
        <url-pattern>/image</url-pattern>
    </servlet-mapping>

package com.maze.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;
    
    /** * @Title: * @Package * @Description: * @author: maze * @date 2020/11/29下午 15:34 */
    public class ImageServlet extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            // 浏览器每3s 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);
            Graphics2D g = (Graphics2D) image.getGraphics();
    
            //设置背景图片颜色
            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/jpg");
            //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
            boolean write = ImageIO.write(image,"jpg" ,resp.getOutputStream());
        }
        private String MakeNum(){ 
            Random random = new Random();
            String s = random.nextInt(999999)+"";
            StringBuffer sb = new StringBuffer();
            for (int i=0;i<7-s.length();++i){ 
                sb.append("0");
            }
            s = sb.toString() + s;
            return s;
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

## 实现重定向 ##

当 B 服务器收到 A 客户端的请求后，B 会通知 A 去访问另外一个资源C ，这个过程就叫 web 中的重定向

常见的常见：登录页面，注册页面等等

重写 doGet 方法中，

resp.sendRedirect("/img");

会被重定向到 /img 的路径中

其实底层做了两个步骤

resp.setHeader("Location","r/img");
    resp.setStatus(302);

> 重定向和转发的区别？

1.  页面都会实现跳转
2.  请求转发的时候 url 不会变化，重定向 url 会变化

小练习：登录页面的跳转

package com.maze.servlet;
    
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.io.IOException;
    
    /** * @Title: * @Package * @Description: * @author: maze * @date 2020/11/29下午 21:35 */
    public class RequestTest extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            System.out.println("进入了这个请求");
            String username = req.getParameter("username");
            String password = req.getParameter("password");
    
            System.out.println(username+":"+password);
            resp.sendRedirect("/Response_war/success.jsp");
        }
    
        @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>Title</title>
    </head>
    <body>
    <h1>你进入 success 这个页面</h1>
    
    </body>
    </html>

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

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNzYzMzQ0_size_16_color_FFFFFF_t_70 1]  
后台也正确输出了

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[2020112723433220.png_pic_center]: /images/20221120/7ea45b49149e475d868466f5f5da8e38.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNzYzMzQ0_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20221120/b6fb1721244a41f39cb0020c951f4236.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNzYzMzQ0_size_16_color_FFFFFF_t_70]: /images/20221120/06d61b8e6bdd4bbfa7c91fd00ab2a8e3.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNzYzMzQ0_size_16_color_FFFFFF_t_70 1]: /images/20221120/25beba6a692841d3b4b31bff3a821c90.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQzNzYzMzQ0_size_16_color_FFFFFF_t_70 2]: /images/20221120/c90e5fbf779f4f308c3f4c9794b70d71.png