来谈谈Servlet~~

灰太狼 2023-02-23 05:23 11阅读 0赞

### 目录 ###

*   *   *  1、Servlet简介
         *  2、第一个Servlet程序
         *  3、Servlet原理
         *  4、servlet-mapping的配置
         *  5、关于ServletContext
         *   *  1、共享数据
             *  2、获取初始化参数
             *  3、请求转发
             *  4、读取资源文件
         *  6、HttpServletResponse
         *   *  1、简单分类
             *  2、常见应用
         *  7、HttpServletRequest

### 1、Servlet简介 ###

*  Servlet就是sun公司开发动态web的一门技术
 *  Java Servlet 是运行在 Web 服务器或应用服务器上的程序，它是作为来自 Web 浏览器或其他 HTTP 客户端的**请求**和 HTTP 服务器上的**数据库或应用程序**之间的**中间层**。

![image-20200718154542534][]

*  Sun在这些API提供了一个接口叫做：Servlet，如果你想开发一个Servlet程序，只需要完成两个小步骤：
    
     *  编写一个类，实现Servlet接口
     *  把开发好的Java类部署web服务器中

**把实现了Servlet接口的Java程序叫做 Servlet**

--------------------

### 2、第一个Servlet程序 ###

**Servlet接口SUN公司定义了两个默认实现类，分别为：GenericServlet、HttpServlet。**

![image-20200718153837930][]

1.  构建一个maven项目，删掉里面的src目录，以后就在这里面建立Moudel；这个空的工程就是Maven的主工程
    
    --------------------
2.  关于Maven父子工程的理解
    
    ![image-20200718153854227][]
    
    父项目中会有
    
        <modules>
            <module>servlet_01</module>
        </modules>
    
    子项目中会有
    
        <parent>
            <artifactId>HelloServlet</artifactId>
            <groupId>org.example</groupId>
            <version>1.0-SNAPSHOT</version>
        </parent>
    
    **父项目中的jar包子项目可以直接使用，但子项目中的jar包父项目不能直接使用**
    
    --------------------
3.  Maven环境优化
    
    1.  修改子工程web.xml为最新的，默认版本太老，我们替换为webapp4.0版本和tomcat一致!
        
        ![format_png][]
        
            <?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">
            
            </web-app>
    2.  将maven的结构搭建完整（添加java目录和resource目录，并做相关标记）
    
    ![image-20200718153943411][]
    
    --------------------
4.  编写一个Servlet程序
    
     *  编写一个普通类实现Servlet接口，这里我们直接继承HttpServlet
    
    ![image-20200718154002395][]
    
        package com.zsr.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.io.PrintWriter;
        
        public class HelloServlet extends HttpServlet { 
            //由于get或者post只是请求实现的不同方式，可以相互调用，业务逻辑都一样
            @Override
            protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
                PrintWriter writer = resp.getWriter();//响应流
                writer.print("Hello,Servlet");
            }
        
            @Override
            protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
                super.doPost(req, resp);
            }
        }
5.  编写Servlet映射
    
     *  为什么需要映射？？
        
         *  因为我们写的是Java程序，但是要通过浏览器访问，而浏览器需要连接web服务器，所以我们需要在web服务器中注册我们写的Servlet，还需要给他一个浏览器能够访问的路径
    
        <?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>hello</servlet-name>
                <servlet-class>com.zsr.servlet.HelloServlet</servlet-class>
            </servlet>
            
            <servlet-mapping>
                <servlet-name>hello</servlet-name>
                <url-pattern>/hello</url-pattern>
            </servlet-mapping>
        </web-app>
6.  配置Tomcat
7.  启动测试

--------------------

### 3、Servlet原理 ###

![image-20200718154028691][]

### 4、servlet-mapping的配置 ###

1.  一个Servlet可以指定一个映射路径
    
        <servlet-mapping>
            <servlet-name>hello</servlet-name>
            <url-pattern>/hello</url-pattern>
        </servlet-mapping>
2.  一个Servlet可以指定多个映射路径
    
        <servlet-mapping>
            <servlet-name>hello</servlet-name>
            <url-pattern>/hello1</url-pattern>
        </servlet-mapping>
        <servlet-mapping>
            <servlet-name>hello</servlet-name>
            <url-pattern>/hello2</url-pattern>
        </servlet-mapping>
        <servlet-mapping>
            <servlet-name>hello</servlet-name>
            <url-pattern>/hello3</url-pattern>
        </servlet-mapping>
        <servlet-mapping>
            <servlet-name>hello</servlet-name>
            <url-pattern>/hello4</url-pattern>
        </servlet-mapping>
3.  一个Servlet可以指定通用映射路径
    
        <servlet-mapping>
            <servlet-name>hello</servlet-name>
            <url-pattern>/hello/*</url-pattern>
        </servlet-mapping>
4.  默认请求路径（尽量不用）
    
        
        <servlet-mapping>
            <servlet-name>hello</servlet-name>
            <url-pattern>/*</url-pattern>
        </servlet-mapping>
5.  指定一些后缀或者前缀登等…
    
        
        <servlet-mapping>
            <servlet-name>hello</servlet-name>
            <url-pattern>*.zsr</url-pattern>
        </servlet-mapping>
6.  优先级问题
    
    指定了固有映射路径的优先级最高，如果找不到就会走默认的处理请求

--------------------

### 5、关于ServletContext ###

web容器在启动的时候，它会为每一个web程序创建一个对应的ServletContext对象，它代表了当前的web应用；

ServletContext实例是通过 \*\*getServletContext()\*\*方法获得

*  **是一个域对象**
    
     *  域对象是服务器在内存上创建的存储空间，用于在不同动态资源（servlet）之间传递与共享数据。
     *  凡是域对象都有如下3个方法：
        
            setAttribute(name,value);//name是String类型，value是Object类型；往域对象里面添加数据，添加时以key-value形式添加
            
            getAttribute(name);//根据指定的key读取域对象里面的数据
            
            removeAttribute(name);//根据指定的key从域对象里面删除数据
 *  **可以读取全局配置参数**
    
     *      getServletContext().getInitParameter(name);//根据指定的参数名获取参数值
            
            getServletContext().getInitParameterNames();//获取所有参数名称列表
 *  **可以搜索当前工程目录下面的资源文件**
    
     *      getServletContext().getRealPath(path);//根据相对路径获取服务器上资源的绝对路径
            
            getServletContext().getResourceAsStream(path);//根据相对路径获取服务器上资源的输入字节流
 *  **可以获取当前工程名字**
    
     *      getServletContext().getContextPath();//获取当前工程名字

--------------------

#### 1、共享数据 ####

我在这个Servlet中保存的数据，可以在另外一个Servlet中拿到；

*  **放置数据的类 HellowServlet：**

public class HelloServlet extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
    // this.getInitParameter() 初始化参数
    // this.getServletConfig() Servlet配置
    // this.getServletContext() Servlet上下文参数
            ServletContext context = this.getServletContext();
            String name = "zsr";
            context.setAttribute("username", name);//将一个数据放在了ServletContext中,名字为username,值为name
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            super.doPost(req, resp);
        }
    }

*  **读取数据的类 GetServlet：**

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 { 
            super.doPost(req, resp);
        }
    }

*  **配置web.xml，对HelloServlet和GetServlet添加对应的注册和请求路径**

<servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.zsr.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    
    
    <servlet>
        <servlet-name>getusername</servlet-name>
        <servlet-class>com.zsr.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getusername</servlet-name>
        <url-pattern>/username</url-pattern>
    </servlet-mapping>

*  **测试访问结果：**

打开浏览器，先访问**localhost:8080/s1/hello**，在访问**localhost:8080/s1/username**，就可以获取名字

![image-20200718154046894][]

--------------------

#### 2、获取初始化参数 ####

*  **在web.xml里配置以下web应用的初始化参数**

<context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

*  **获取初始化参数的类Servlet03：**

public class Servlet03 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);
        }
    }

*  **配置web.xml，给Servlet03类添加注册和请求路径**

<servlet>
        <servlet-name>initial</servlet-name>
        <servlet-class>com.zsr.servlet.Servlet03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>initial</servlet-name>
        <url-pattern>/initial</url-pattern>
    </servlet-mapping>

*  **测试访问结果：**

打开浏览器，访问**localhost:8080/s1/initial**，获取到初始参数

![image-20200718154100298][]

--------------------

#### 3、请求转发 ####

*  **请求转发的类Servlet04**

public class Servlet04 extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            ServletContext context = this.getServletContext();
            System.out.println("进入了s4");
    // RequestDispatcher requestDispatcher = context.getRequestDispatcher("/initial");//转发请求路径
    // requestDispatcher.forward(req, resp);//调用forward方法请求转发
            context.getRequestDispatcher("/initial").forward(req, resp);
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

*  **配置web.xml，对Servlet04添加对应的注册和请求路径**

<servlet>
        <servlet-name>dispatch</servlet-name>
        <servlet-class>com.zsr.servlet.Servlet04</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatch</servlet-name>
        <url-pattern>/dispatch</url-pattern>
    </servlet-mapping>

*  **测试访问结果：**

打开浏览器，访问**localhost:8080/s1/dispatch**，显示与**localhost:8080/s1/initial**同样的页面，实现了请求与转发（**路径不用改变，重定向的路径需要改变**！！！）

![image-20200718154112104][]

--------------------

#### 4、读取资源文件 ####

Properties

*  在java目录下新建properties
 *  在resources目录下新建properties

发现：都被打包到了同一个路径下：classes，我们俗称这个路径为类路径（classpath）

![image-20200718154125679][]

思路：需要一个文件流

**db.properties/db1.properties:**

username=zsr
    password=200024

public class Servlet05 extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            //第一个/代表当前web项目
            InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/zsr/servlet/db1.properties");
            Properties prop = new Properties();
            prop.load(is);
            String user = prop.getProperty("username");
            String pwd = prop.getProperty("password");
            resp.getWriter().print(user + "：" + pwd);
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

添加对应的注册和请求路径，访问测试即可；

--------------------

### 6、HttpServletResponse ###

web服务区接收到客户端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；

*  我们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

#### 1、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：

void setCharacterEncoding(String var1);
    
        void setContentLength(int var1);
    
        void setContentLengthLong(long var1);
    
        void setContentType(String var1);
    
        void setDateHeader(String var1, long var2);
    
        void addDateHeader(String var1, long var2);
    
        void setHeader(String var1, String var2);
    
        void addHeader(String var1, String var2);
    
        void setIntHeader(String var1, int var2);
    
        void addIntHeader(String var1, int var2);

响应的状态码：

int SC_CONTINUE = 100;
        int SC_SWITCHING_PROTOCOLS = 101;
        int SC_OK = 200;
        int SC_CREATED = 201;
        int SC_ACCEPTED = 202;
        int SC_NON_AUTHORITATIVE_INFORMATION = 203;
        int SC_NO_CONTENT = 204;
        int SC_RESET_CONTENT = 205;
        int SC_PARTIAL_CONTENT = 206;
        int SC_MULTIPLE_CHOICES = 300;
        int SC_MOVED_PERMANENTLY = 301;
        int SC_MOVED_TEMPORARILY = 302;
        int SC_FOUND = 302;
        int SC_SEE_OTHER = 303;
        int SC_NOT_MODIFIED = 304;
        int SC_USE_PROXY = 305;
        int SC_TEMPORARY_REDIRECT = 307;
        int SC_BAD_REQUEST = 400;
        int SC_UNAUTHORIZED = 401;
        int SC_PAYMENT_REQUIRED = 402;
        int SC_FORBIDDEN = 403;
        int SC_NOT_FOUND = 404;
        int SC_METHOD_NOT_ALLOWED = 405;
        int SC_NOT_ACCEPTABLE = 406;
        int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
        int SC_REQUEST_TIMEOUT = 408;
        int SC_CONFLICT = 409;
        int SC_GONE = 410;
        int SC_LENGTH_REQUIRED = 411;
        int SC_PRECONDITION_FAILED = 412;
        int SC_REQUEST_ENTITY_TOO_LARGE = 413;
        int SC_REQUEST_URI_TOO_LONG = 414;
        int SC_UNSUPPORTED_MEDIA_TYPE = 415;
        int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
        int SC_EXPECTATION_FAILED = 417;
        int SC_INTERNAL_SERVER_ERROR = 500;
        int SC_NOT_IMPLEMENTED = 501;
        int SC_BAD_GATEWAY = 502;
        int SC_SERVICE_UNAVAILABLE = 503;
        int SC_GATEWAY_TIMEOUT = 504;
        int SC_HTTP_VERSION_NOT_SUPPORTED = 505;

#### 2、常见应用 ####

1.  **向浏览器输出消息**
2.  **下载文件**
    
    1.  获取文件下载的路径
    2.  获取下载的文件名
    3.  设置浏览器支持我们所下载的东西
    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写出到缓冲区
    8.  使用OutputStream将缓冲区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、获取下载文件的路径
            String path = "D:/学习/IDEA project/HelloServlet/response/src/main/resources/钟嗣儒.jpg";
            System.out.println("下载的文件路径为：" + path);
            //2、获取下载的文件名
            String fileName = path.substring(path.lastIndexOf("\\") + 1);
            //3、设置让浏览器支持下载我们所需要的东西，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、获取下载文件到输入流
            FileInputStream in = new FileInputStream(path);
            //5、创建缓冲区
            int len = 0;
            byte b[] = 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(b)) != -1) { 
                out.write(b, 0, len);
            }
            out.close();
            in.close();
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

1.  **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，生成一个图片

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");
    
            //把图片交给浏览器
            ImageIO.write(image, "jpg", resp.getOutputStream());
        }
    
        public String makeNum() { 
            Random random = new Random();
            String num = random.nextInt(9999999) + "";
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < 7 - num.length(); i++) { 
                sb.append("0");
            }
            num = sb.toString() + sb;
            return num;
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

1.  **实现重定向**

一个web资源收到客户端请求，他会通知客户端会访问另外一个web资源，这个过程叫做重定向

常见场景：

*  用户登录

public void sendRedirect(String location) throws IOException;

*  测试代码

public class RedirectServlet extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            resp.sendRedirect("/imageServlet");//重定向
            /** * 等价于两步 * resp.setHeader("Location", "/imageServlet"); * resp.setStatus(302); */
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

**面试题：请你聊聊重定向和转发的区别？**

*  相同点：
    
     *  页面都会跳转
 *  不同点
    
     *  请求转发的时候，url不会发生变化
     *  重定向的时候，url地址栏会发生变化

**案例：表单提交，获取用户名和密码**

**index.jsp:**

<html>
    <body>
    <h2>Hello World!</h2>
    <%--这里提交的路径，需要找到项目的路径--%>
    <%--${pageContext.request.contextPath}代表当前项目--%>
    <form action="${pageContext.request.contextPath}/login" method="get">
        用户名: <input type="text" name="username"> <br>
        密码: <input type="password" name="password">
        <input type="submit">
    </form>
    </body>
    </html>

**success.jsp:**

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

**RequestTest.java:**

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404
            resp.sendRedirect("/success.jsp");
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

**在web.xml添加对应的注册**

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

**测试：**

![image-20200718154147696][]

![image-20200718154158251][]

--------------------

### 7、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中，通过这个HttpServletRequest的方法，可以获取客户端的所有信息

**获取前端传递的参数，并且请求转发**

![image-20200718154208999][]

**index.jsp:（通过Post方式请求）**

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
    <html>
    <head>
        <title>Title</title>
    </head>
    <body>
    <h1>登录</h1>
    <div style="text-align: center">
        <%--这里表单表示的意思:以post的方式提交表单,提交到我们的login请求--%>
        <form action="${pageContext.request.contextPath}/login" method="post">
            用户名:<input type="text" name="username">
            登录:<input type="password" name="password">
            爱好:
            <input type="checkbox" name="hobbies" value="Coding">Coding
            <input type="checkbox" name="hobbies" value="Sing">Sing
            <input type="checkbox" name="hobbies" value="Playing">Playing
            <input type="checkbox" name="hobbies" value="Guitar">Guitar
            <input type="submit">
        </form>
    </div>
    </body>
    </html>

**重定向页面success.jsp:**

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
    <html>
    <head>
        <title>你成功啦！</title>
    </head>
    <body>
    <h1>SUCCESS~</h1>
    </body>
    </html>

**获取前端传递的参数，并且请求转发的类：LoginServlet**

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 hobbies[] = req.getParameterValues("hobbies");
            System.out.println(username);
            System.out.println(password);
            System.out.println(Arrays.toString(hobbies));
            //通过请求转发
            req.getRequestDispatcher("/success.jsp").forward(req, resp);
        }
    
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doGet(req, resp);
        }
    }

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[image-20200718153854227]: https://gitee.com/zhong_siru/images/raw/master/img/20200718153854.png
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[image-20200718154125679]: https://gitee.com/zhong_siru/images/raw/master/img/20200718154125.png
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