JavaWeb【Servlet】

Servlet

6.1、Servlet简介

Servlet就是sun公司开发动态web的一门技术

Sun在这些API中提供的一个接口叫做：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类部署到web服务器中

把实现了Servlet接口的java程序叫做，Servlet

6.2、HelloServlet

Serlvet接口Sun公司有两个默认的实现类：HttpServlet，GenericServlet

1.构建一个普通的Maven项目，删掉里面的src目录，以后我们的学习就在这个项目里面建立Modle，这个空的工程就是Maven主工程

主Model的pop.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.jiang</groupId>
    <artifactId>javaweb-02-servlet</artifactId>
    <packaging>pom</packaging>
    <version>1.0-SNAPSHOT</version>
    <modules>
        <module>servlet-01</module>
    </modules>

    <dependencies>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>4.0.1</version>
        </dependency>
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>jsp-api</artifactId>
            <version>2.2</version>
        </dependency>
    </dependencies>


</project>

servlet-01中的pom.xml

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <parent>
    <groupId>com.jiang</groupId>
    <artifactId>javaweb-02-servlet</artifactId>
    <version>1.0-SNAPSHOT</version>
  </parent>
  <modelVersion>4.0.0</modelVersion>
  <artifactId>servlet-01</artifactId>
  <packaging>war</packaging>

  <name>servlet-01 Maven Webapp</name>
  <!-- FIXME change it to the project's website -->


  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.7</maven.compiler.source>
    <maven.compiler.target>1.7</maven.compiler.target>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.11</version>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <finalName>servlet-01</finalName>
    <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
      <plugins>
        <plugin>
          <artifactId>maven-clean-plugin</artifactId>
          <version>3.1.0</version>
        </plugin>
        <!-- see http://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_war_packaging -->
        <plugin>
          <artifactId>maven-resources-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-compiler-plugin</artifactId>
          <version>3.8.0</version>
        </plugin>
        <plugin>
          <artifactId>maven-surefire-plugin</artifactId>
          <version>2.22.1</version>
        </plugin>
        <plugin>
          <artifactId>maven-war-plugin</artifactId>
          <version>3.2.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-install-plugin</artifactId>
          <version>2.5.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-deploy-plugin</artifactId>
          <version>2.8.2</version>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
</project>

2.关于Maven父子工程的理解

父项目中会有

<modules>
    <module>servlet-01</module>
</modules>

子项目会有

<parent>
    <artifactId>javaweb-02-servlet</artifatId>
    <groupId>com.jiangyou</groupId>
    <version>1.0-SNAPSHOT</version>
</parent>

父项目中的java子项目可以直接使用

son extends father

3.Maven环境优化

1.修改web.xml为最新的

2.将maven的结构搭建完毕

4.编写一个servlet程序

1.编写一个普通类

2.实现Servlet接口，直接继承HttpServlet

package com.jiang.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.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的方式，可以相互调用，业务逻辑都一样

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

        ServletOutputStream outputStream = resp.getOutputStream();
        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的映射

为什么需要映射：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">
    
    <!--注册Servlet-->
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.jiang.servlet.HelloServlet</servlet-class>
    </servlet>
    
    <!--Servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

</web-app>

6.配置Tomcat

注意：配置项目发布的路径就可以

7.启动测试，OK!

6.3、Servlet原理

Servlet是由Web服务器调用，web服务器在收到浏览器请求之后，会：

6.4、Mapping问题

1.一个Servlet可以指定一个映射路径

2.一个Servlet可以指定多个映射路径

3.一个Servlet可以指定通用映射路径

4.默认请求路径

5.指定一些后缀或者前缀等等

6.优先级问题

指定了固有的映射路径优先级最高，如果找不到就会走默认的处理请求

6.5、ServletContext

web容器在祁东的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用

1、共享数据

我在这个Servlet中保存的数据，可以再另一个servlet中拿到

GetServlet.java

package com.jiang.servlet;

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.io.IOException;

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);
    }
}

HelloServlet.java

package com.jiang.servlet;

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.io.IOException;

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

        ServletContext context = this.getServletContext();

        String username="酱油";
        context.setAttribute("username",username);

    }

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

web.xml

<?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.jiang.servlet.HellowServlet</servlet-class>
    </servlet>
    <servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>get</servlet-name>
        <servlet-class>com.jiang.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>get</servlet-name>
        <url-pattern>/get</url-pattern>
    </servlet-mapping>
    
    
</web-app>

需要配置镜像路径

测试效果

2、获取初始化参数

获取url

 <!--配置一些web应用初始化参数-->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

ServletDemo03.java

package com.jiang.servlet;

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.io.IOException;

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

        ServletContext context = this.getServletContext();

        String username="酱油";
        context.setAttribute("username",username);

    }

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

3、请求转发

package com.jiang.servlet;

import javax.servlet.RequestDispatcher;
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.io.IOException;

public class ServletDemo04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//转发请求的路径
requestDispatcher.forward(req,resp);//调用forword实现请求的转发
    }

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

    }
}

<servlet>
        <servlet-name>s4</servlet-name>
        <servlet-class>com.jiang.servlet.ServletDemo04</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>s4</servlet-name>
        <url-pattern>/s4</url-pattern>
    </servlet-mapping>

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classpath;

思路：需要一个文件流

<!--在Build中配置resources，来防止我们资源导出失败的问题-->
  <build>
    <resources>
      <resource>
        <directory>src/main/java</directory>
        <includes>
          <include>**/*.properties</include>
          <include>**/*.xml</include>
        </includes>
        <filtering>true</filtering>
      </resource>
    </resources>
  </build>

username=jiangyou
password=123456

 <servlet>
        <servlet-name>s5</servlet-name>
        <servlet-class>com.jiang.servlet.ServletDemo05</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>s5</servlet-name>
        <url-pattern>/s5</url-pattern>
    </servlet-mapping>

package com.jiang.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.InputStream;
import java.util.Properties;

public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.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 {
        super.doPost(req, resp);
    }
}

6.6、HttpServletResponse

web服务器接收到客户端的http请求，针对这个请求，分别简历一个代表请求的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

1、简单分类

负责向浏览器发送数据的方法

ServletOutputSteam 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

2、常见应用

1.向浏览器输出消息

2.下载文件

要获取下载的文件路径
下载的文件名是啥
设置想办法让浏览器能够下载我们需要的东西
获取下载文件的输入流
创建缓冲区
获取OutputStream对象
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

package com.jiang.response;

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 doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        1.要获取下载的文件路径
        String realPath = "F:\\Javaweb_Test\\javaweb-02-servlet\\response\\target\\classes\\1.png";
        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下载我们需要的东西
        resp.setHeader("Content-Disposition","attachment;filename="+fileName);
//        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
        while((len=in.read(buffer))>0){
            out.write(buffer,0,len);
        }
//        8.使用OutputStream将缓冲区中的数据输出到客户端
        in.close();
        out.close();
    }

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

3、验证码功能

验证码怎么来的？

前端实现

后端实现，需要用到java的图片类，生成一个图片

ImageServlet.java

package com.jiang.response;

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.BufferedReader;
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3秒刷新一次
        resp.setHeader("refresh","3");
        
        //在内存中创建一个图片
        BufferedImage Image = new BufferedImage(100,20, 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(0,0,10,20);
        //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());
    }

    //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 < 6-num.length();i++) {
            sb.append("0");
        }
        num=sb.toString()+num;
        return num;


    }

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

测试效果

4、实现重定向

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

常见场景：

用户登录

 void sendRedirect(String var1) throws IOException;

package com.jiang.response;

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","/response_war/is");
       resp.setStatus(302);
       */
       resp.sendRedirect("/response_war/is");
    }

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

面试题：请你聊聊重定向和转发的区别？

相同点：

页面都会实现跳转

不同点：

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

package com.jiang.response;

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 {
        System.out.println("进入这个请求");
        //处理请求
        String username=req.getParameter("username");
        String password=req.getParameter("password");
        System.out.println(username+" "+password);
        resp.sendRedirect("/r/success.jsp");
    }

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

<%--
  Created by IntelliJ IDEA.
  User: Administrator
  Date: 2022/3/29 0029
  Time: 10:56
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>恭喜您进入到这个页面</h1>

</body>
</html>

6.7、HttpServletRequest

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方法，获取客户端的所有信息

https://www.bilibili.com/video/BV12J411M7Sj?p=16