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javaweb

6、Servlet

6.1、Servlet简介

• Servlet就是sun公司开发动态web的一门技术
• sun公司在这些API中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类部署到web服务器中

把实现了Servlet接口的java程序叫做,Servlet

6.2、HelloServlet

Servlet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建立module;这个空的工程就是Maven主工程

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</artifactId>
        <groupId>org.example</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

父项目中的java子项目可以直接用

3、maven环境优化

​ 1.修改web.xml为最新的

​ 2.将maven的结构搭建完整

4、编写一个Servlet程序

​ 1.编写一个普通类

​ 2.实现Servlet接口，这里我们直接继承HttpServlet

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.println("hello,servlet");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

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服务器中注册我们写的servlet,还要给他一个浏览器能够访问的路径；

    <!--注册servlet-->
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.lc.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>

6、配置Tomcat

注意：配置项目发布的路径就可以了

7、启动测试

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后，会：

6.4、Mapping问题

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>hello</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>

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

<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/*</url-pattern>
</servlet-mapping>

5.指定一个后缀或者前缀等等....

<!--注意点：*.前面不能加项目映射路径-->
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>*.liang</url-pattern>
</servlet-mapping>

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容器在启动的时候，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，可以在另外一个Sercvlet中拿到；

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
        ServletContext servletContext = this.getServletContext();
        String  username="梁聪";   //数据
        servletContext.setAttribute("username",username);//将一个数据保存在了ServletContext中，名字为username。值username

    }
}

public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        String username = (String) servletContext.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 {

    }
}

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

2、获取初始化参数

  <!--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>

 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        String url = servletContext.getInitParameter("url");
        resp.getWriter().print(url);
    }

3、请求转发

 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        System.out.println("进入了ServletDemo02");
        //RequestDispatcher requestDispatcher = servletContext.getRequestDispatcher("/gp");//转发的请求路径
        //requestDispatcher.forward(req,resp);//调用forward实现请求转发；
        servletContext.getRequestDispatcher("/gp").forward(req,resp);
    }

4、读取资源文件

Properties

• 在java目录下新建properties
• 在resource目录下新建properties

如果遇到配置文件无法导出或者生效，解决方案：

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

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

public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/lc/servlet/db.properties");
        Properties prop = new Properties();
        prop.load(is);
        String user = prop.getProperty("username");
        String paw = prop.getProperty("password");
        resp.getWriter().print(user+":"+paw);

    }

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

    }
}

访问测试即可。

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

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、下载文件

1、向浏览器输出消息

2、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流写入到buffer缓冲区

​ 8.使用OutputStream将缓冲区中的数据输出到客户端

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//     	1.要获取下载文件的路径
        String realPath = "D:\\IdeaProjects\\java-web-servlet\\src\\main\\resources\\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支持（Content-Disposition）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);
        }
        in.close();
        out.close();

    }

3、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

package com.lc.servlet;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
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;

@WebServlet("/image")
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(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.BLUE);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.BLACK);
        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 < 7-num.length() ; i++) {
            sb.append("0");
        }
        String s = sb.toString() + num;
        return num;
    }

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

    }
}

4、实现重定向

常见场景：

• 用户登录

void sendRedirect(String var1) throws IOException;

测试：

@WebServlet("/red")
public class RedirectServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        /*
        resp.setHeader("location","/r/image");
        resp.setStatus(302);
        */
        String name=req.getContextPath();
        System.out.println(name);
        resp.sendRedirect(name+"/image")//重定向
    }

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

• 请求转发的时候，url不会发生变化; 307
• 重定向的时候，url地址栏会发生变化; 302

<html>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<body>
<h2>Hello World!</h2>
<%--这里提交的路径，需要找到项目的路径--%>
<form action="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>

@WebServlet("/login")
public class RequestTest extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username+":"+password);
        resp.sendRedirect(req.getContextPath()+"/success.jsp");
    }

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>Success</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的方法，获得客户端的所有信息。

1、获取前端传递的参数

<%@ 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">
        <form action="login" method="get">
            用户名：<input type="text" name="username"> <br>
            密码：<input type="password" name="password"> <br>
            爱好：
            <input type="checkbox" name="hobby" value="女孩">女孩
            <input type="checkbox" name="hobby" value="唱歌">唱歌
            <input type="checkbox" name="hobby" value="代码">代码
            <input type="checkbox" name="hobby" value="电影">电影
            <br>

            <input type="submit">

        </form>
</div>
</body>
</html>

@WebServlet("/login")
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("hobby");
        System.out.println("-----------------------------------------------------------");
        System.out.println(username);
        System.out.println(password);
        System.out.println(Arrays.toString(hobbies));
        System.out.println("-----------------------------------------------------------");
        //通过请求转发
        req.getRequestDispatcher("success.jsp").forward(req,resp);
    }

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

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