050_Servlet详解

Servlet

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

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse response) throws ServletException, IOException {
        //响应的类型：html
        response.setContentType("text/html");
        //响应的编码
        response.setCharacterEncoding("utf-8");
        //获取响应的输出流
        PrintWriter out = response.getWriter();
        out.println("<html>");
        out.println("<head>");
        out.println("<title>Hello World!</title>");
        out.println("</head>");
        out.println("<body>");
        out.println("<h1>Hello World!</h1>");
        out.println("</body>");
        out.println("</html>");
    }
}

<?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.xml中配置web的核心应用-->
    <!--注册Servlet-->
    <servlet>
        <!--名称，可以随意写-->
        <servlet-name>helloServlet</servlet-name>
        <servlet-class>com.qing.servlet.HelloServlet</servlet-class>
    </servlet>
    <!--映射：一个Servlet对应一个mapping-->
    <servlet-mapping>
        <servlet-name>helloServlet</servlet-name>
        <!--请求路径，可以随意写-->
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>

Servlet简介

Servlet是sun公司开发动态web的一门技术
sun公司在这些API中提供一个接口：Servlet，如果想开发一个Servlet程序，只需要完成两个步骤
1. 编写一个类，实现Servlet接口
2. 把开发好的java类部署到web服务器中
我们把实现了Servlet接口的java程序叫做Servlet
Servlet接口sun公司有两个默认的实现类：HttpServlet和GenericServlet

HelloServlet

构建一个普通maven项目，删掉src目录，这个空的工程就是Maven主工程，可以在里面建立Module；

配置pom.xml，添加依赖；

<?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.qing</groupId>
    <artifactId>javaweb-03-servlet</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>4.0.1</version>
        </dependency>
    </dependencies>

</project>

在父工程中新建子模块Module；

关于父子工程的理解：父项目中的jar包子项目可以直接使用；
1. 父项目中会有
2. 子项目中会有

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

    <parent>
        <artifactId>javaweb-03-servlet</artifactId>
        <groupId>com.qing</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

Maven环境优化：
1. 修改web.xml为最新版本：修改子工程web.xml中的版本；
2. 将maven的结构搭建完整：子工程添加java和resources目录；

<?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>

编写一个Servlet程序；
1. 编写一个普通类
2. 实现Servlet接口，这里继承HttpServlet

package com.qing.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 {
        System.out.println("进入doGet方法");
        PrintWriter writer = resp.getWriter();//响应流
        writer.print("hello,servlet");
    }

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

在web.xml中编写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可以访问的路径

<?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>helloServlet</servlet-name>
        <servlet-class>com.qing.servlet.HelloServlet</servlet-class>
    </servlet>
    <!--Servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>helloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>

配置Tomcat

启动测试

Servlet原理

Servlet是由web服务器调用，web服务器收到浏览器请求后，会：

servlet-mapping Servlet请求路径

一个Servlet指定一个映射路径

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

一个Servlet指定多个映射路径

    <servlet-mapping>
        <servlet-name>helloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>helloServlet</servlet-name>
        <url-pattern>/hello1</url-pattern>
    </servlet-mapping>

一个Servlet指定通用映射路径

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

默认请求路径，会跳过index.jsp，直接进Servlet

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

指定后缀

<!--可以自定义后缀实现请求映射，注意：*前面不能加任何字符，否则会报错-->
<servlet-mapping>
  <servlet-name>helloServlet</servlet-name>
  <url-pattern>*.qing</url-pattern>
</servlet-mapping>

优先级问题：指定固定映射路径优先级最高，如果找不到就会走默认请求路径

<!--404-->
<servlet>
  <servlet-name>error</servlet-name>
  <servlet-class>com.qing.servlet.ErrorServlet</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>error</servlet-name>
  <url-pattern>/*</url-pattern>
</servlet-mapping>

ServletContext Servlet上下文

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

ServletContext的应用

共享数据：在一个Servlet设置的数据，可以在另一个Servlet获取到

package com.qing.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 HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("hello");
        //Servlet上下文
        ServletContext context = this.getServletContext();
        //设置数据
        String username = "清风";
        context.setAttribute("username", username);
    }

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

package com.qing.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 {
        System.out.println("get");
        ServletContext context = this.getServletContext();
        String username = (String) context.getAttribute("username");
        System.out.println(username);
    }

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

<?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.qing.servlet.HelloServlet</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.qing.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>get</servlet-name>
        <url-pattern>/get</url-pattern>
    </servlet-mapping>
</web-app>

获取初始化参数

<?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">

    <context-param>
        <param-name>url</param-name>
        <param-value>127.0.0.1</param-value>
    </context-param>

    <servlet>
        <servlet-name>get</servlet-name>
        <servlet-class>com.qing.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>get</servlet-name>
        <url-pattern>/get</url-pattern>
    </servlet-mapping>
</web-app>

package com.qing.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 {
        System.out.println("get");
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().println(url);
    }

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

请求转发

package com.qing.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 {
        System.out.println("get");
        ServletContext context = this.getServletContext();
        //转发到/hello
        context.getRequestDispatcher("/hello").forward(req, resp);
    }

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

读取Properties文件

username=root
password=123456

package com.qing.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 PropertiesServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取流，根路径为打包后target下的项目名称
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties properties = new Properties();
        properties.load(in);
        String user = properties.getProperty("username");
        String pwd = properties.getProperty("password");
        resp.getWriter().print(user + ":" + pwd);
    }

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

<?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>prop</servlet-name>
        <servlet-class>com.qing.servlet.PropertiesServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>prop</servlet-name>
        <url-pattern>/prop</url-pattern>
    </servlet-mapping>
</web-app>

HttpServletResponse

web服务器接收到浏览器的http请求，针对这个请求，分别创建一个代表请求的HttpServletRequest对象和一个代表响应的HttpServletResponse对象
- 如果要获取浏览器请求过来的参数：找HttpServletRequest
- 如果要给浏览器响应一下信息：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

void setCharacterEncoding(String var1);
void setContentLength(int var1);

响应的状态码

int SC_OK = 200;
int SC_CREATED = 201;

常见应用
- 向浏览器输出消息

下载文件

  - 获取下载文件的路径
  - 获取文件名
  - 设置浏览器支持下载
     - 文件下载的消息头和编码问题 [https://www.cnblogs.com/hackxiyu/p/9046247.html](https://www.cnblogs.com/hackxiyu/p/9046247.html)

response.setHeader("Content-Disposition", "attachment;filename="+new String(AREA_FILE.getBytes("gb2312"), "ISO-8859-1"));

  - 获取下载文件的输入流
  - 创建缓冲区
  - 获取OutputStream对象
  - 将输入流写入buffer缓冲区，使用输出流将缓冲区中的数据输出到客户端

package com.qing.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;

public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取下载文件的路径
        String realPath = this.getServletContext().getRealPath("/WEB-INF/classes/静.jpg");
        System.out.println("下载文件的路径为：" + realPath);
        //获取文件名
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        System.out.println("文件名为：" + fileName);
        //设置浏览器支持下载
        resp.setHeader("Content-Disposition", "attachment;filename=" + URLEncoder.encode(fileName, "UTF-8"));
        //获取下载文件的输入流
        FileInputStream fileInputStream = new FileInputStream(realPath);
        //创建缓冲区
        int len = 0;
        byte[] buffer = new byte[1024];
        //获取OutputStream对象
        ServletOutputStream outputStream = resp.getOutputStream();
        //将输入流写入buffer缓冲区，使用输出流将缓冲区中的数据输出到客户端
        while ((len=fileInputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, len);
        }
        fileInputStream.close();
        outputStream.close();
    }

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

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

验证码功能

package com.qing.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;

public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //设置浏览器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);
        //得到画笔
        Graphics2D g = (Graphics2D) image.getGraphics();
        //设置画笔颜色，画矩形背景
        g.setColor(Color.WHITE);
        g.fillRect(0,0,80,20);
        //设置画笔颜色和字体，画随机数
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeRandom(),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());
    }

    //生产随机数
    private String makeRandom() {
        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() + num;
        return num;
    }

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

<?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>imageServlet</servlet-name>
        <servlet-class>com.qing.servlet.ImageServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>imageServlet</servlet-name>
        <url-pattern>/image</url-pattern>
    </servlet-mapping>
</web-app>

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外一个web资源，这个过程叫重定向。

常见场景：用户登录

package com.qing.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 {
        /*
        重定向相当于
        resp.setHeader("Location","/resp/image");
        resp.setStatus(302);
         */
        //重定向
        resp.sendRedirect("/resp/image");
    }

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

<?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>imageServlet</servlet-name>
        <servlet-class>com.qing.servlet.ImageServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>imageServlet</servlet-name>
        <url-pattern>/image</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>redirectServlet</servlet-name>
        <servlet-class>com.qing.servlet.RedirectServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>redirectServlet</servlet-name>
        <url-pattern>/redirect</url-pattern>
    </servlet-mapping>
</web-app>

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不会发生变化状态码：307
- 重定向的时候，URL会发生变化状态码：302

登录demo

pom.xml添加jsp依赖：javax.servlet.jsp

<?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.qing</groupId>
    <artifactId>javaweb-03-servlet</artifactId>
    <packaging>pom</packaging>
    <version>1.0-SNAPSHOT</version>
    <modules>
        <module>servlet-01</module>
        <module>servlet-02</module>
        <module>response</module>
    </modules>

    <dependencies>
        <!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>4.0.1</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/javax.servlet.jsp/javax.servlet.jsp-api -->
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>javax.servlet.jsp-api</artifactId>
            <version>2.3.3</version>
        </dependency>
    </dependencies>

</project>

编辑登录页，${pageContext.request.contextPath}代表当前项目

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

编辑登录成功页面

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

处理登录请求，重定向到成功页面

package com.qing.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 LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进入登录请求");
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username + ":" + password);
        //重定向
        resp.sendRedirect("/resp/success.jsp");
    }

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

在web.xml中添加Servlet映射

<?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>loginServlet</servlet-name>
        <servlet-class>com.qing.servlet.LoginServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>loginServlet</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>
</web-app>

HttpServletRequest

HttpServletRequest代表客户端的请求，用户通过http协议访问服务器，http请求中的所有信息都会被封装到HttpServletRequest，通过HttpServletRequest的方法，可以获得客户端的所有信息。

获取浏览器传递的参数

String contextPath = req.getContextPath();//项目路径
String servletPath = req.getServletPath();//Servlet路径
String requestURI = req.getRequestURI();//请求路径
String name = req.getParameter("name");
String[] names = req.getParameterValues("name");

请求转发

//请求转发
//这里的/代表当前的web应用，即contextPath
req.getRequestDispatcher("/success.jsp").forward(req,resp);

本文链接：https://www.cnblogs.com/wl3pb/p/14546863.html

050_Servlet详解

Servlet

Servlet简介

HelloServlet

Servlet原理

servlet-mapping Servlet请求路径

ServletContext Servlet上下文

ServletContext的应用

共享数据：在一个Servlet设置的数据，可以在另一个Servlet获取到

获取初始化参数

请求转发

读取Properties文件

HttpServletResponse

下载文件

验证码功能

重定向

重定向和转发的区别

登录demo

HttpServletRequest

获取浏览器传递的参数

请求转发

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