HttpServletRequest类的介绍以及Servlet-Request常用API、获取请求参数、请求转发

a）HttpServletRequest类有什么作用？

每次只要有请求进入Tomcat服务器，Tomcat服务器就会把请求过来的HTTP协议信息解析好封装到Request对象中。
然后传递到service方法（doGet和doPost）中给我们使用。我们可以通过HttpServletRequest对象，获取到所有请求的信息。

b）HttpServletRequest类的常用方法

RequestAPIServlet.java

package com.atguigu.servlet;

import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

import java.io.IOException;

/** * Create By LiFang on 2020/7/14. */
public class RequestAPIServlet extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
// super.doGet(req, resp);
        // i.getRequestURL() 获取请求的资源路径
        System.out.println("URI => " + req.getRequestURI());
        // ii.getRequestURL 获取请求的统一资源定位符（绝对路径）
        System.out.println("URL => " + req.getRequestURL());
        // iii.getRemoteHost() 获取客户端的ip地址
        // iv.getHeader() 获取请求头
        // vii.getMethod() 获取请求的方式GET或POST
    }
}

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">
    <servlet>
        <servlet-name>RequestAPIServlet</servlet-name>
        <servlet-class>com.atguigu.servlet.RequestAPIServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>RequestAPIServlet</servlet-name>
        <url-pattern>/requestAPIServlet</url-pattern>
    </servlet-mapping>
</web-app>

控制台结果:

浏览器客户端：
对程序进行热部署，直接刷新浏览器后，得到结果 :

------------------------------------------------------------分割线-----------------------------------------------------------------
控制台运行结果：

此时，你就会发现客户端 ip地址为什么不是 127.0.0.1 ？
别慌，看这篇文章： https://www.cnblogs.com/m3Lee/p/3808002.html 你会得到答案。

继续获取：

结果：
服务器结果：

与浏览器（客户端）获取的是一样的：

在浏览器敲回车，得到GET请求，

控制台信息：

c)如何获取请求参数

ParameterServlet.java

package com.atguigu.servlet;

import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

import java.io.IOException;

/** * Create By LiFang on 2020/7/14. */
public class ParameterServlet extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
// super.doGet(req, resp);
		//设置请求体的字符集为UTF-8,从而解决post请求的中文乱码问题
		 //也要在获取请求参数之前调用才有效
        req.setCharacterEncoding("UTF-8");
        System.out.println("--------Get-----------");
        //获取请求参数
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobby = req.getParameterValues("hobby");

        System.out.println("用户名：" + username);
        System.out.println("密码：" + password);
        System.out.println("兴趣爱好：" + hobby);
    }
     @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
// super.doPost(req, resp);

        //设置请求体的字符集为UTF-8,从而解决post请求的中文乱码问题
         //也要在获取请求参数之前调用才有效
        req.setCharacterEncoding("UTF-8");
        System.out.println("---------Post-----------");
        //获取请求参数
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobby = req.getParameterValues("hobby");

        System.out.println("用户名：" + username);
        System.out.println("密码：" + password);
        System.out.println("兴趣爱好：" + Arrays.asList(hobby));
    }
}

form.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="http://localhost:8080/04_servlet/parameterServlet" method="get">
    用户名：<input type="text" name="username"><br/>
    密码：<input type="password" name="password"><br/>
    兴趣爱好：<input type="checkbox" name="hobby" value="cpp">C++
    <input type="checkbox" name="hobby" value="java">Java
    <input type="checkbox" name="hobby" value="js">Js<br/>
    <input type="submit">
</form>


</body>
</html>

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">
    <servlet>
        <servlet-name>RequestAPIServlet</servlet-name>
        <servlet-class>com.atguigu.servlet.RequestAPIServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>RequestAPIServlet</servlet-name>
        <url-pattern>/requestAPIServlet</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>ParameterServlet</servlet-name>
        <servlet-class>com.atguigu.servlet.ParameterServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ParameterServlet</servlet-name>
        <url-pattern>/parameterServlet</url-pattern>
    </servlet-mapping>