什么是请求的转发?
请求转发是指，服务器收到请求后，从一次资源跳转到另一个资源的操作叫请求转发。

可以访问当前服务器内受保护的内容（WEB-INF），不能访问其他远程服务器

package top.qaqaq.P149;

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

import java.io.IOException;

/**
 * @author RichieZhang
 * @create 2022-12-05 下午 5:31
 */
public class Servlet1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        super.doGet(req, resp);

        /*
        viii. setAttribute(key, value);     设置域数据
        ix. getAttribute(key);              获取域数据
        x. getRequestDispatcher()           获取请求转发对象
         */

        //获取请求的参数（办事的材料）查看
        String username = req.getParameter("username");
        System.out.println("在Servlet1（柜台1）中查看参数（材料）：" + username);

        // 给 材料 盖一个章，并传递到Servlet2（柜台2）去查看
        req.setAttribute("key1","柜台1的章");

        // 问路：Servlet2（柜台 2）怎么走
        /*
        请求转发必须要以斜杠打头，/ 斜杠表示地址为：http://ip:port/工程名/ ，映射到IDEA代码的web目录
         */
        RequestDispatcher requestDispatcher = req.getRequestDispatcher("/servlet2");
//        RequestDispatcher requestDispatcher = req.getRequestDispatcher("/WEB-INF/form.html");

        // 走向Servlet2（柜台2）
        requestDispatcher.forward(req,resp);
    }
}

package top.qaqaq.P149;

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

import java.io.IOException;

/**
 * @author RichieZhang
 * @create 2022-12-05 下午 5:31
 */
public class Servlet2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        super.doGet(req, resp);

        //获取请求的参数（办事的材料）查看
        String username = req.getParameter("username");
        System.out.println("在Servlet2（柜台2）中查看参数（材料）：" + username);

        //查看 柜台1 是否有盖章
        Object key1 = req.getAttribute("key1");
        System.out.println("柜台1是否有章：" + key1);

        // 处理自己的业务
        System.out.println("Servlet2 处理自己的业务 ");
    }
}

    <servlet>
        <servlet-name>Servlet1</servlet-name>
        <servlet-class>top.qaqaq.P149.Servlet1</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>Servlet1</servlet-name>
        <url-pattern>/servlet1</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>Servlet2</servlet-name>
        <servlet-class>top.qaqaq.P149.Servlet2</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>Servlet2</servlet-name>
        <url-pattern>/servlet2</url-pattern>
    </servlet-mapping>