什么是域对象?
域对象，是可以像 Map 一样存取数据的对象，叫域对象。
这里的域指的是存取数据的操作范围，整个 web 工程。

                  存数据            取数据             删除 数据
Map               put()             get()             remove()
域对象            setAttribute()    getAttribute()     removeAttribute();

ServletContext 像 Map 一样存取数据：

package top.qaqaq.P134; /**
 * @author RichieZhang
 * @create 2022-12-05 下午 1:36
 */

import jakarta.servlet.*;
import jakarta.servlet.http.*;

import java.io.IOException;

public class ContextServlet1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        /*
        什么是域对象?
        域对象，是可以像 Map 一样存取数据的对象，叫域对象。
        这里的域指的是存取数据的操作范围，整个 web 工程。

                    存数据             取数据             删除 数据
        Map         put()              get()              remove()
        域对象      setAttribute()     getAttribute()     removeAttribute();
         */

        // 获取ServletContext对象
        ServletContext context = getServletContext();
        System.out.println(context);
        System.out.println("保存之前：Context1 获取key1的值是：" + context.getAttribute("key1"));

        context.setAttribute("key1","value1");

        System.out.println("Context1 中获取域数据key1的值是：" + context.getAttribute("key1"));
        System.out.println("Context1 中获取域数据key1的值是：" + context.getAttribute("key1"));
        System.out.println("Context1 中获取域数据key1的值是：" + context.getAttribute("key1"));
        System.out.println("Context1 中获取域数据key1的值是：" + context.getAttribute("key1"));
        System.out.println("Context1 中获取域数据key1的值是：" + context.getAttribute("key1"));
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

package top.qaqaq.P134; /**
 * @author RichieZhang
 * @create 2022-12-05 下午 1:45
 */

import jakarta.servlet.*;
import jakarta.servlet.http.*;

import java.io.IOException;

public class ContextServlet2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = getServletContext();
        System.out.println(context);
        System.out.println("Context2 中获取域数据key1的值是：" + context.getAttribute("key1"));

    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

    <servlet>
        <servlet-name>ContextServlet1</servlet-name>
        <servlet-class>top.qaqaq.P134.ContextServlet1</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ContextServlet1</servlet-name>
        <url-pattern>/context1</url-pattern>
    </servlet-mapping>
    
    <servlet>
        <servlet-name>ContextServlet2</servlet-name>
        <servlet-class>top.qaqaq.P134.ContextServlet2</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ContextServlet2</servlet-name>
        <url-pattern>/context2</url-pattern>
    </servlet-mapping>