<!-- servlet标签给Tomcat配置Servlet程序   -->
    <servlet>
        <!-- servlet-name 标签 Servlet程序起一个别名（一般是类名）       -->
        <servlet-name>HelloServlet</servlet-name>
        <!-- servlet-class 标签 Servlet程序的全类名       -->
        <servlet-class>com.servlet.HelloServlet</servlet-class>
        <!--init-param是初始化参数        -->
        <init-param>
            <!--是参数名-->
            <param-name>username</param-name>
            <!--是参数值-->
            <param-value>root</param-value>
        </init-param>
        <init-param>
            <param-name>url</param-name>
            <param-value>jdbc:mysql://localhost:3306/test</param-value>
        </init-param>
    </servlet>

 @Override
    public void init(ServletConfig servletConfig) throws ServletException {
        System.out.println("2 执行init 初始化");
//        1、可以获取Servlet程序的别名servlet-name 的值
        System.out.println("HelloServlet程序的别名是："+servletConfig.getServletName());
//        2、获取初治化参数 init-param
        System.out.println("初始化参数username的值是："+servletConfig.getInitParameter("username"));
        System.out.println("初始化参数url的值是："+servletConfig.getInitParameter("url"));
//        3、获取 ServletContext对象
        System.out.println(servletConfig.getServletContext());
    }

package com.servlet;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;
public class ContextServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//        1、获取web.xml中配置的上下文参数context-param
        ServletContext context = getServletConfig().getServletContext();
        String username = context.getInitParameter("username");
        System.out.println("context-param参数username的值是："+username);
        System.out.println("context-param参数username的值是："+context.getInitParameter("password"));
        System.out.println("init-param参数url的值是："+context.getInitParameter("url"));//null
//        2、获取当前的工程路径，格式:工程路径
        System.out.println("当前工程路径："+context.getContextPath());
//        3、获取工程部署后在服务器硬盘上的绝对路径
        /**
         *   / 斜杠被服务器解析地址为：http://ip:prot/工程名/     映射到IDEA代码的web目录
         *
         */
        System.out.println("工程部署的路径是："+context.getRealPath("/"));
        System.out.println("工程下css目录的绝对路径是："+context.getRealPath("/css"));
        System.out.println("工程下imgs目录1.jpg的绝对路径是："+context.getRealPath("/imgs/1.jpg"));
    }
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    }
}

 <!--    Context-Param是上下文参数（它属于整个web工程）-->
    <context-param>
        <param-name>username</param-name>
        <param-value>context</param-value>
    </context-param>
    <context-param>
        <param-name>password</param-name>
        <param-value>root</param-value>
    </context-param>
   <servlet-mapping>
        <servlet-name>ContextServlet</servlet-name>
        <url-pattern>/contextServlet</url-pattern>
    </servlet-mapping>

package com.servlet;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;
public class ContextServlet1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext context = getServletContext();
        System.out.println(context);
        System.out.println("保存之前：Context1 中获取域数据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"));
    }
}

package com.servlet;
import javax.servlet.*;
import javax.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"));
    }
}