Servlet学习（六）：ServletContext对象的作用

package demo01;
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 ContextServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    }
    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);
        String url = context.getInitParameter("url");
        System.out.println("context-param参数url的值是："+url);
//        2、获取当前的工程路径，格式：/工程路径
        System.out.println("当前工程路径是"+context.getContextPath());
//        3、获取工程部署后在服务器硬盘上的绝对路径
//              / 斜杠映射到IDEA工程下的web目录
        System.out.println("工程部署的绝对路径"+context.getRealPath("/"));
        System.out.println("工程目录下css目录的绝对路径"+context.getRealPath("/css"));
//        4、像Map一样存取数据
    }
}

<?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">
<!--    context-param是上下文参数，它属于整个web项目-->
    <context-param>
        <param-name>username</param-name>
        <param-value>context</param-value>
    </context-param>
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/test</param-value>
    </context-param>   
    <servlet>
        <servlet-name>ContextServlet</servlet-name>
        <servlet-class>demo01.ContextServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ContextServlet</servlet-name>
        <url-pattern>/contextServlet</url-pattern>
    </servlet-mapping>
</web-app>