【ServletContext】获取全局配置参数

ServletContext可以看做是servlet上下文，每个web工程都只有一个ServletContext对象，不管在哪个servlet里面，获取到的类的对象都是同一个。
具体使用方法及作用：
1.用来获取全局配置参数
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">

    <context-param>
        <param-name>address</param-name>
        <param-value>深圳</param-value>
    </context-param>

    <servlet>
        <servlet-name>HelloServlet02</servlet-name>
        <servlet-class>HelloServlet02</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloServlet02</servlet-name>
        <url-pattern>/HelloServlet02</url-pattern>
    </servlet-mapping>
 </web-app>

HelloServlet02类文件：

  ...
  //get请求方法
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取对象
        ServletContext context = getServletContext();
        String address = context.getInitParameter("addresss");
        System.out.print(address);
    }
    ...