ServletContext

***使用ServletContext可实现不同客户端获取到相同的服务器数据 (如文章浏览次数, 点赞数等)

实例:

1. 先访问下面servlet在服务器创建ServletContext以便让不同客户端使用:

public class TestServletContext extends HttpServlet {

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        req.setCharacterEncoding("utf-8");

        //获取ServletContext对象 (三种方式)
        ServletContext sc = this.getServletContext();
        ServletContext sc2 = this.getServletConfig().getServletContext();
        ServletContext sc3 = req.getSession().getServletContext();

        //使用ServletContext对象完成数据共享    键的类型为String, 值的类型为Object
        sc.setAttribute("name", "尼古拉斯 凯奇");

        resp.getWriter().write("ServletContext对象已经创建, 其他用户都可以获取到此值--尼古拉斯 凯奇");

    }
}

2. 访问之后, 清除浏览器缓存(即cookie, 模拟不同浏览器访问), 再访问下面servlet, 获取到name的值, 实现了不同客户端访问相同的服务器数据:

public class TestServletContext2 extends HttpServlet {

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");

        ServletContext sc = this.getServletContext();
        Object name = sc.getAttribute("name");

        resp.getWriter().write("获取到的ServletContext的值是: " + name);
    }
}

 

***使用ServletContext也可以获取到web.xml中的全局配置数据:

web.xml中配置如下:

<?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">

注:一组<context-param>标签只能存储一组键值对数据, 多组可声名多个<context-param>进行存储

    <context-param>
        <param-name>name</param-name>
        <param-value>Ryan</param-value>
    </context-param>
    <context-param>
        <param-name>city</param-name>
        <param-value>杭州</param-value>
    </context-param>

    <!--配置servlet类路径-->
    <servlet>
　　　　......

public class TestServletContext3 extends HttpServlet {

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        req.setCharacterEncoding("utf-8");

        ServletContext sc = this.getServletContext();
        //根据键的名字返回web.xml中配置的全局数据的值, 返回String类型, 如果数据不存在返回null.
        String name = sc.getInitParameter("name");
        resp.getWriter().write("name的值是: " + name);
        //返回键名的枚举.
        Enumeration<String> ipa = sc.getInitParameterNames();
        resp.getWriter().write("获取的键名枚举为: " + ipa);

    }
}

浏览器访问此servlet后可获取web.xml中的相关值:

***获取项目根目录下资源的绝对路径

        String rp = sc.getRealPath("/aaa.text");
        System.out.println(rp);

另外还可以获取项目根目录下资源的流对象: