ServletContext作用-获取web的上下文路径：

public class ContextDemo extends HttpServlet {
    private static final long serialVersionUID = 1L;
     
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("到啦");
        ServletContext context = this.getServletContext();
        String path = context.getContextPath();
        /*结果显示：/myFirstServlet
         但是注意，这个返回的是网站的路径，并不是我们这个项目的名称*/
    
        //作用：可以让这个获取文件的路径更加灵活
        /*当我们需要重定向到一个页面，但是我们的网站可能要修改，这样的话就不会造成什么影响*/
        response.sendRedirect(context.getContextPath()+"/hello.html");
    }
}

ServletContext作用-获取全局参数：
因为一个网站只存在一份ServletContext，所以该网站中所有的servlet都可以获取到该全局参数

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protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //获取所有参数
        Enumeration<String> enums = context.getInitParameterNames();
        //遍历参数名
        while(enums.hasMoreElements()){
            String paramName = enums.nextElement();
            //通过参数名获取参数值
            String paramValue = context.getInitParameter(paramName);
            System.out.println(paramName+":"+paramValue);
        }
    }

ServletContext作用-servlet域对象：
域对象相当于一个容器，可以从一个servlet中用ServletContext存入数据；
从另一个servlet中用ServletContext读取数据；

用ServletContext写入属性和值

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        context.setAttribute("name", "丁昌江");
        context.setAttribute("age", 22);//22会自动装箱为对象
    
        ArrayList list = new ArrayList();
        list.add("丁昌江");
        list.add("杨燕语");
        list.add("黄智欣");
        context.setAttribute("list", list);
        
        System.out.println("写入成功");
}

用ServletContext读取属性和值

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        ServletContext context = this.getServletContext();
        String name = (String) context.getAttribute("name");
        Integer age = (Integer) context.getAttribute("age");
        List list = (List) context.getAttribute("list");
        System.out.println("name:"+name);
        System.out.println("age:"+age);
        System.out.println("list:"+list);
        System.out.println("读取成功");
}

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