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ServletContext对象

概述

全局对象，拥有作用域，对应Tomcat的Web应用Web服务器启动时，会为每一个Web应用程序创建一块共享的存储区域（ServletContext）ServletContext在Web服务器启动时创建，服务器关闭时销毁

获取ServletContext对象

GenericServlet提供 getServletContext(); 方法 this.getServletContext();HttpServletRequest提供 getServletContext(); 方法HttpSession提供 getServletContext(); 方法

//获取ServletContext对象ServletContext servletContext = this.getServletContext();//通过request对象，获取ServletContext对象ServletContext servletContext1 = request.getServletContext();//通过request获取session对象，获取ServletContext对象HttpSession session = request.getSession();ServletContext servletContext2 = session.getServletContext();

ServletContext 作用

获取项目真实路径（获取当前项目在服务器发布的真实路径）

String realpath = servletContext.getRealPath("/")；

        //获取ServletContext对象        ServletContext servletContext = this.getServletContext();        //获取项目真实路径        System.out.println(servletContext.getRealPath("/"));/*输出结果D:\Eclipse\apache-tomcat-8.5.82-windows-x64\apache-tomcat-8.5.82\webapps\Servlet_Projects_war\*/

获取项目上下文路径（获取当前项目上下文路径：应用程序名称）

System.out.println(servletContext.getContextPath());System.out.println(request.getContextPath());

//获取ServletContext对象ServletContext servletContext = this.getServletContext();//获取项目上下文路径System.out.println(servletContext.getContextPath());System.out.println(request.getContextPath());/*输出结果：/Servlet_Projects_war/Servlet_Projects_war*/

全局容器

ServletContext拥有作用域，可以存储数据到全局容器中

存储数据： servletContext.setAttribute(“name”,“value”);获取数据： servletContext.getAttribute(“name”);移除数据： servletContext.removeAttribute(“name”);

ServletContextController类

@WebServlet(name = "ServletContextController", value = "/ctxController")public class ServletContextController extends HttpServlet {    @Override    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {    doPost(request, response);    }    @Override    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        //获取ServletContext对象        ServletContext servletContext = this.getServletContext();        //获取项目真实路径        System.out.println(servletContext.getRealPath("/"));        //存储数据        servletContext.setAttribute("context","info");    }}

ShowContextController类

@WebServlet(name = "ShowContextController", value = "/ShowController")public class ShowContextController extends HttpServlet {    @Override    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {    doPost(request, response);    }    @Override    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        //获取ServletContext对象        ServletContext servletContext = this.getServletContext();                //获取数据        String context = (String) servletContext.getAttribute("context");        System.out.println(context);                 /*        输出结果：        D:\Eclipse\apache-tomcat-8.5.82-windows-x64\apache-tomcat-8.5.82\webapps\Servlet_Projects_war\        info        */    }}

ServletContext 特点

唯一性：一个应用对应一个ServletContext生命周期：只要容器不关闭或者应用不卸载，ServletContext就一直存在

ServletContext 应用场景

CounterController类

@WebServlet(name = "CounterController", value = "/counterController")public class CounterController extends HttpServlet {    @Override    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        doPost(request, response);    }    @Override    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        //获取ServletContext对象        ServletContext servletContext = request.getServletContext();        //获取计数器        Integer counter = (Integer) servletContext.getAttribute("counter");        //计数器初始化        if (counter == null) {            counter = 1;            //counter存入计数器            servletContext.setAttribute("counter", counter);        } else {            counter++;            servletContext.setAttribute("counter", counter);        }                System.out.println("counter:" + counter);    }}

ShowCounterController类

@WebServlet(name = "ShowCounterController", value = "/ShowCounterController")public class ShowCounterController extends HttpServlet {    @Override    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        doPost(request, response);    }    @Override    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        //获取ServletContext对象        ServletContext servletContext = request.getServletContext();        //获取计数器        Integer counter = (Integer) servletContext.getAttribute("counter");                //计数器初始化        if (counter == null) {            counter = 1;            //counter存入计数器            servletContext.setAttribute("counter", counter);        } else {            counter++;            servletContext.setAttribute("counter", counter);        }        System.out.println("show:" + counter);    }}

结果

counter:1show:2show:3show:4show:5show:6show:7

作用域总结

HttpServletRequest：一次请求，请求响应之前有效HttpSession：一次会话开始，浏览器不关闭或不超时之前有效ServletContext：服务器启动开始，服务器停止之前有效

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