Javaweb | ServletContext对象

绝地灬酷狼 2024-04-22 19:12 54阅读 0赞

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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:1
>     show:2
>     show:3
>     show:4
>     show:5
>     show:6
>     show: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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