ServletContext对象

ServletContext概述

• 全局对象，也拥有作用域，对应一个Tomcat中的web应用
• 当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提供了getServletContext()方法，（推荐使用） this.getServletContext();
• HttpServletRequest提供了getServletContext()方法（推荐）
• HttpSession提供了getServletContext()方法 

ServletContextController.java

package com.newer.servletProject.servlet.controller;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;

@WebServlet(name = "ServletContextController",value = "/ctxController")
public class ServletContextController extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.通过 this.getServletContext()
        ServletContext servletContext=this.getServletContext();
        //2.通过request对象获取
        ServletContext servletContext1=request.getServletContext();
        //3.通过session获取
       /* HttpSession session=request.getSession();
        ServletContext servletContext2=session.getServletContext();*/

        //获取当前项目路径
        System.out.println(servletContext.getRealPath("/"));
        //获取当前项目上下文路径(应用程序名称)
        System.out.println(servletContext.getContextPath());
        //存值
        servletContext.setAttribute("context","info");


    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request,response);
    }
}

 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("/");

获取项目上下文路径

获取当前项目上下文路径（应用程序名称）

 servletContext.getContextPath();

request.getContextPath();

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.java

package com.newer.servletProject.servlet.controller;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;

@WebServlet(name = "ServletContextController",value = "/ctxController")
public class ServletContextController extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.通过 this.getServletContext()
        ServletContext servletContext=this.getServletContext();
        //2.通过request对象获取
        ServletContext servletContext1=request.getServletContext();
        //3.通过session获取
       /* HttpSession session=request.getSession();
        ServletContext servletContext2=session.getServletContext();*/

        //获取当前项目路径
        System.out.println(servletContext.getRealPath("/"));
        //获取当前项目上下文路径(应用程序名称)
        System.out.println(servletContext.getContextPath());
        //存值
        servletContext.setAttribute("context","info");


    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request,response);
    }
}

ShowContextController.java

package com.newer.servletProject.servlet.controller;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(name = "ShowContextController",value = "/showController")
public class ShowContextController extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //通过 this.getServletContext获取servletContext
        ServletContext servletContext=this.getServletContext();
        //取值
       String s= (String)servletContext.getAttribute("context");
       System.out.println(s);

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request,response);
    }
}

ServletContext特点

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

 ServletContext应用场景

ServletContext统计当前项目访问次数

 CountController.java

package com.newer.counter;
/**
 * 统计项目访问次数
 */

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(name = "CountController",value = "/counterController")
public class CountController extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
       //获取ervletContext对象
        ServletContext servletContext=request.getServletContext();
        //获取计数器
        Integer counter=(Integer)servletContext.getAttribute("counter");
        //判断是否是第一次访问
        if(counter==null){
            counter=1;
            servletContext.setAttribute("counter",counter);
        }else {
            counter++;
            servletContext.setAttribute("counter",counter);
        }
        System.out.println("counter:"+counter);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request,response);
    }
}

ShowCountController.java

package com.newer.counter;
/**
 * 作用域：
 * HttpServletRequest:一次请求，请求响应之前有效
 * HttpSession:一次会话，浏览器不关闭或者不超时之前有效---登录验证码
 * ServletContext:服务器启动开始，服务器停止之前有效
 */

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(name = "ShowCounterController",value = "/showCounterController")
public class ShowCounterController extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ervletContext对象
        ServletContext servletContext=request.getServletContext();
        //获取计数器
        Integer counter=(Integer)servletContext.getAttribute("counter");

        //判断是否是第一次访问
        if(counter==null){
            counter=1;
            servletContext.setAttribute("counter",counter);
        }else {
            counter++;
            servletContext.setAttribute("counter",counter);
        }
        System.out.println("show:"+counter);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request,response);
    }
}

作用域总结

• HttpServletRequest:一次请求，请求响应之前有效（数据传递）
• HttpSession:一次会话开始，浏览器不关闭或者不超时之前有效（登录权限认证）
• ServletContext:服务器启动开始，服务器停止之前有效（上升到全局容器，如计数器）

本文地址：https://blog.csdn.net/weixin_44364444/article/details/109555819