一、什麼是servletContext：

servletContext可以理解為伺服器端的一個共享空間，它可以被所有的客戶端訪問。基於這種特性，我們就可以利用servletContext做計數器等應用。
我們用個圖來描述cookie、session、servletContext的區別.
　　區別在於session是區域性的,servletContext是共享的.當換了另一個視窗開始訪問又從0開始計數,而servletContext則是接下去計數.
　　
上圖中，下面的A、B、C是代表客戶端的cookie，伺服器端的A、B、C代表session，D代表servletContext。

二、怎樣使用ServletContext

• 1、首先得到ServletContext: this.getServletContext();
• 2、ServletContext也是和session一樣像一張表，分為屬性、值
  新增屬性：setAttribute(String name,Object obj);
  得到值：getAttribute(String name);
  刪除屬性：removeAttribute(String name);
• 3、生命週期
  ServletContext中的屬性的生命週期從建立開始，到伺服器關閉而結束

三、Servlet實現訪問次數的例子!

/*Servlet實現訪問次數的例子!*/
 

public class ShowTimesServlet extends HttpServlet {

    public  doGet(HttpServletRequest request,HttpServletResponse response)
            throws ServletException, java.io.IOException {
        response.setContentType(" text/html");
        HttpSession session = request.getSession();
        String heading;
        Integer accessCount =(Integer)session.getAttribute(" accessCount"
 
);
        (accessCount == ) {
            accessCount =  Integer(0);
            heading = " Welcom,You are first time to visit!";
        }
        {
            heading = " Welcome Backer";
            accessCount =  Integer(accessCount.intValue()+1);
        }
        session.setAttribute(" accessCount",accessCount);
        PrintWriter out = response.getWriter();
        out.println(" The title:"+heading);
        out.println(" Access count: "+accessCount);
    }

    public  doPost(HttpServletRequest request,HttpServletResponse response)
            throws ServletException, java.io.IOException {
        doGet(request,response);
    }
}

下面是通過servleContext來做訪問次數
Java程式碼：

package org.linweihan.test;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class TestServletContext extends HttpServlet {

    @Override
    protected  doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext servletContext = .getServletContext();
        Integer accessCount =(Integer)servletContext.getAttribute(" accessCount");
        (accessCount == ){
            accessCount = 0;
        }{
            accessCount = accessCount + 1;

        }
        servletContext.setAttribute(" accessCount", accessCount);
        PrintWriter out = response.getWriter();
        out.println(" < html> < body> < h2> "+accessCount+" < /body> < /html> ");
    }
    protected  doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    }
}