一、什么是servletContext：
servletContext可以理解為服務器端的一個共享空間，它可以被所有的客戶端訪問。基于這種特性，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

//ShowTimesServlet.java /*Servlet實現訪問次數的例子!*/

import java.io.*;

import java.util.zip.*;

import javax.servlet.*;

import javax.servlet.http.*;

/*

這個類實現訪問次數。顯示訪問次數！看是第幾次訪問！

*/

public class ShowTimesServlet extends HttpServlet {

public void 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" );

if (accessCount == null ) {

accessCount = new Integer( 0 );

heading = " Welcom,You are first time to visit!" ;

}

else {

heading = " Welcome Backer" ;

accessCount = new Integer(accessCount.intValue()+ 1 );

}

session.setAttribute( " accessCount" ,accessCount);

PrintWriter out = response.getWriter();

out.println( " The title:" +heading);

out.println( " Access count: " +accessCount);

}

public void 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

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 void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

ServletContext servletContext = this .getServletContext();

Integer accessCount =(Integer)servletContext.getAttribute( " accessCount" );

if (accessCount == null ){

accessCount = 0 ;

} else {

accessCount = accessCount + 1 ;

}

servletContext.setAttribute( " accessCount" , accessCount);

PrintWriter out = response.getWriter();

out.println( " < html> < body> < h2> " +accessCount+ " < /body> < /html> " );

}

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

}

}

區別 在于session是局部的,servletContext是共享的.
當換了另一個窗口開始訪問又從0開始計數,而servletContext則是接下去計數.

ssession和servletContext的比較，Servlet實現訪問次數的例子