一、什么是servletContext：
servletContext可以理解為服務(wù)器端的一個(gè)共享空間，它可以被所有的客戶端訪問(wèn)。基于這種特性，我們就可以利用servletContext做計(jì)數(shù)器等應(yīng)用。
我們用個(gè)圖來(lái)描述cookie、session、servletContext的區(qū)別

//ShowTimesServlet.java /*Servlet實(shí)現(xiàn)訪問(wèn)次數(shù)的例子!*/

import java.io.*;

import java.util.zip.*;

import javax.servlet.*;

import javax.servlet.http.*;

/*

這個(gè)類實(shí)現(xiàn)訪問(wèn)次數(shù)。顯示訪問(wèn)次數(shù)！看是第幾次訪問(wèn)！

*/

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(guò)servleContext來(lái)做訪問(wèn)次數(shù)

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(qū)別 在于session是局部的,servletContext是共享的.
當(dāng)換了另一個(gè)窗口開始訪問(wèn)又從0開始計(jì)數(shù),而servletContext則是接下去計(jì)數(shù).

ssession和servletContext的比較，Servlet實(shí)現(xiàn)訪問(wèn)次數(shù)的例子