session和servletContext的比较，Servlet实现访问次数的例子

一、什么是servletContext：   

　　servletContext可以理解为服务器端的一个共享空间，它可以被所有的客户端访问。基于这种特性，我们就可以利用servletContext做计数器等应用。
我们用个图来描述cookie、session、servletContext的区别.

　　区别在于session是局部的,servletContext是共享的.当换了另一个窗口开始访问又从0开始计数,而servletContext则是接下去计数.

  上图中，下面的A、B、C是代表客户端的cookie，服务器端的A、B、C代表session，D代表servletContext。

二、怎样使用ServletContext

   1、首先得到ServletContext

      this.getServletContext();

    2、ServletContext也是和session一样像一张表，分为属性、值

      添加属性：setAttribute(String name,Object obj);
      得到值：getAttribute(String name);
      删除属性：removeAttribute(String name); 

  3、生命周期

      ServletContext中的属性的生命周期从创建开始，到服务器关闭而结束

三、Servlet实现访问次数的例子!

ShowTimesServlet.java 

//ShowTimesServlet.java /*Servlet实现访问次数的例子!*/
import java.io.*;
import java.util.zip.*;
import javax.servlet.*;
import javax.servlet.http.*;
/*
这个类实现访问次数。显示访问次数！看是第几次访问！
*/
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);
}    
}

下面是通过servleContext来做访问次数
Java代码：

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 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 {         
}
}

2011年11月25日 21:20:18ujn