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
//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 {
}
}
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.
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