一.application概念：
application即ServletContext，能够被所有的客户端页面共享，不同的浏览器，不同电脑上的浏览器
演示：
//ServletContextTest.java
package com.servlet;
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 ServletContextTest extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
out
.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
out.println("<HTML>");
out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>");
out.println(" <BODY>");
//这块内存可以被所有客户端共享
ServletContext application = this.getServletContext();
Integer count = (Integer)application.getAttribute("accessCount");
if(count==null){
count = new Integer(0);
}else{
count ++;
}
application.setAttribute("accessCount", count);
out.println(count);
out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doGet(request, response);
}
}
将这个页面配置好，访问： http://localhost:8080/Servlet_0500_Application/ServletContextTest ，不断刷新，发现数值在增长；换个浏览器，重新访问，发现值还在增加
注意application实际上指的是ServletContext接口，HttpServlet继承自GenericServlet，GenericServlet实现了ServletConfig接口，ServletConfig接口中有个getServletContext()的抽象方法，ServletContext application = this.getServletContext()，得到的就是这个ServletContext具体实现。
证明application是能被所有的客户端共享的